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In this study, we investigated four different ferroan dolomite samples using in situ Raman spectroscopy and powder X-ray diffraction (XRD) at pressures up to 48 GPa and room temperature. Our results show that the transition from Dolomite-I (Dol-I) to Dolomite-II (Dol-II) occurs above 13–16 GPa, and the transition pressure depends on the composition of the solid solution. Compression above 32–35 GPa results in the appearance of Dolomite-IIIc (Dol-IIIc) or Dolomite-IIIb (Dol-IIIb). In the high-pressure XRD study, we found that the XRD patterns of Ca0.97(Mg0.77Fe0.23Mn0.03)(CO3)2 (xFe = 0.23, Ank23) can be indexed as Dol-IIIc at 44 GPa, while the rhombohedral Dol-IIIb structure matches better with the XRD patterns of the xFe = 0.40 (Ank40) and 0.64 (Ank64) solid solutions. Additionally, in the Raman spectra of the Fe-richest sample (Ank64), we observed an abrupt frequency downshift of the CO3-stretching vibrations between 40–42 GPa, which may reflect a pressure-induced Fe2+ spin transition. We further investigated two samples with xFe = 0.19 (Ank19) and 0.23 (Ank23) at high pressure and high temperatures, up to at least 2600 K. The experiments revealed that the unquenchable Dol-IIIc structure could be a stable high-pressure/high-temperature polymorph in ferroan dolomite up to at least 2600 K. [ABSTRACT FROM AUTHOR]

New petrographic and microanalytical studies of mineral inclusions in the Purang ophiolitic chromitites (SW Tibet) are used to scrutinize the evolution of the associated Cretaceous sub-oceanic lithospheric mantle section. Silicate inclusions in the chromite grains include composite and single-phase orthopyroxene, clinopyroxene, amphibole, and uvarovite. Most inclusions are sub-rounded or globular, whereas a few inclusions exhibit cubic/octahedral crystal morphologies. The latter are randomly distributed in the large chromite grains, though discrete aggregates are consistently confined to the grain centers. Abundant micrometer-scale, clinopyroxene inclusions are topotaxially aligned along crystallographic planes. Less-abundant sulfide, wüstite, apatite, and uvarovite inclusions are observed in some samples. The trace element geochemistry of the Purang chromitite evoke parental MORB- and boninitelike melts, consistent with the supra-subduction zone setting. The δ26Mg values of the high-Cr and high-Al chromitites range from –0.25 to –0.29‰ and –0.05 to –0.32‰, respectively. The associated harzburgite has nearly overlapping δ26Mg values of –0.13 to –0.37‰, but pyroxenite sills show distinct δ26Mg values (–0.61 to –0.67‰). The variable Mg isotope signatures, combined with abundant exotic, ultrahigh-pressure and super reduced (UHP-SuR) mineral inclusions in the chromite grains, suggest that recycling and recrystallization under different mantle conditions played an important role in the genesis and evolution of these rocks. Furthermore, discrete silicate, sulfide, and metal alloy inclusions in the Purang chromitites are comparable to those reported in other Tethyan ophiolites, and collectively suggest a common geodynamic evolution. [ABSTRACT FROM AUTHOR]

The new minerals wenjiite, Ti10(Si,P,D)7 (IMA2019-107c) and kangjinlaite, Ti„(Si,P)10 (IMA2019-112b) occur with badengzhuite, zhiqinite, and a K-bearing dmisteinbergite-like mineral in a spheroid 20 ^m across enclosed in corundum from the Cr-11 podiform chromitite orebody near the Kangjinla, Luobusa ophiolite, Tibet, China. In addition, wenjiite occurs with deltalumite, jingsuiite, osbornite-khambaraevite, and the K-bearing dmisteinbergite-like mineral in a lamellar intergrowth 100 ^m long, also enclosed in corundum from the same locality. The new minerals were characterized by energy-dispersive spectroscopy and three-dimensional electron diffraction, which enabled us to obtain an ab initio structure solution and dynamical refinement from grains a few micrometers across hosted in a FIB lamella. Four analyses of wenjiite from the spheroid gave in wt% Si 21.67, P 6.24, Ti 66.39, V 1.37, Cr 2.20, Mn 0.97, and Fe 1.17 (normalized to 100), which corresponds to (Ti093Cr0.03Mn0.01Fe0.01V0.02)10 (Si0.79P0.21)6.51 on the basis of 10 cations excluding Si and P. The simplified formula is Ti10(Si,P)65, or more generally Ti10SixPy, where x > y and 6 < (x + y) < 7, i.e., Ti10(Si,P,D)7. Wenjiite has hexagonal symmetry, space group: P63/mcm (no. 193), with a = 7.30(10) A, c = 5.09(10) A, V = 235(6) A3, Z = 1, and is isostructural with xifengite, mavlyanovite, synthetic Ti5Si3, and synthetic Ti5P315. Four analyses of kangjinlaite gave in wt% Si 25.56, P 9.68, Ti 62.35, V 0.21, Cr 0.83, Mn 0.42, and Fe 0.95 (normalized to 100), which corresponds to (Ti10.65V0.03Cr0.13Mn0.06Fe0.14)m.01(Si7.43P2.55)S9.99. The simplified formula is Tin(Si,P)10. Kangjinlaite is tetragonal, with space group: I4/mmm (no. 139), a = 9.4(2) A, c = 13.5(3) A, V = 1210(50) A3, Z = 4, and is isostructural with synthetic compounds of the HonGe10 type, being the most compact of these phases. Despite there now being over 70 compounds containing 38 elements isostructural with HonGe10, synthesis of an analog of kangjinlaite has not been previously reported in either the Ti-P or Ti-Si binary systems or in a multicomponent system. The previously deduced crystallization sequence with decreasing temperature of the four minerals in the spheroid is wenjiite ^ kangjinlaite ^ zhiqinite + badengzhuite. This sequence is consistent with relationships reported in 9 binary systems containing intermetallic compounds of Ge and Sn isostructural with Mn5Si3 and HouGe10. In eight of these systems the Mn5Si3 analog melts congruently, whereas the HonGe10 analog never does. Instead, the HonGe10 analog melts peritectically, generally to an Mn5Si3 analog and less commonly to compounds with 5:4 stoichiometry. Final crystallization of the spheroid to zhiqinite + badengzhuite is expected to be well below the temperature of 1500 °C for the congruent melting of zhiqinite in the Ti-Si system, i.e., in the range of ~1100-1300 °C. [ABSTRACT FROM AUTHOR]

Chemical zoning according to the substitution Al + (OH,F) = Ti + O is a common phenomenon in natural titanite and can be used to reconstruct its growth conditions. We synthesized titanite, which has a zoning pattern similar to that found in natural titanite that grew at the expense of rutile, visible in element distribution maps as a patchy irregular pattern. Run conditions in the system CaO-[Al.sub.2][O.sub.3]- Ti[O.sub.2]-Si[O.sub.2]-NaCl-[H.sub.2]O(-Ca[F.sub.2]) were 600[degrees]C, 0.4 GPa, 1 to 107 days duration. Natural rutile crystals were placed in a perforated Pt-tube within an outer Au-capsule containing wollastonite, [gamma]-[Al.sub.2][O.sub.3], optionally powdered Ca[F.sub.2], and a NaCl brine, to simulate conditions, where ruffle is transformed into titanite by a hydrous Ca-Si-Al(-F) fluid, driven by a chemical potential gradient between inner and outer capsule. Spontaneous nucleation of titanite is restricted to a small number of crystals on the rutile surface in the inner capsule. Growth proceeds from sparse isolated titanite crystals after 1 day to assemblages of several 100 [micro]m large crystals in a reaction rim in the long-time runs. Titanite is strongly zoned in Al-Ti and shows up to ~0.5 Al per formula unit (pfu) in experiments containing F; without F, substitution is limited to ~0.25 Al pfu. The range of the Ti-Al exchange is already large in short run times and the same heterogeneity is observed in long-time runs. The Al-Ti distribution (with and without F) in compositional domains (up to several tens of micrometers) is patchy and irregular. No relation of the zoning with time (core-rim) or preferred growth directions was observed. In the case of fluid-mediated growth, mineral growth zoning represents a disequilibrium at supersaturated conditions. The concentration of Ti in the fluid is likely the limiting factor for titanite growth considering the low solubility compared to that of Al, Ca, and Si. Titanium is not transported in significant amounts into the outer capsule, indicating low mobility, in contrast to Al, Ca, and Si, which are transported into the inner capsule. Assuming buffered Al concentration from dissolution of the Al-source, we speculate that the Ti/Al of the fluid at the precipitation site was controlled by the local Ti concentration, which varies over the distance from the rutile surface to the growing titanite crystal and produces the irregular patchy zoning. Keywords: Dissolution-precipitation reaction, Al-Ti substitution, mineral zoning, experiments DOI: 10.2138/am.2010.3518

Anhydrous and hydrous wadsleyite were synthesized at 13.3-13.5 GPa and 1150-1200[degrees]C in a multianvil press and investigated by Fourier transform infrared (FTIR) spectroscopy, single-crystal X-ray refinement (SC-XRD), and electron microprobe analyses (EMPA). The FTIR spectra agree with previous data, i.e., the spectra are dominated by a broad band around 3380 [cm.sup.-1], resolvable in three bands 3326 ([v.sub.2]), 3382 ([v.sub.3]), and 3546 ([v.sub.4]) [cm.sup.-1] besides some weaker OH-bands around 3600 [cm.sup.-1]. We confirm that wadsleyite incorporates water in the wt% range and that the concentration strongly increases with decreasing temperature when using secondary ion mass spectrometry (SIMS) and Raman spectroscopy. The quantifications combined with FTIR spectra led us to develop the first IR calibration for water in wadsleyite, i.e., calculating an [[epsilon].sub.i,tot] of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. A SC-XRD determination of hydrous wadsleyite FD0718, bearing 8000 [+ or -] 1000 wt ppm [H.sub.2]O, certifies the presence of Mg vacancies at the M3 sites as previously suggested. Furthermore, we found maxima in the electron density map close to the O atoms O1 and O3 of an M3 octahedron assuming the anhydrous structure. Based on our new data we suggest that the main protonation in wadsleyite occurs along the O1 ... O4 (3.1 [Angstrom]) and O3 ... O4 (3.05 [Angstrom]) edges of a vacant M3 octahedron. H-incorporation seems to be random leading to protonation of either two O1, two O3, or one O1 and one O3 of the vacant M3 octahedra. With this assignment, the observed ambient and high-pressure IR pattern can now be explained. Keywords: Wadsleyite, FTIR spectroscopy, Raman spectroscopy, protonation, SIMS, absorption coefficient for water DOI: 10.2138/am.2010.3267

Complex textures in zircon associated with Ta oxides have been used to assess the processes at the origin of zircon crystallization and associated Ta mineralization in pegmatites, in particular the role of magmatic vs. hydrothermal processes. The Tanco pegmatite is used as an example because its zircon is devoid of post-magmatic alteration and its petrogenesis is well constrained. Zircon in primary units is metamict with high U-Pb-Th contents. By contrast, in secondary assemblages affected by late-magmatic micaceous alteration, zircon is devoid of radiogenic elements, but it may contain abundant [Ta.sub.2][O.sub.5] (up to 4.7 wt%). The incorporation of Ta into zircon may occur through coupled substitution mechanisms involving other minor elements such as P, Al, Mn, or Li. The presence of Ta accounts for the distorted structures in the Ta-rich zircon, as revealed in high-resolution TEM images. Four zircon types occur sequentially in single zircon crystals, which permits a new model for zircon growth and evolution in rare element pegmatites to be advanced: (1) zircon (Z1) is Ta-rich and crystallized possibly before discrete Ta phases; (2) Z1 was overgrown by regularly zoned zircon Z2 and Z3, which show lower Ta and extreme Zr/Hf fractionation (Hf[O.sub.2] up to 38.9 wt%), suggesting crystallization from a highly fractionated melt, possibly at the onset of Ta mineralization; and (3) close to the solidus, the aqueous fluid at the origin of micaceous alteration corroded the distorted structure of Z1, and a low-Ta zircon (Z4) replaced Z1 by dissolution-reprecipitation, whereas Z2 and Z3 resisted this alteration. Tantalum was no longer stable in the zircon structure and crystallized as Ta-oxide inclusions in the reequilibrated zircon (Z4). Keywords: Zircon, tantalum mineralization, hydrothermal alteration, pegmatite, Tanco

To gain insight into hydroxyl solubilities and possible hydration mechanisms of mantle silicates, as well as to test the utility of germanium analog models in studies of water-related defects, our present work is focused on the protonation of germanium analogs of silicates. For this purpose Ge-analogs of ringwoodite, anhydrous phase B (anhB), and for the first time, superhydrous phase B (shyB), were synthesized in a piston cylinder device at 2 GPa and 950-1000 [degrees]C under water-excess conditions. Electron probe microanalysis (EPMA), transmission electron microscopy (TEM), and X-ray diffraction as well as Raman and infrared (IR) spectroscopy were used to characterize the experimental products. Ge-ringwoodite incorporates from 900 to 2200 ppm [H.sub.2]O by weight, which is much less than Smyth et al. (2003) observed for the Si-equivalent synthesized at 22 GPa and 1500[degrees]C, but 200X more than published for [gamma]-[Mg.sub.2]Ge[O.sub.4] by Hertweck and Ingrin (2005). In addition to this discrepancy, the incorporation mechanism of H in Ge-ringwoodite also differs from that of Si-ringwoodite. Ge-anhB, which is currently believed to be anhydrous in the Si-system, contains from 2400 to 5300 ppm water by weight. A hydration model for germanate anhB was constructed based on single-crystal X-ray diffraction analysis and IR spectroscopy, in which OH is incorporated via the hydrogarnet substitution [[[V.sub.Ge] x 4[(OH).sub.O]].sup.x] and via vacant Mg sites [[[V.sub.Mg] x 2[(OH).sub.O]].sup.x]. For Ge-shyB the water concentration and incorporation mechanism obtained in this study are identical to results reported for the silicate phase synthesized at 22 GPa and 1200[degrees]C (Koch-Muller et al. 2005). Thus, germanates are good low-pressure analogs for hydrous mantle silicates in which protonation is controlled by stoichiometry. However, for nominally anhydrous minerals we cannot recommend the use of germanates as high-pressure models in water-related studies. In these Ge-analogs, which are usually synthesized at much lower pressures, i.e., lower water fugacities, OH incorporation seems to differ from the high-pressure silicate equivalents qualitatively and quantitatively, as hydroxyl solubility is governed by other factors such as water fugacity and intrinsic defects. Keywords: Germanates, ringwoodite, nominally anhydrous minerals, FTIR, anhydrous phase B, superhydrous phase B

The mineral eskolaite ([Cr.sub.2][O.sub.3]) has been discovered in association with natural diamond from the Udachnaya pipe in Yakutia, where it is intergrown with an octahedral diamond, mostly as an inclusion in the diamond, but also exposed at its surface. A detailed study was performed on fragments extracted from the outer surface of the diamond, using single-crystal X-ray diffraction (XRD), high-resolution electron microscopy (HRTEM), analytical electron microscopy (AEM), including line-scan and elemental-mapping, EMP, and SIMS. These applied techniques confirmed the nature of the eskolaite with 86.8 wt% [Cr.sub.2][O.sub.3] and notable impurities of Ti[O.sub.2] (3.99 wt%), [Al.sub.2][O.sub.3] (2.00 wt%), [Fe.sub.2][O.sub.3] (5.83 wt%), and MgO (1.11 wt%). Trace elements, including V (4900 ppm), Mn (129 ppm), Zr (56 ppm), and Nb (32 ppm) were also detected. The entire range of REE is just at or below the limits of detection. A small picrochromite inclusion ([X.sub.Mg] 81.2; [Y.sub.Cr] 94.7) was detected in the eskolaite; its chemistry is typical of chromite diamond inclusions. It also contains minute inclusions of perovskite, corundum, and an unidentified Ti-phase. Nano-sized cavities in picrochromite were determined to consist of carbonate and quench products, including Si, Mg, Ca, P, K, and Cl. This may represent relics of the diamond-forming metasomatic fluids. The eskolaite, containing a picrochromite inclusion, was formed at high pressure within the diamond stability field from C-O-H-bearing fluids containing Ca, K, Cl, P, and possibly even peridotitic (U-type) oxides and silicates. Keywords: Diamond association, intergrowth, eskolaite, picrochromite, Udachnaya kimberlite pipe, Yakutia, Russia

Monazite is both partially replaced and overgrown by a ThSi[O.sub.4] phase along grain rims in a series of experiments from 300 to 900 [degrees]C and 200 to 1000 MPa. All experiments consisted of 10 mg of 100-500 [micro]m size, euhedral to subhedral crystals of a natural Th-free monazite-(Ce), 5 mg of Th[(N[O.sub.3]).sub.4]*5[H.sub.2]O, 2.5 mg of Si[O.sub.2], and 5 mg of [H.sub.2]O loaded into 3 mm wide, 1 or 1.3 cm long platinum capsules that were arc welded shut. Experimental conditions were: 300 [degrees]C at 200 and 500 MPa; 300, 400, 500, 600, and 700 [degrees]C at 500 MPa (cold seal hydrothermal autoclave); and 900[degrees]C at 1000 MPa (Catz assembly; pistoncylinder press). Back-scattered electron (BSE) imaging, electron back-scattered diffraction (EBSD) analysis, and transmission electron microscopy (TEM) indicates that in the experiments from 500 to 900[degrees]C, the ThSi[O.sub.4] phase took the form of monoclinic huttonite implying that huttonite, associated with monazite, could exist metastably over a much greater P-T range than previously thought. TEM analysis of a foil cut perpendicular to the monazite-huttonite interface from the 600 [degrees]C, 500 MPa experiment using a focused ion beam (FIB) indicates that the huttonite as well as the interface between the huttonite and monazite is characterized by fluid inclusions. High-resolution TEM analysis indicates that the huttonite-monazite interface is coherent. In the case of replacement of monazite by huttonite, fluid-aided dissolution-reprecipitation is proposed as the most likely mechanism responsible. Keywords: Monazite, huttonite, dissolution-reprecipitation, metastability, metasomatism, EBSD, TEM, pseudomorphism

The release of Zr, U, and Pb from nearly metamict zircon and its recrystallized analog and of Zr from fully crystalline and slightly radiation-damaged zircon in [H.sub.]O + HCl [+ or -] Si[O.sub.2] fluids was investigated in situ at temperatures between 200 and 500[degrees]C using a hydrothermal diamond-anvil cell and time-resolved synchrotron-radiation XRF analyses. Dissolution of nearly metamict zircon proceeded much faster than that of zircon with little or no radiation damage and resulted in a 1.5 to 2 log units higher Zr molality in 6 to 7 m HCl fluids. Extensive recrystallization of the almost fully amorphous material started at 260 to 300[degrees]C in [H.sub.2]O + HCl, and at about 360[degrees]C if quartz was added, and was coupled with a decrease of the Zr concentration in the fluid by more than an order of magnitude. Recrystallization in 7 m HCl had little effect on the aqueous U and Pb concentrations, whereas addition of quartz caused a more sluggish decrease of the Zr concentration in the fluid upon recrystallization and lowered the release of U. The data presented here support the interpretation that enhanced Zr mobility in low-grade metamorphic rocks may be related to dissolution of metamict zircon by aqueous fluids and illustrate the significance of the silica activity on the kinetics of dissolution and recrystallization during zircon-fluid interaction. Keywords: High-temperature studies, zircon-aqueous fluid interactions, hydrothermal diamond-anvil cell, in-situ SR-XRF analysis, dissolution kinetics, order-disorder, metamictization, recrystallization, fluid phase, [H.sub.2]O + HCl [+ or -] Si[O.sub.2], recrystallization kinetics

In the Monte Rosa area, Northern Italy, the assemblage garnet + phengite + quartz was formed in polymetamorphic metapelites from pre-existing biotite and plagioclase during an eclogite-facies Alpine metamorphic overprint. While phengite nucleated within plagioclase, garnet formed l0 to 20 [micro]m wide continuous rims along the original biotite-plagioclase interphase boundaries. Garnet formation involved diffusion of Ca-, Fe-, and Mg-bearing species across the growing rims. The garnet rims show an overall asymmetric compositional zoning and distinct nanometer scale chemical patterns across grain boundaries within the garnet polycrystal. The compositional patterns resulted from the interplay of diffusion along grain and interphase boundaries and volume diffusion. Individual garnet crystals are separated by low-angle grain boundaries, which are predominantly oriented perpendicular to the reaction rim giving rise to an overall palisade microstructure. Grain boundaries contain arrays of closely spaced channels about 2 nm wide, which are filled with an amorphous phase. These channels represent direct links between the garnet-biotite and garnet-plagioclase reaction fronts and provide pathways for fast diffusion. From numerical simulation of the observed chemical patterns, diffusion of Ca and Fe is inferred to have been 5-6 orders of magnitude faster within grain boundaries than through the interior of garnet grains. Although the amorphous grain boundary phase only takes up a small fraction (2-5 per rail) of the total volume of the garnet polycrystal, short-circuit diffusion along grain boundaries contributed substantially to material transfer across the growing garnet rim. Despite the presence of fast pathways, bulk diffusion was too slow to allow for chemical equilibration of the phases involved in garnet rim formation, even on a micrometer scale. Relying on published garnet volume diffusion data, [D.sub.gb] (where gb = grain boundary), is estimated to be on the order of [l0.sup.-(19-20)] [m.sup.2]/s at the estimated reaction conditions of 650 [degrees]C and 12.5 kbar, where [D.sup.Ca.sub.gb] is slower than [D.sup.Fe.sub.gb] by a factor of ten. Keywords: Garnet, high-pressure metamorphism, grain boundary, diffusion, TEM

Fluid-bearing microinclusions in diamonds ( Carbonates appear in all assemblages, together with either halides or silicates, indicative of the importance of carbonatitic high-density fluid during diamond growth and fluid evolution. The presence of halide-carbonate or silicate-carbonate assemblages is in agreement with the bulk composition of the microinclusions as detected by EPMA. The high K content of some microinclusions detected by EPMA cannot be accounted for by the solid phases analyzed by TEM. This discrepancy suggests that K is concentrated in the residual fluid that is lost during TEM sample preparation. In addition to microinclusions, large cavities containing amorphous phases were found in the inner parts of one Siberian and one Canadian diamond. An Al-rich phase is the most abundant, and it is accompanied by Ca-rich and Si-rich phases. These phases may be explained by amorphization of crystalline phases. A breakdown of a single melt into three immiscible components is less likely. Keywords: Crystal structure, high-density fluid, high-silica mica, carbonate, halide, secondary minerals, multi-phase assemblage

Metastable coesite is an important pressure indicator tier ultrahigh-pressure rocks. However, in many cases coesite does not survive exhumation, but reacts back to quartz. Although it was shown experimentally that incorporation of H in coesite increases with increasing pressure, most coesite relics found in nature are essentially dry (i.e., OH concentrations are below detection limit,

For more than 100 years mineralogists, physicists, chemists, geologists, and biologists have discussed the formation of periodic Liesegang patterns observed in natural and experimental systems. Spectacular examples of minerals showing complex periodic patterns are agate, malachite, and sphalerite. Here we report the first observation of Liesegang-like patterns in hydrothermally treated metamict (i.e., amorphous) zircon. The structures observed show curved bands, radial sets of pocket-like wave fronts or irregular curved patterns in both cathodoluminescence and backscattered electron images. They are composed of alternating zones of crystallographically well-aligned, polycrystalline zircon along with remnant amorphous pockets and a phase assemblage of randomly oriented zircon crystallites, monoclinic Zr[O.sub.2], and amorphous Si[O.sub.2], as revealed by transmission electron microscopy. Analyses by secondary ion mass spectrometry and electron microprobe reveal that the latter zones are characterized by higher hydrogen concentrations and higher Zr-Si ratios. Both zones are also distinguishable by a distinctly different crystallite size. We propose a possible pattern-forming mechanism that is based on a feedback of hydrogen diffusion, zircon nucleation, and the displacement of hydrogen atoms from growing crystallites.

A series of clinopyroxenes from the lower crust and upper mantle beneath the Siberian platform was investigated by Fourier-transform infrared (FTIR) spectroscopy and transmission electron microscopy (TEM). The IR spectra of all our samples exhibit three groups of absorption bands at (1) 3445-3465, (2) 3500-3540, and (3) 3600-3624 [cm.sup.-1]. Using synchrotron IR radiation, which utilizes a spot size of only 5 x 5 [micro]m, we realized that the intensities of the absorption bands, mostly those of group 3, showed extreme variation within one crystal. TEM as well as polarized and high-pressure IR spectroscopy indicated that the OH groups that cause the bands of group 3 were not intrinsic but due to nm-sized inclusions of sheet silicates. The intensities and peak positions of the bands of group 2 correlate with the amount of tetrahedral [Al.sup.3+] indicating that the charge-deficient substitution of Al for Si is responsible for the bands of group 2. The intensities and peak positions of the bands of group 1 correlate with the concentration of vacancies at M2 indicating that the cation vacancies at M2 control the incorporation of hydroxyl responsible for the bands of group 1. The bands of groups 1 and 2 are caused by the same type of OH dipole, however, occurring in different structural environments. The concentration of the structurally bound water of the omphacitic clinopyroxene is in the range from 31 to 514 ppm [H.sub.2]O (by weight). Surprisingly, the lowest concentration was found in clinopyroxene, that comes from the highest pressure region, i.e., the diamond-bearing eclogite xenoliths of the Mir kimberlite pipe. The highest values were obtained in omphacites of the lower-pressure grospydites of the Zagadochnaya kimberlite pipe and in omphacitic clinopyroxene of the lower pressure granulites of the Udachnaya kimberlite pipe, respectively. The low water content of clinopyroxene from the high-pressure region seems to be controlled by low water activity during crystallization. However, hydrogen loss during the uplift cannot be ruled out.

A suite of 18 zircon gemstones from placers in the Highland/Southwestern Complex, Sri Lanka, were subjected to a comprehensive study of their radiation damages and ages. The investigation included X-ray diffraction. Raman and PL spectroscopy, electron microprobe, PIXE and HRTEM analysis, as well as (U-Th)/He and SHRIMP U-Th-Pb age determinations. Zircon samples described in this study are virtually homogeneous. They cover the range from slightly metamict to nearly amorphous. Generally concordant U-Th-Pb ages averaging 555 [+ or -] 11 Ma were obtained. Late Ordovician zircon (U-Th)/He ages scattering around 443 [+ or -] 9 Ma correspond reasonably well with previously determined biotite Rb-Sr ages for rocks from the HSWC. Slightly to moderately metamict zircon has retained the radiogenic He whereas only strongly radiation-damaged zircon (calculated total fluences exceeding ~3.5 x [10.sup.18] [alpha]-events/g) has experienced significant He loss. When compared to unannealed zircon from other localities, Sri Lanka zircon is about half as metamict as would correspond to complete damage accumulation over a ~555 m.y. lasting self-irradiation period, suggesting significant annealing of the structural radiation damage. Insufficient consideration of this has often resulted in significant underestimation of radiation effects in zircon. We suggest to estimate 'effective [alpha]-doses' for Sri Lanka zircon by multiplying total [alpha]-fluences, which were calculated using the zircon U-Th-Pb age, by a correction factor of 0.55. This conversion may be applied to literature data as well, because all gem-zircon samples from Sri Lanka (this work and previous studies) seem to reveal the same general trends of property changes depending on the radiation damage. The use of 'effective [alpha]-doses' for Sri Lanka zircon contributes to more reliable quantitative estimates of radiation effects and makes possible direct comparison between natural and synthetic radiation-damaged zircon.

Aluminum orthoposphate (AlP[O.sub.4]) has polymorphs isostructural with tridymite, cristobalite, and quartz. Berlinite is the low-temperature form that corresponds to [alpha]-quartz. We report berlinite-quartz solid solutions to crystallize from a synthetic P-rich peraluminous granitic melt, similar in composition to the most volatile-rich silicate melt inclusions found in pegmatites. The crystallization took place in experiments performed in cold-seal pressure vessels at 450-700 [degrees]C and 0.1-0.2 GPa [H.sub.2]O pressure. At these conditions, the berlinite-quartz mutual solubility is limited to 5-7 mol% Si[O.sub.2] on the phosphate side of the solvus and to the maximum of 1 mol% AlP[O.sub.4] on the silica-rich side. The mutual solubility appears to decrease with falling temperature. At low T the crystals of berlinitequartz solid solutions are strongly zoned and show complex intergrowths between the P-rich and silica-rich phases. They were studied by electron microprobe, transmission electron microscopy, and Raman spectroscopy. In the light of our new experimental results, the extreme P enrichment reported earlier for some natural quartz-hosted melt inclusions may be explained as a post-entrapment contamination by the berlinite-bearing host.

The new mineral jingsuiite (TiB2, IMA-2018-117b), together with osbornite-khamrabaevite solid solution (TiN-TiC), deltalumite, and a potential new mineral, hexagonal Ti10(Si,P,□)7, constitute four inclusions up to 50 μm across in corundum recovered from the Cr-11 podiform chromitite orebody near Kangjinla, Luobusa ophiolite, Tibet, China. EELS, EDS, and 3D electron diffraction were applied to study the phases. In one inclusion, jingsuiite forms a rounded grain 40 μm across. Associated osbornite-khamrabaevite solid solution forms an irregular mass up to 10 μm across having the composition Ti(N0.5C0.5) and the Ti10(Si,P,□)7 phase forms an incomplete overgrowth up to 20 μm thick around the grain of jingsuiite. In a second inclusion, jingsuiite, osbornite-khamrabaevite solid solution, Ti10(Si,P,□)7 and deltalumite form a lamellar intergrowth 100 μm long composed of tablets of the four phases up to 50 μm long × 4 μm in thickness. Jingsuiite has a primitive hexagonal cell with a = 3.04(6), b = 3.04(6), c = 3.22(6) Å, α = 90°, β = 90°, γ = 120°, V = 25.8 (9) Å3, space group P6/mmm, Z = 1. Its structure was determined ab initio and dynamically refined on the basis of three-dimensional electron diffraction data; it is equivalent to that of synthetic TiB2. Results of EELS analyses of jingsuiite in foil no. 5357 (N = 20) gave B 61.87(1.22), C 1.53 (1.26), Ti 36.62 (1.45) at% from which an empirical formula of Ti1.10(B1.86C0.05)Σ1.91 was calculated on the basis of 3 atoms. The ideal formula is TiB2. Our preferred scenario is that corundum with entrapped Ti-Si-P-Fe intermetallic melts was precipitated from basaltic magmas during exhumation following deep subduction. Enrichment of B in the melt pockets is attributed to the highly reducing conditions that led to the segregation of siderophile elements into intermetallic melts and to the siderophile behavior of B, thereby concentrating it in the intermetallic melts in preference to silicate melt. Experimental work on the Ti-Fe-Si system indicates that minerals enclosed in corundum grains such as Ti, FeTiSi2, and TiSi2 could have crystallized from alloy melts at the lowest T accessible on the liquidus, i.e., <1300 °C. The presence of TiB2 in four inclusions in the Cr-11 orebody suggests incorporation of crustal sediments in the ophiolite followed by deep subduction to the Transition Zone where qingsongite (cubic BN) is inferred to have crystallized and subsequently exhumed to shallower levels where hexagonal BN and jingsuiite presumably crystallized. [ABSTRACT FROM AUTHOR]

The migrating fluid-mineral interface provides an opportunity for the uptake of trace elements as solid solutions in the newly formed crystal lattice during the non-equilibrium growth of the crystal. However, mineral nanoparticles could precipitate directly from the interfacial fluid when it evolves to a supersaturated situation. To better understand the role of mineral nanoparticles in this scenario, this study focuses on a well-documented magnetite with oscillatory zoning from a skarn deposit by using high-resolution transmission electron microscopy (TEM). Our results show that the Al concentration in magnetite measured on a micrometer-scale is caused by three different effects: Al solid solution, Al-rich nanometer-sized lamellae, and zinc spinel nanoparticles in the host magnetite. Here, we propose a genetic relationship among the three different phases mentioned above. At first, a continuous increase of the Al concentration in the interfacial fluid can be incorporated into the crystal lattice of magnetite forming a solid solution. During cooling in a later stage, aluminum in magnetite is oversaturated and exsolution of hercynite (Al-rich lamellae) occurs from the host magnetite. If the Al concentration at the fluid-magnetite interface still increases during further growth of magnetite, the substitution of Fe by Al has gradually reached saturation so that aluminum cannot be incorporated in the magnetite crystal structure any longer. Using the magnetite lattice as a template, nucleation of abundant zinc spinel nanoparticles occurs. This will, in turn, lead to a gradual depletion of Al concentration in the interfacial fluid until the available ions for zinc spinel nucleation and growth have been used up. As a result, the migrating fluid-magnetite interface will enrich the Al concentration in the interfacial fluid until the available ion concentration is sufficient for nucleation of zinc spinel phase again. The fluid-mineral interface in this mechanism has been repeatedly utilized during crystal growth, providing an efficient way for the uptake of trace element from a related undersaturated bulk fluid. [ABSTRACT FROM AUTHOR]

A series of polycrystalline diamond grains were found within the Valizhgen Peninsula in Koryakia, northern Kamchatka, Russia. A grain from the Aynyn River area is studied in detail with TEM. Diamond crystallites, 2–40 μm in size are twinned and have high dislocation density. They are cemented with tilleyite Ca5(Si2O7)(CO3)2, SiC, Fe-Ni-Mn-Cr silicides, native silicon, graphite, calcite, and amorphous material. Among SiC grains, three polymorphs were discriminated: hexagonal 4H and 6H and cubic C3 (β-SiC). Silicides have variable stoichiometry with (Fe,Ni,Mn,Cr)/Si = 0.505–1.925. Native silicon is an open-framework allotrope of silicon S24, which has been observed, to date, as a synthetic phase only; this is a new natural mineral phase. Three types of amorphous material were distinguished: a Ca-Si-C-O material, similar in composition to tilleyite; amorphous carbon in contact with diamond, which includes particles of crystalline graphite; and amorphous SiO2. No regularity in the distribution of the amorphous material was observed. In the studied aggregate, diamond crystallites and moissanite are intensively twinned, which is characteristic for these minerals formed by gas phase condensation or chemical vapor deposition (CVD) processes. The synthetic analogs of all other cementing compounds (β-SiC, silicides, and native silicon) are typical products of CVD processes. This confirms the earlier suggested CVD mechanism for the formation of Avacha diamond aggregates. Both Avacha and Aynyn diamond aggregates are related not to "classic" diamond locations within stable cratons, but to areas of active and Holocene volcanic belts. The studied diamond aggregates from Aynyn and Avacha, by their mineralogical features and by their origin during the course of volcanic eruptions via a gas phase condensation or CVD mechanism, may be considered a new variety of polycrystalline diamond and may be called "kamchatite." Kamchatite extends the number of unusual diamond localities. It increases the potential sources of diamond and indicates the polygenetic character of diamond. [ABSTRACT FROM AUTHOR]

The nucleation and growth processes of titanian clinohumite (Ti-C1) and titanian chondrodite (TiCh) during hydrous alteration of olivine at 8 GPa, 1300 K are described in detail. We observed two different nucleation sites and orientation relationships for Ti-C1: (1) heterogeneous nucleation at olivine grain or phase boundaries, yielding semicoherent (001) lamellae with (001)[sub.o1]//(001)[sub.Ti-C1] and (010)[sub.o1]//(010)[sub.Ti-C1]; (2) formation of coherent nanometer-sized nuclei inside olivine with the orientation relationship (100)[sub.o1]//(001)[sub.Ti.C1] and (011)[sub.o1]//(020)[sub.Ti.C1]. Only the former, for which the interface with olivine facilitates the transport of OH- and Ti necessary for the formation of the humite-group minerals, survives to large sizes. These results suggest that Ti-C1 should form readily in a Ti- and Hrich environment at mantle temperatures, independent of F content, from shallow environments to those at depths exceeding 250 km, and could play a role in recycling water back into the deep mantle.

Several micro-techniques (confocal laser-Raman microprobe, optical absorption micro-spectroscopy, high-resolution transmission electron microscopy, electron microprobe analysis) were employed in the detailed characterization of radiohaloes in biotites from two Variscan rocks from Germany. The studied biotites are intermediate members of the phlogopite-annite series with Mg/ [Fe.sup.2+] ratios in the range 1.6-1.0. Radiohaloes in biotite resulted from the impact of [sup.4]He cores ([Alpha]-particles) emitted from actinide-bearing inclusions. Monte Carlo simulations yielded [Alpha] ([sup.238]U, [sup.235]U, and [sup.232]Th series) penetration ranges in biotite between 12.5 and 37.3 [micro]m, which are in reasonable agreement with the observed radii of radiohaloes in natural biotites. The coloration pattern of a radiohalo closely correlates with the calculated distribution pattern of point defects generated in displacive events. Calculated point defect densities in the range from [is less than] [10.sup.-5] to at most [10.sup.-2] dpa (displacements per lattice atom) suggest that there are only scattered point defects in a mainly preserved biotite lattice. This is consistent with HRTEM studies that did not reveal any indication for initial volume amorphization in the haloes. However, general Raman band broadening and intensity loss suggest that the short-range order in radiohaloes is significantly disturbed. The darkened color of radiohaloes, when compared with the un-irradiated host biotite, is caused by increased light absorption over the complete visible range due to increased point defect density. No additional color centers were found, and the absorbances of the [sup.VI][Fe.sup.2+], [Fe.sup.2+]-[Fe.sup.3+], and [Fe.sup.2+]-[Ti.sup.4+] centers seem hardly to be changed. Both Raman and optical absorption spectra obtained from radiohaloes retain a clear orientational dependence. The results suggest that the formation of point defects rather than ionization is the main process causing the coloration of radiohaloes in natural biotites. The haloes represent an early stage of structural radiation damage, characterized by significantly disturbed short-range order but still widely preserved long-range order of the structure.

Two samples from the augite syenite unit of the 1.13 Ga alkaline to peralkaline Ilimaussaq intrusion in South Greenland contain olivine ([Fa.sub.75-85]) with micrometer-sized exsolution lamellae of intergrown augite and magnetite. The exsolved olivine contains only about 0.6 wt% CaO, whereas the reintegrated magmatic composition contained about 1.4% CaO corresponding to about 3 mol% of a larnite (La) component. The exsolved olivine bears no measurable Na, Al, or Ti, but the reintegrated magmatic olivine contained up to 0.4 wt% [Na.sub.2]O, 0.2 wt% [Al.sub.2][O.sub.3], and 0.3 wt% Ti[O.sub.2]. Magmatic baddeleyite in the same samples indicates an extremely low silica activity of below 0.4 during the early stages of magmatic crystallization at [is greater than] 900 [degrees] C and 1 kbar. According to the equilibrium [Ca.sub.2]Si[O.sub.4] (La in Ol) + Si[O.sub.2] = 2 CaSi[O.sub.3] (Wo in Cpx), the low silica activity is thought to be responsible for the unusually Ca-rich compositions of the primary olivines, which, in these samples, formed at 875 [+ or -] 15 [degrees] C and at oxygen fugacities between 1.2 and 2.3 log units below the QFM buffer. The composite exsolution lamellae formed during cooling due to an overstep of the schematic reaction Fa + Kst (Ca-rich component in olivine) + 1/3 [O.sub.2] = Hd + 2/3 Mt. Phase equilibria indicate temperatures between 300 and 750 [degrees] C and oxygen fugacities close to QFM for this exsolution process. The lamellae only developed in samples where the olivines were both especially high in Ca and presumably rapidly cooled (although not chilled!), as the lamella-bearing samples were collected at the outermost rim of the augite syenite unit and close to the top of the Ilimaussaq intrusion. Schreinemakers analysis shows that Ca-rich olivine of the kirschsteinite-monticellite solid solution series [Ca(Fe,Mg)Si[O.sub.4]] would exsolve from Ca-bearing olivine of the fayalite-forsterite series only at unusually low silica activities, whereas at higher silica activities, the observed exsolutions of clinopyroxene with magnetite are more stable. Hence, both cooling history and evolution of intensive variables during cooling are important for the formation of olivine exsolution textures.

To define the liquidus and solidus of the system CaCO3-MgCO, rotating multi-anvil experiments were performed at 6 GPa in the temperature range 1300 to 1800 °C under anhydrous conditions. Additionally, experiments under hydrous conditions were performed in the Mg-rich part of the phase diagram. To determine the melting point of the end-member magnesite at 6 GPa falling sphere/body experiments were performed. The run products were analyzed using electron microprobe, Raman spectroscopy, and X-ray diffraction. Some of the run products were investigated by transmission electron microscopy (TEM). Previous studies report tremendous quenching problems in melting experiments of carbonates, as the primary grown carbonates could not be distinguished from the quenched melt. With the help of rotating multi-anvil experiments the primary grown crystals could be separated from the melt phase and the compositions of both phases could be analyzed by electron microprobe. Compared with the results of static experiments the corresponding phase diagram under anhydrous conditions is significantly different. The anhydrous melting point of MgCO3 at 6 GPa could be located between 1750 and 1800 °C. Under hydrous conditions liquidus and solidus moved to lower temperatures compared to anhydrous conditions and the melting point of hydrous MgCO3 at 6 GPa is located between 1700 and 1750 °C. [ABSTRACT FROM AUTHOR]

Calcite is one of the most ubiquitous minerals in the Earth's crust and is mostly present as calcite or the slightly denser polymorph aragonite. In addition five different phases of CaCO3 (calcite II-VI), which display similar structural features as calcite, have been observed with increasing pressure in different experiments by several authors. Experimentally, the CaCO3-III and CaCO3-IIIb polymorphs have recently been observed by Merlini et al. (2012) applying pressures between 2.5-15 GPa on natural samples of calcite using single-crystal synchrotron X-ray diffraction. Here we report an occurrence of metastable authigenic CaCO3-III and CaCO3-IIIb nanocrystals for the first time in nature. Using transmission electron microscopy, idiomorphic, 50-150 nm sized crystals were observed within several meters from the surface in quaternary loess deposits in Central Asia. Nanocrystals contain higher surface energy per volume compared to coarse-grained materials due to their larger surface area. The internal pressure of a solid, PS, is at equilibrium with the surface stress, which increases with decreasing particle size. We estimated internal pressures inside the observed nanocrystals between 2.54-4.06 GPa, assuming spherical crystals with 1 nm diameter and specific surface energies, between 1.27-2.03 J/m2 (Forbes et al. 2011). [ABSTRACT FROM AUTHOR]

The occurrence of a trioctahedral analog of illite, the dioctahedral interlayer-deficient K-mica, has long been debated. Due to the inherent difficulties of determining structure and chemical composition of the extremely fine-grained material, earlier descriptions based on separated material are equivocal. Here we describe low-temperature (diagenetic) formation of fluorophlogopite, which is interlayer-deficient and therefore analogous to illite, using high-resolution in situ methods (transmission electron microscopy, TEM, with preparation by focused ion beam milling, combined with wavelength-dispersive analysis by field-emission gun electron microprobe). The average composition is K0.5Mg2.8V0.01Fe0.005 [Si3.15Al0.85O10 (OH) 0.65F1.35], including minor amounts of NH4 for charge compensation as determined by electron energy loss spectroscopy. The K-deficient Mg-mica occurs in layer packages of ~10 layers, and no indications for interlayering with other sheet silicate layers such as chlorite or vermiculite could be identified with TEM. X-ray powder diffraction patterns of separated material confirm the absence of smectite components. The mineral was identified in phosphorites from the lowermost Cambrian Tal Group, Mussoori Syncline, Lesser Himalayas, India. The rocks are alternating phosphatic mudstones and phosphatic dolostones, at times interbedded with phosphate-poor carbonate layers, which are rich in organic matter. Sedimentary fluorophlogopite occurs in both rock types and in two textural associations; one in vesicles filled with amorphic organic matter, the other as reaction rims around illite, which contains up to 5 wt% V2O3 in its rims. Textural arguments favor an early diagenetic formation of both, V-bearing illite and fluorophlogopite, closely associated with organic matter and linked to dolomitization. The high-F content stabilizes phlogopite to low temperatures. Our findings confirm that the stability field of fluorophlogopite extends from magmatic to metamorphic and sedimentary conditions. [ABSTRACT FROM AUTHOR]

Ringwoodite [(Mg,Fe)2SiO4] is the high-pressure polymorph of olivine stable in the upper mantle between ~525 to 660 km. Information on its temperature-dependent water content and Fe-oxidation state bears important implications on the hydrogen cycle and oxidation state of the Earth's interior. We conducted several multi-anvil experiments to synthesize iron-bearing (0.11 ≤ xFe ≤ 0.24) hydrous ringwoodite under oxidizing and reducing conditions. The experiments were performed at 1200 °C and pressures between 16.5 and 18.3 GPa. The incorporation of hydrogen and iron in ringwoodite was studied using Fourier transform infrared (FTIR), Mössbauer (MB), ultraviolet-visible (UV-VIS), and electron energy loss (EEL) spectroscopy. For MB spectroscopy, ringwoodite enriched in 57Fe was synthesized. The IR spectra of ringwoodite show a broad OH band around 3150 cm-1 and two shoulders on the high-energy side: one intense at 3680 cm-1 and one weak at around 3420 cm-1. The water content of the samples was determined using FTIR spectroscopy to have a maximum value of 1.9(3) wt% H2O. UV-VIS spectra display a broad band around 12 700 cm-1 and a shoulder at 9900 cm-1 representing the spin-allowed dd-transitions of VIFe2+. The weaker band around 18 200 cm-1 is a distinct feature of Fe2+-Fe3+ intervalence charge transfer indicating the presence of Fe3+ in the samples. EEL spectra yield Fe3+ fractions ranging from 6(3)% at reducing conditions to 12(3)% at oxidizing conditions. We performed heating experiments up to 600 °C in combination with in situ FTIR spectroscopy to evaluate the temperature-dependent behavior of ringwoodite, especially with respect to hydrogen incorporation. We observed a color change of ringwoodite from blue to green to brown. The heat-treated samples displayed hydrogen loss, an irreversible rearrangement of part of the hydrogen atoms (FTIR), as well as oxidation of Fe2+ to Fe3+ evidenced by the appearance of the spin-forbidden dd-transition band for Fe3+ and the ligand-metal (O2--Fe3+) transition band in the optical spectra. An increased Fe3+ fraction was also revealed by EEL and MB spectroscopy (up to 16% Fe3+/ΣFe). Analyses of MB data revealed the possibility of tetrahedral Fe3+ in the annealed ringwoodite.These results lead to a reinterpretation of the broad OH band, which is a combination of several bands, mainly [VMg(OH)2]x), a weaker high-energy band at 3680 cm-1 ([VSi(OH)4]x) and a shoulder at 3420 cm-1 ([(Mg/Fe)Si(OH)2]x). [ABSTRACT FROM AUTHOR]

The assemblage strontium anorthite, quartz, and kyanite was reacted with H2O+CaCl2 solutions at 500 °C and pressures between 460 and ~1300 MPa using a hydrothermal diamond-anvil cell. Information on the kinetics was obtained in situ based on time-resolved synchrotron-radiation X-ray fluorescence analyses of the Sr concentration in the fluid. The reaction products (anorthite or zoisite) were studied using transmission electron microscopy to obtain information on the reaction mechanism and mineral-fluid partitioning of strontium. The time required for equilibration was primarily controlled by the reaction mechanism, but not discernibly affected by pressure or chloride concentration. Nucleation and growth of zoisite at the expense of strontium anorthite was much faster than the Sr-Ca exchange reaction of strontium anorthite to anorthite, and resulted in chemically homogeneous crystals. The anorthite had developed a high nanoporosity during the reaction, which is indicative of coupled dissolution-precipitation. A zoisite-fluid exchange coefficient ... was obtained for the Sr-Ca fractionation at 500 °C and ~1300 MPa. At low bulk Sr/Ca, this value is in very good agreement with literature data, which are based on zoisite syntheses from oxide and hydroxide mixtures in chloridic fluids at 600 °C, 2 GPa and analyses after quench. This suggests that the Ca-Sr ratios in fluid and zoisite were not affected by back reactions during quenching. The constrained anorthite-fluid Sr partition coefficient for 500 °C, 460 MPa is, likewise, consistent with literature data, but determination of mineral-fluid partition and exchange coefficients can be hampered by quench phases in nanopores if coupled dissolution-precipitation acted as reaction mechanism. [ABSTRACT FROM AUTHOR]

Fluid-assisted mass transport reactions by dissolution-precipitation, where a precursor mineral reacts with a fluid, play an important role in metamorphism and metasomatism. We investigated titanite growth on rutile in time series experiments between one and 107 days at constant P-T conditions of 600 °C and 400 MPa in the system TiO2-CaO-SiO2-Na2O-HCl-H2O. A two-capsule assemblage allows for transport of Ca and Si from dissolving wollastonite to dissolving rutile, the Ti-source, in a NaCl-bearing aqueous fluid, according to the general reaction CaSiO3 + TiO2 = CaTiSiO5. Complete overgrowth of rutile by titanite occurred after just one day of experiment. Fine-grained lozenge-shaped titanite crystals of short-time runs (up to 14 days) reorganize to larger predominantly prismatic crystals after >14 days. After investigation by scanning electron microscopy, the titanite overgrowth was removed from the rutile by hydrofluoric acid, to provide a three-dimensional view of the dissolution-reaction front on the rutile surface. The morphology of the rutile surface is dominated by humps or ridges beneath the central region of a titanite crystal and valleys at the grain boundaries between adjacent titanite crystals. The dissolution pattern on the rutile surface mimics the titanite overgrowth and changes with changing grain size and shape of the titanite with longer run times. The preferred dissolution of rutile in the valleys is clearly linked to the position of the titanite grain boundaries, which served as pathways for fluid-assisted element transport. Rutile-titanite and titanite-titanite boundaries show a significant porosity in transmission electron microscopy images of foils prepared by focused ion beam milling. The large-scale dissolution pattern on the rutile surface is independent of the crystallographic orientation of the rutile and entirely dominated by the arrangement of titanite crystals in the overgrowth. Dissolution features on a scale smaller than ~1 µm are dominated by stepwise dissolution and etch-pits following the crystallographic orientation of the rutile. Similar observations were made in experiments with an additional Al-source, although these experiments result in a different overgrowth pattern; i.e., an exposed rutile surface is always present and solitary titanite crystals are accompanied by partial overgrowths. Quantitative characterization of the surface morphology by white-light interference microscopy demonstrates that, with increasing grain size of titanite, dissolution of rutile is strongly enhanced at the titanite grain boundaries. Natural examples of titanite overgrowths on rutile show the same relations between element pathways, arrangement of titanite crystals and 3D dissolution pattern on rutile as in the experimental systems. We conclude that the transport of Ti away from the rutile and of Ca + Si into the reaction rim occurred in a grain boundary fluid, the composition of which must have been strongly different from the composition of the bulk fluid in the experiment, as well as in the natural system. The reaction progress depends on the availability of a fluid, and relicts of rutile in titanite indicate restricted availability of fluid in the natural system (e.g., a fluid pulse that was consumed by the reactions). The reaction examined here can serve as a proxy for other reactions of the conversion of oxide minerals (e.g., spinel or corundum) into silicates. [ABSTRACT FROM AUTHOR]