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In this work we address the stability of riebeckite at high temperatures and compare the different behaviors observed under various oxidation conditions. For this purpose, we annealed powders of a sample from Mt. Malosa (Malawi), which is compositionally close to the end-member; the run products obtained after annealing in air vs. in vacuum were studied by Mössbauer spectroscopy and powder X-ray diffraction. The results show that riebeckite follows two distinct paths depending on the external environment. Under oxidizing conditions, it is stable in the hydrous form up to relatively low temperatures (400–450 °C), then it undergoes a rapid (within ~50 °C) dehydrogenation, forming oxo-riebeckite, which is stable up to ~900 °C. The final breakdown products of the oxo-amphibole include aegirine + cristobalite + hematite. Based on the relative intensity of the (310) Bragg reflection, the activation energy (Ea) for the riebeckite to oxo-riebeckite transition is 166 ± 6 kJ/mol. Under vacuum conditions, no Fe oxidation is observed, and riebeckite is stable up to much higher temperatures (750–800 °C); however, in the 550 < T < 700 °C range, it undergoes a significant re-arrangement of the C cations (those hosted in the strip of octahedra). Indeed, the amphibole stable in the 700–800 °C range has the same chemical formula as riebeckite but has a disordered and nonstandard cation distribution at the octahedra, i.e., M(1)(Fe3+Fe2+)M(2)(Fe3+Fe2+)M(3)Fe2+; we call this phase “CR3+ disordered riebeckite”. For T ≥ 800 °C, it decomposes to aegirine + fayalite + cristobalite + H2O. External oxygen is required for the release of water into the surrounding system, being a prerequisite for the Fe-amphiboles to be a carrier of H2O in the lower crust and upper mantle. One important implication of our results is that characterization of the overall oxidation state of iron does not necessarily provide the redox conditions of the environment of formation because a crystal-chemical re-arrangement under reducing conditions allows riebeckite to maintain its Fe3+/Fe2+ composition up to higher temperatures.

High-temperature Fourier transform infrared (HT-FTIR) spectroscopy was used to characterize the deprotonation process of synthetic potassic-ferro-richterite of composition A ( K 0 . 90 Na 0 . 07 ) B ( Ca 0 . 54 Na 1 . 46 ) C ( Fe 4 . 22 2 + Fe 0 . 78 3 + ) T Si 8 O 22 W ( OH 1 . 70 O 0 . 30 2 − ) . Unpolarized single-crystal spectra were collected both in situ and on quenched samples, and heating experiments were conducted in air, at a rate of 10 °C/min. The room- T spectrum measured before annealing shows a main band at 3678 cm −1 and a minor band at 3622 cm −1 ; these are assigned to local configurations involving Fe 2+ at M (1) M (1) M (3) and facing a filled and an empty alkali-site, respectively. At 400 °C, a new band grows at 3656 cm −1 ; this is the most intense feature in the pattern at 450 °C. At T ≥ 500 °C, all peaks decrease drastically in intensity, and finally disappear at T > 600 °C. The total absorbance measured in situ increases significantly in the 25 T T range. This feature is consistent with an increase of the absorption coefficient (e) with T , the reason for which is still unclear. However, this feature has significant implications for the quantitative use of FTIR data in HT experiments. Examination of the relevant OH-stretching bands shows that iron oxidation occurs preferentially at the M (1,3) sites associated with occupied A sites. The deprotonation temperature indicated by FTIR for single-crystals is around 100 °C higher that that obtained by HT-X-ray diffraction (XRD) on single crystal by Oberti et al. (2016), whereas that obtained by HT-XRD on powders is intermediate. This unexpected observation can be explained by considering that: (1) the iron oxidation process, which is coupled to deprotonation and is probed by XRD, occurs preferentially at the crystal surface where it is triggered by the availability of atmospheric oxygen; (2) the proton diffusion, probed by FTIR, is slower that the electron diffusion probed by XRD; thus, the temperature shift may be explained by a much longer escape path for H in single-crystals than in powders. These results suggest that possible effects due to crystals size should be carefully considered in HT experiments done on Fe-rich silicates.

Microanalysis of Fe 3+ /∑Fe in geological samples using synchrotron-based X-ray absorption spectroscopy has become routine since the introduction of standards and model compounds. Existing calibrations commonly use least-squares linear combinations of pre-edge data from standard reference spectra with known coordination number and valence state acquired on powdered samples to avoid preferred orientation. However, application of these methods to single mineral grains is appropriate only for isometric minerals and limits their application to analysis of in situ grains in thin sections. In this work, a calibration suite developed by acquiring X-ray absorption near-edge spectroscopy (XANES) data from amphibole single crystals with the beam polarized along the major optical directions (X, Y, and Z) is employed. Seven different methods for predicting %Fe 3+ were employed based on (1) area-normalized pre-edge peak centroid, (2) the energy of the main absorption edge at the location where the normalized edge intensity has the highest R 2 correlation with Fe 3+ /∑Fe, (3) the ratio of spectral intensities at two energies determined by highest R 2 correlation with Fe 3+ /∑Fe, (4) use of the slope (first derivative) at every channel to select the best predictor channel, (5 and 6) partial least-squares models with variable and constant numbers of components, and (7) least absolute shrinkage and selection operator models. The latter three sophisticated multivariate analysis techniques for predicting Fe 3+ /∑Fe show significant improvements in accuracy over the former four types of univariate models. Fe 3+ /∑Fe can be measured in randomly oriented amphibole single crystals with an accuracy of ±5.5–6.2% absolute. Multivariate approaches demonstrate that for amphiboles main edge and EXAFS regions contain important features for predicting valence state. This suggests that in this mineral group, local structural changes accommodating site occupancy by Fe 3+ vs. Fe 2+ have a pronounced (and diagnostic) effect on the XAS spectra that can be reliably used to precisely constrain Fe 3+ /∑Fe.

Following the characterization of the new amphibole species fluoro-leakeite, ideally ANa BNa2 C(Mg2Al2Li) TSi8 O22 WF2, at Norra Karr (Sweden), so far considered the type locality of eckermannite, re-examination of the holotype material of eckermannite deposited at the Museum of Natural History in London (BM 1949.151) and of the original sample analyzed by Tornebohm (1906) confirmed that they both are actually fluoro-leakeite. A survey of literature data showed that the only analysis reported for eckermannite is that of sample AMNH 108401 from the Jade Mine Tract, Myanmar. Complete characterization of that sample has led to the approval of a new holotype for eckermannite (IMA-CNMNC 2013-136), ideally ANa BNa2 C(Mg4Al) TSi8 O22 W(OH)2, which is described in this work. Holotype eckermannite from Myanmar has the empirical unit formula A(Na0.87K0.06)∑=0.93 B(Na1.89Ca0.11)∑=2.00 C(Mg3.87Fe2+0.09Mn0.01Fe3+0.38Al0.62)∑=4.97 TSi8.00 O22 W(F0.03OH1.97)∑=2.00. It is monoclinic, C 2/ m , with a = 9.8087(7), b = 17.8448(13), c = 5.2905(4) A, β = 103.660(1), V = 899.8(1) A3; Z = 2, D calc = 3.02 g/cm3. Optics: biaxial (−); α = 1.605, β = 1.630, γ = 1.634 all ±0.002 (λ = 590 nm). The 10 strongest reflections in the X-ray powder pattern [ d values (in A), I , ( hkl )] are: 2.702, 100, [(331) (151)]; 3.395, 59, (131); 3.128, 56, (310); 2.525, 56, (202); 8.407, 42, (110); 2.574, 36, [(061) (002)]; 3.257, 34, (240); 2.161, 33, (261); 2.966, 33, (060); 4.460, 30, (040). The reason for the rarity of eckermannite compositions are examined and discussed based on considerations on the short-range order of A cations and W anions.

A new classification and nomenclature scheme for the amphibole-supergroup minerals is described, based on the general formula AB 2 C 5 T 8 O 22 W 2 , where A = □, Na, K, Ca, Pb, Li; B = Na, Ca, Mn 2+ , Fe 2+ , Mg, Li; C = Mg, Fe 2+ , Mn 2+ , Al, Fe 3+ , Mn 3+ , Ti 4+ , Li; T = Si, Al, Ti 4+ , Be; W = (OH), F, Cl, O 2− . Distinct arrangements of formal charges at the sites (or groups of sites) in the amphibole structure warrant distinct root names , and are, by implication, distinct species; for a specific root name, different homovalent cations (e.g., Mg vs. Fe 2+ ) or anions (e.g., OH vs. F) are indicated by prefixes (e.g., ferro-, fluoro-). The classification is based on the A, B, and C groups of cations and the W group of anions, as these groups show the maximum compositional variability in the amphibole structure. The amphibole supergroup is divided into two groups according to the dominant W species: W (OH,F,Cl)-dominant amphiboles and W O-dominant amphiboles (oxo-amphiboles). Amphiboles with (OH, F, Cl) dominant at W are divided into eight subgroups according to the dominant charge-arrangements and type of B-group cations: magnesium-iron-manganese amphiboles, calcium amphiboles, sodium-calcium amphiboles, sodium amphiboles, lithium amphiboles, sodium-(magnesium-iron-manganese) amphiboles, lithium-(magnesium-iron-manganese) amphiboles and lithium-calcium amphiboles. Within each of these subgroups, the A- and C-group cations are used to assign specific names to specific compositional ranges and root compositions. Root names are assigned to distinct arrangements of formal charges at the sites, and prefixes are assigned to describe homovalent variation in the dominant ion of the root composition. For amphiboles with O dominant at W, distinct root-compositions are currently known for four (calcium and sodium) amphiboles, and homovalent variation in the dominant cation is handled as for the W (OH,F,Cl)-dominant amphiboles. With this classification, we attempt to recognize the concerns of each constituent community interested in amphiboles and incorporate these into this classification scheme. Where such concerns conflict, we have attempted to act in accord with the more important concerns of each community.

This paper describes a multi-technique approach to the complete crystal-chemical characterization of a gadolinite-(Y) sample found in a volcanic holocrystalline ejectum near the Vico lake (Latium, Italy). Gadolinite-(Y) occurs as poly-twinned crystals forming rounded short-prismatic aggregates (generally 0.1-0.3 mm in size, with the largest ever found >1 mm), associated with zircon, thorite, danburite, betafite, and tourmaline. Both the chemical and the structural characterization of gadolinite-(Y) from Vico required nonstandard procedures. After correction for (100) twinning, the structure of a crystal with unit-cell dimensions a = 4.7708(4) angstrom, b = 7.6229(7) angstrom, c = 9.8975(9) angstrom, beta = 90.017(7)degrees, and V= 359.95(6)angstrom(3) was refined in the P2(1)/c space group down to R = 2.3%. Electron microprobe (EMP) analyses failed to give accurate quantification of major elements, due to the presence of light and volatile elements as well as of rare earth elements (REE) and actinides. Secondary ion-mass spectrometry (SIMS) analysis done with accurate calibrations on well-characterized minerals allowed quantification of light, volatile, REE, and actinide elements, and also of Ca and Si. The derived chemical composition was interpreted with reference to the site-scattering values obtained from single-crystal structure refinement. The resulting unit formula is (Ca0.81REE0.66Y0.39Th0.13U0.02)(Sigma 2.01)(Fe0.292+Li0.14 Fe0.123+Mn0.02Mg0.01)(Sigma 0.58)(Si1.98Be1.09B0.81Li0.12)(Sigma 4.00) O-8(O1.20F0.51OH0.29)(Sigma 2.00,) which yields a calculated density of 4.267 g cm(-3). Fourier transform infrared spectroscopy (FTIR) single-crystal spectrum of gadolinite-(Y) shows several absorptions in the OH-stretching region that can be assigned to the different local configurations involving Ca and (REE,Y) at the A site and Be, B, and Li at the Z site. Lithium incorporation in gadolinite-group minerals is proposed to occur according to the exchange vectors: (1) Fe-x(2+) + Y-A -> Li-x + (A)(Th + U) and (2) Be-z + Fe-x(2+) -> Li-z + Fe-x(3+); the maximum amount of Li allowed in the gadolinite structure is 1.0 apfu. This work provides the first evidence that Li is a significant component in gadolinite-group minerals, particularly in geochemical environments enriched in actinides. This conclusion suggests that materials having the composition of Li-rich gadolinite may be considered as possible forms for radioactive waste disposal.

Here we present 45 Sc and 27 Al NMR results on Sc-doped pyrope (Mg 3 Al 2 Si 3 O 12 ), grossular (Ca 3 Al 2 Si 3 O 12 ), and an 80% grossular-20% pyrope garnet (grs80) that have recently been well-studied by X-ray diffraction and X-ray spectroscopies. Clearly distinct NMR peaks are observed for Sc in the eight-coordinated X site (pyrope and grs80) and in the six-coordinated Y site (grossular and grs80). X-ray and NMR data agree that only eight-coordinated Sc is present in pyrope and that six-coordinated Sc is predominant in grossular; however, the XRD results also indicated significant X and Z site (four-coordinated) Sc in the Ca-rich garnet. Possible reasons for this apparent discrepancy are discussed. We demonstrate that 45 Sc NMR is potentially a useful new method for studies of the site occupancies of Sc 3+ in oxides and silicates, at least in experimental systems where its concentration is a few percent or greater.

This paper reports a single-crystal unpolarized- light FTIR study in the OH-stretching region of a suite of well-characterized Ti-rich pargasites from Lherz (French Pyrenees). All amphiboles studied have fairly constant M-site composition, with Al-[6](tot) similar to 0.55 atoms per formula unit (apfia), Ti-[6] similar to 0.45 apfia, and Fe-[6](3+) similar to 0.40 apfu. SIMS and SREF data show all samples to have an 03 anion composition of OH approximate to O2- approximate to 1.0 apfu, with negligible F. The FTIR spectra show for all samples a broad absorption consisting of several overlapping bands; three main components can be recognized: similar to 3710, 3686, and 3660 cm(-1), respectively, with an asymmetric tail extending to lower frequency. Six Gaussian components can be fitted to the spectra; comparison with spectra of both synthetic and natural pargasites allows five of these components to be assigned to local configurations involving OH-O-2- at the 03 site, thus showing that coupling with an O2- anion through an A-cation significantly affects band position. Infrared spectroscopy can detect the presence of O2- in amphiboles in chemically favorable cases, i.e., in the absence of F. Moreover, the FTIR spectra show that all octahedral configurations involving Ti-M1(4+) or Fe-M1(3+) Fe-M3(3+) are associated with O2- at both adjacent 03 sites, and that Al-M3 is focally associated with OH, confirming SRO models based on structure refinement results.

Comprehensive knowledge of the order-disorder relations in amphiboles is essential to (1) complete understanding of the crystal chemistry of these minerals, and to (2) the use of amphiboles in thermodynamic calculations (e.g., of temperature and pressure of equilibration) where accurate activity models are critical to the accuracy of such treatments. Moreover, such information is essential to our understanding of phase relations, optical and electrical properties and dehydrogenation mechanisms. As a result, more effort has been expended on characterizing site occupancies in amphiboles than in any other group of minerals. Hawthorne (1983a,b) reviewed in detail all work prior to 1983. Here, we will briefly summarize this work, and focus more on what has been learned since then. We will briefly review the common methods of deriving site populations. It is important that everyone who uses site populations has an appreciation of the methods used to derive this information, as a significant fraction of the data in the literature is wrong, and the user has to be in a position to assess the accuracy and precision of the data that they will use. The most comprehensive method of deriving site populations is crystal-Structure REFinement (SREF), as this method senses every atom (in significant amounts) in a crystal. However, this universality has a negative side. Some atomic species cannot be distinguished as they scatter radiation in a very similar way. Different types of radiation (e.g., X-rays, neutrons) can compensate for this drawback: for example, X-ray scattering cannot easily distinguish between Fe and Mn or Fe and Ti (differences in size of these species can be used for this purpose, but this becomes inaccurate to ineffective in more complicated compositions), whereas neutron scattering can distinguish these species. However, there are many instances where such differentiation is not possible. It is here that …

This chapter provides an introduction to the crystal structure, crystal chemistry and chemical composition of the amphiboles. It is not an exhaustive treatment; it is intended as an introduction to the material discussed in the following chapters. More extensive discussion of many points is given in Hawthorne (1981, 1983a), although all later developments are discussed in some detail here. Published crystal-structure refinements are listed in Appendix I. The general chemical formula of the amphiboles can be written as \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[A\ B\_{2}\ C\_{5}\ T\_{8}\ O\_{22}\ W_{2}\] \end{document} where Minor elements such as Zn, Ni2+, Co2+, V3+, Sc, Cr3+ and Zr are also observed as C cations. In a mineral group as chemically complicated as the amphiboles, there are many problems connected with (1) the measurement of chemical composition, and (2) calculation of the chemical formula. ### Chemical composition The chemical composition of an amphibole is most commonly produced by electron microprobe analysis (EMPA). Instrumentation is very reliable and data reduction (including matrix corrections) are accurate. The main source of error is almost certainly errors in standards, a problem that can be dealt with in a simple but tedious fashion by cross-analyzing all standards. A more serious problem involves the components that cannot be analyzed (or analyzed accurately) by EMPA. Of particular relevance with regard to amphiboles are FeO vs. Fe2O3, Li2O and H2O, all of which occur commonly as highly variable constituents in amphiboles. We will focus on microbeam methods of …

The crystal-chemical formula and the major-element composition of amphiboles have been discussed in Hawthorne and Oberti (2007a), and the amphibole compositional space has been defined in terms of root-names and allowed charge arrangements in Hawthorne and Oberti (2007b). Although the number of possible homovalent substitutions occurring at the various structural sites is very high, geochemically exotic cations generally do not reach the limits required to assign a distinct prefix to the root-name. Thus prefixes are so far confined to the presence of Na, K, Mg, Al, Ti, Mn, Fe, F and Cl. In this chapter, we examine the new amphibole compositions which have been found and characterized since 1981, i.e. since the publication of volume 9A of the MSA Reviews in Mineralogy. We divide the chapter in two sections. The first section describes new charge arrangements found in Nature, and the second section describes the extensive studies of synthetic amphiboles of novel composition done to clarify the crystal chemistry of exotic cations which are normally found as minor components. This chapter deals exclusively with monoclinic amphiboles, as the compositional space of orthorhombic amphiboles has not expanded since the former review. Although natural occurrences of protoamphibole (with space group Pnmn ) have been described only in the last twenty years (Sueno et al. 1998; Konishi et al. 2003), their composition is within the compositional range previously defined for Pnma amphiboles. The main extensions of the traditional amphibole compositional space recognized in between the 1978 and the 1997 classification schemes (Leake 1978; Leake et al. 1997) are due to the discovery of: (1) sadanagaite, calcic amphiboles of ideal compositions AR+ BCa2 C(R2+3R3+2) T(Si5Al3) O22 W(OH)2 (Shimazaki et al. 1984; Sokolova et …

Aluminotaramite, ideally Na(CaNa)(Fe 3 2+ Al 2 )(Si 6 Al 2 )O 22 (OH) 2 , and alumino-magnesiotaramite, ideally Na(CaNa)(Mg 3 Al 2 )(Si 6 Al 2 )O 22 (OH) 2 , occur in retrogressed eclogites in the Liset kyanite-eclogite pod, near Selje, Vestlandet, Norway. Fluoro-alumino-magnesiotaramite, ideally Na(CaNa)(Mg 3 Al 2 ) (Si 6 Al 2 )O 22 F 2 , occurs in the Jianchang eclogite, Su-Lu coesite-eclogite province, China. These aluminotaramites always replace other higher pressure amphiboles (nyboite and fluoronyboite), and their higher Al content derives from resorbed garnets and lowered pressure during retrogression from the eclogite to the amphibolite facies. This paper reports complete mineral data for the three new holotypes as obtained by EMP analysis, structure refinement, and optical measurements. The three new minerals and mineral names have been approved with votes 2006-023, 2006-024, and 2006-025, respectively, by the IMA Commission on New Minerals, Nomenclature and Classification. Holotype aluminotaramite has the unit formula: A (Na 0.89 K 0.01 ) ∑0.90 B (Fe 0.11 2+ Na 0.82 Ca 1.07 ) ∑2.00 C (Fe 1.75 2+ Mg 1.62 Al 1.12 Fe 0.42 3+ Ti 0.07 Zn 0.01 Mn 0.01 ) ∑5.00 T (Si 6.23 Al 1.77 ) ∑8.00 O 22 W (OH 1.86 F 0.14 ) ∑2.00 , and a = 9.7489(5), b = 17.9377(7), c = 5.3233(3) A, β = 104.539(5)°, V = 901.1(2) A 3 ; the calculated density is 3.29 g/cm 3 . Holotype alumino-magnesiotaramite has the unit formula: A Na 1.07 B (Fe 0.06 2+ Na 0.73 Ca 1.21 ) ∑2.00 C (Fe 1.06 2+ Mg 2.40 Al 1.20 Fe 0.31 3+ Ti 0.03 ) ∑5.00 T (Si 6.09 Al 1.91 ) ∑8.00 O 22 W (OH) 2.00 , and a = 9.7899(7), b = 17.8991(9), c = 5.3192(5) A, β = 104.900(7)°, V = 900.7(3) A 3 ; the calculated density is 3.21 g/cm 3 . Holotype fluoro-alumino-magnesiotaramite has the unit formula: A Na 0.99 B (Fe 2+ 0.02 Na 0.77 Ca 1.21 ) ∑2.00 C (Fe 1.11 2+ Mg 2.12 Al 1.04 Fe 0.68 3+ Ti 0.03 Mn 0.02 ) ∑5.00 T (Si 6.00 Al 2.00 ) ∑8.00 O 22 W (F 1.04 OH 0.96 ) ∑2.00 , and a = 9.7414(8), b = 17.9095(13), c = 5.3335(4) A, β = 104.672(1)°, V = 900.2(3) A 3 ; the calculated density is 3.26 g/cm 3 .

A re-examination of the chemistry and structure of nearly all the known occurrences of arrojadite and related minerals (dickinsonite and sigismundite) allowed understanding of the main substitution vectors and cation ordering schemes ruling the crystal-chemistry of these very complex phosphates. Electron microprobe analyses were done with a careful choice of the standards and of experimental conditions, and were coupled with LA-ICP-MS in situ analysis for Li, Be, and B. Structure refinement was done in a space group ( Cc ) with a lower symmetry than those used in previous studies ( C 2/ c and its equivalents), which allowed a better understanding of the structure details and of cation ordering. The combined approach yielded a new formula for the arrojadite group, namely A 2 B 2 Ca 1 Na 2+x M 13 Al (PO 4 ) 11 (PO 3 OH 1−x )W 2 , where A are either large divalent cations (Ba, Sr, Pb) plus vacancy, or monovalent (K, Na) cations; and B are either small divalent cations (Fe, Mn, Mg) plus vacancy, or monovalent (Na) cations. The number of hydroxyl groups in the arrojadite formula is generally 3 apfu, and can be lowered to 2 apfu in particular when the sum of non-(P,Al) cations is higher than 20 apfu. We present in this paper the complete characterization of three samples (two of which are new members) that are crucial to fix the cornerstones of arrojadite crystal-chemistry. The sample from Rapid Creek (Yukon Territory) is the holotype for arrojadite-(KNa), and has unit formula K 0.83 Na 5.01 (Ca 0.91 Sr 0.01 ) ∑ = 0.92 (Fe 9.34 2+ Mg 2.69 Mn 1.03 2+ Zn 0.01 Li 0.01 ) ∑ = 13.08 (Al 1.04 Ti 0.02 ) ∑ = 1.06 (OH 1.97 F 0.03 ) ∑ =2.00 [(P 11.99 Si 0.01 T 1 )O 47 (OH) 1.00 ] [ideally, A1 K A2 Na B1 Na B2 Na Na1,2 Na 2 Na3 □ Ca Ca M Fe 13 Al (PO 4 ) 11 P1x (PO 3 OH) W(OH,F) 2 ] and unit-cell dimensions: a = 16.5220(11), b = 10.0529(7), c = 24.6477 (16) A, β = 106.509(2)°, V = 3932.2(7) A 3 ( Z = 4). The sample from Horrsjoberg (Varmland, Sweden) is the holotype material for arrojadite-(SrFe), and has unit formula Sr 0.93 Na 3.20 (Ca 0.59 Ba 0.20 Pb 0.03 K 0.03 ) ∑ = 0.85 (Fe 6.64 2+ Mg 3.61 Mn 3.33 2+ Zn 0.07 Li 0.01 ) ∑ = 13.66 (Sc 0.04 Al 1.00 ) ∑ = 1.04 (OH 1.10 F 0.90 ) ∑ = 2.00 [(P 11.95 Si 0.02 ) ∑ = 11.97 O 47 (OH) 1.00 ] [ideally, A1 Sr A2 □B1Fe 2+ B2 □ Na1,2 Na 2 Na3□ Ca Ca M Fe 13 2+ Al (PO 4 ) 11 P1x (PO 3 OH) W(OH,F) 2 ], and unit-cell dimensions a = 16.3992(7), b = 9.9400(4), c = 24.4434(11) A, β = 105.489(1)°, V = 3839.76(46) A 3 . The sample from Branchville (Connecticut) is the holotype material for dickinsonite-(KMnNa), and has unit formula K 0.50 Na 5.78 (Ca 0.51 Sr 0.05 Ba 0.01 Pb 0.01 ) ∑ = 0.58 (Mn 9.70 2+ Fe 3.72 2+ Li 0.31 Mg 0.06 Zn 0.01 ) ∑=13.80 (Al 0.91 Fe 0.09 3+ Ti 0.01 ) ∑=1.00 (OH 1.97 F 0.03 ) ∑=2.00 [(P 12.02 Si 0.01 ) ∑ = 12.03 O 47 (OH) 0.21 ] [ideally, A1 K A2 Na B1 Mn B2 □ Na1,2 Na 2 Na3 Na Ca Ca M Mn 13 Al (PO 4 ) 11 P1x (PO 4 ) W (OH, F) 2 ] and unit-cell dimensions a = 16.6900 (9), b = 10.1013 (5), c = 24.8752 (13) A, β = 105.616(2)°, V = 4038.9(7) A 3 .

Investigation of scandium incorporation in garnets along the synthetic Ca3Fe2 3+Si3O12–Ca3Sc2Si3O12 (adr–CaSc) join, based on the same multi-technique approach used in the companion paper (Oberti et al. 2006a), shows that (1) Sc is incorporated exclusively at the Y octahedron; (2) the local coordination of Sc is slightly different in Sc-poor than in Sc-rich compositions (Sc-O = 2.06 Å in CaSc10 vs. 2.10 Å in CaSc30–90); (3) the local coordination of Ca is also slightly different in Sc-poor than in Sc-rich compositions [Ca1,2-O are 2.34(2) and 2.48(2) Å in CaSc10 and 2.36(2) and 2.50(2) Å in CaSc90, with ∆ Þ xed at 0.14 Å in all the samples]; (4) the linear increase of the unit-cell edge along the join derives from multiple changes in the geometry of the different polyhedra and from the rotation of the tetrahedron around the⎯4 axis (α rotation), and cannot be modeled from extrapolation of the behavior observed along the Ca3Al2Si3O12–Ca3Fe2 3+Si3O12 (grs–adr) join. CaSc-rich garnets, where a large X dodecahedron coexists with a large Y octahedron and a Z tetrahedron occupied by Si, similar to pyrope-grossular garnets, have the highest α values observed to date in calcium silicate garnets. Slightly lower α values are observed in pyrope and almandine, but correspond to a different structural arrangement, where a small X dodecahedron coexists with a small Y octahedron. These results further conÞ rm the efÞ ciency of a combined short- and long-range approach for understanding the properties of garnet solid solutions.

Synthetic NaNa 2 Mg 5 Si 8 O 21 (OH) 3 is the first triclinic member of the amphibole group, and has a tripling of the unit cell in the b direction. The space group is C 1 and the triple- b repeat gives Z = 6. The unit-cell parameters are: a = 9.883(2), b = 54.082(9), and c = 5.277(1) A, α = 90.045(4)°, β = 103.068(3)°, γ = 89.960(4)°, and V = 2748(1) A 3 . The crystal structure has been refined to R 1 = 7.6 and w R 2 = 16.7% for the 1835 reflections with F o > 4σ F and for 4832 supercell reflections in the 2𝛉 range 2–25°, respectively. The structure is pseudo-monoclinic, but both the intensity distribution and refined model indicate space group C 1. Compared with the common C 2/ m amphibole structure, the two halves of an I-beam unit are no longer mirror-related, and the overall structure can be rationalized in terms of two different types of I-beam occurring in the unit cell. The first (with multiplicity 2) is centrosymmetric, and the second (with multiplicity 4) is non-centrosymmetric. There are also significant displacements of the cations, especially at the M4 sites, from their corresponding locations in the C 2/ m structure. The correlated displacements of Na atoms at the M4 sites permit incorporation of excess protons in pseudo-tetrahedral cavities between two adjacent chains of tetrahedra belonging to different I-beams. Bond-valence calculations and crystal-chemical analysis suggest that excess protons are bonded to O atoms at the O4 sites, and are hydrogen bonded to O atoms at adjacent O2 sites. The infrared spectrum of the amphibole in the principal OH-stretching region has a triplet of sharp bands at 3740, 3727, and 3711 cm −1 , which are assigned to the three independent “normal” O3-H groups in the triclinic structure. There is an additional intense and very broad absorption at 3430 cm −1 that is resolved only when adsorbed moisture is removed. This band is assigned to the extra H in the structure. All the details provided by structure refinement and the proposed location of the excess H atoms is in accord with previous HRTEM and 29 Si- and 1 H MAS NMR studies of this amphibole.

In the present work, we characterize the amphibole Na(NaMg)Mg5Si8O22(OH)2 synthesized at 0.4 GPa and 750, 800, and 850 °C, and 0.5 GPa, 900 °C. Experiments at 800 and 900 °C yielded crystals suitable for single-crystal data collection. Structure refinement shows that synthetic Na(NaMg)Mg5Si8O22(OH)2 has P 21/ m symmetry at room T . The two non-equivalent tetrahedral double-chains differ in their degree of stretching and kinking. The infrared spectrum of synthetic Na(NaMg)Mg5Si8O22(OH)2 has two well-defined absorption bands at 3742 and 3715 cm−1 which can be assigned to O-H bands associated with the two independent anion sites (O3A and O3B) in the structure. The higher frequency band is assigned to the shorter O3B-H2 bond, and the lower frequency band is assigned to the longer O3A-H1 bond. The broader shape of the 3743 cm−1 band is consistent with a stronger interaction of the H2 atom with ANa, which is confirmed by structure refinement. Increasing T of synthesis causes a progressive departure from the ideal stoichiometry via the A□1 BMg1 ANa−1 BNa−1 substitution, as confirmed by EMPA, structure refinement, and FTIR spectroscopy.

Ferripedrizite, ideally A (OH) 2 , is a new amphibole end-member found in episyenites formed after cordierite-bearing porphyritic granites in the Eastern Pedriza Massif (Central System, Spain). It contains the maximum amount of Li that can be incorporated in the amphibole structure. The name was approved by the IMA-CNMMN together with restriction of the use of the prefix sodic in the pedrizite series to compositions with B Na > 0.5 apfu; its use for com- positions with Na tot > 0.5 apfu has been maintained in the rest of the Mg-Fe-Mn-Li group. Complete solid solution between ferripedrizite and leakeite (ideally, A Na B Na 2

Leverage analysis enables identification of reflections with the greatest influence on the estimate of each refined variable, and thus may be an important tool to improve standard structure-refinement procedures, especially in the case of minerals with complex composition. In this work, leverage analysis was used to investigate in detail the influence of each reflection in high-resolution X-ray diffraction data collected from olivine, a mineral often used to model order-disorder processes and to calculate temperatures of closure and cooling rates of host rocks. Particular attention was paid to the estimate of the scattering power at the cation sites, that are crucial for the above studies. Various procedures for data correction and refinement were also investigated, and the different possible choices were compared to choose the strategy that provides the best results. The presence of high-leverage weak reflections in olivine strongly suggests that systematic data truncation according to intensity threshold should be avoided. The estimates of the site-scatterings obtained under the different conditions tested are very close (always ≤3 σ); they are often smaller than those which may be obtained from electron-microprobe analysis under different experimental conditions or on inhomogeneous (zoned) crystals. Chemical data should thus not be routinely used to constrain the refinement procedure and/or to optimize final site-populations, provided that appro- priate errors are given; on the other hand, they are valuable to appreciate the presence and the amounts of very minor, sometimes unexpected, substituents. Our tests show that precision better than 0.001 in site-occupancy determination (sometimes claimed in the literature) is probably not achieved.

Single-crystal structure refinements are presented of the holotype of crystal peprossiite-(Ce) (Monte Cavalluccio) and of a new sample from Cura di Vetralla (Viterbo, Italy) with slightly different composition, together with new EMP-SIMS chemical analyses. These results allow us to propose a new unit formula: [REE 1– x – y (Th,U) x Ca y ](Al 3 O) 2/3 [(B 4– z Si z )]O 10 with x − y + z = 1/3 ( Z = 1) for the peprossiite group. Lattice constants for the holotype crystal are: a = 4.612(1), c = 9.374(3) A, V = 172.6 A 3 , Z = 1, space group P 6 2 m . The crystal structure was solved by Patterson methods and refined to R obs = 1.8% ( R all = 2.2%) for 706 unique reflections in the 2θ-range 6–136°. Lattice constants for the thorian peprossiite-(Ce) from Cura di Vetralla are: a = 4.596(3), c = 9.309(16) A, V = 170.3 A 3 , and the structure was refined to R obs = 2.9% and R all = 3.0% for 271 unique reflections in the 2θ-range 4–80°. The topology of the tetrahedral layer and the site of the inter-layer cation (REE) in peprossiite resembles that of dioctahedral micas. The main difference lies in the presence of layers of pyramids instead of layers of octahedra typical of mica. In peprossiite, Al is coordinated by five O atoms in a nearly square-pyramidal arrangement, the base of which is formed by pairs of apical O atoms from two layers of tetrahedra related by a mirror plane. Three of these pyramids share their apical O forming Al 3 O groups with occupancy of 2/3 according to the structure refinement. A model is proposed that explains the apparent disorder in the pyramidal layer of peprossiite by the stacking within a triple cell (with a ′ = a √ 3 and a ∧ a′ = 30°) of three ordered layers randomly translated by ± a .

Leverage analysis allows detection of the reflections that have the greatest influence on the estimate of a refined variable. We have applied leverage analysis in the structure refinement of some rock-forming minerals, in the attempt of settling the best procedures to obtain accurate and reliable results. Attention has been focused on those variables (e.g., the refined site-scatterings) which are fundamental to the determination of site populations, which in turn are the basis for the study of order-disorder processes and petrogenetic modeling. A garnet (pyrope) and an amphibole (tremolite) were used to test the procedure, being illustrative of different symmetries and structural complexity. As the omission of reflections with high leverage on a particular variable may strongly decrease the accuracy in its estimate, the results indicate that systematic procedures for data truncation [either on the basis of (low or high) sin θ/λ, high Δ F /σ F o or of low I /σ I ] are potentially dangerous. The choice of a correctly ionized model is shown to be critical to obtain accurate estimates of the refined site-scatterings. The set of reflections with the highest leverage value with respect to a peculiar group of variables was found to be nearly invariant within isomorphous solid-solutions; this implies that the results of this work are valid throughout the amphibole and garnet compositional spaces. The warnings proposed to the treatment and refinement of X-ray diffracted data may be of general utility in the structure refinement of minerals.

Constraints on the calculation and use of mineral/melt and two-mineral partition coefficients for Ti (D Ti ) have been derived from current knowledge of the distinct crystal-chemical mechanisms for the incorporation of Ti 4+ in the amphibole structure as follows: (1) mineral/melt partition coefficients for Ti, and other tetravalent high field-strength elements (HFSE), can be compared only when considering the fraction of Ti 4+ that enters the same structural site; (2) accurate two-mineral partition coefficients can be obtained only when considering the fraction of Ti 4+ that is involved in the same crystal-chemical mechanism in the two relevant phases (i.e., M2 Ti 4+ and M1 Ti 4+ for amphibole and clinopyroxene, respectively). The complete crystal-chemical characterization of synthetic titanian pargasite and kaersutite and of synthetic richterite (all crystallized under P , T , X , f O 2 conditions of interest for upper-mantle studies) shows that the site preference of Zr and Hf differs between the two amphibole compositions; these elements are essentially ordered at M2 in pargasite and kaersutite, but preferentially enter M1 in richterite. In the latter case, Ti segregates into the split M1′ site with distorted coordination and shorter Ti-O3 distances, whereas Zr and Hf most likely prefer the larger and more regular M1 site. The observed site preference is strongly controlled by the relative dimensions of the available sites. The crystal-chemical mechanisms that govern the incorporation of octahedral high-charge cations are the local charge balance of [IV] Al (by R 3,4+ at M2) and of dehydrogenation (by R 3,4+ at M1); thus the incorporation of Zr and Hf depends on distinct intensive parameters in the two amphibole compositions. Calculation of partition coefficients and of elastic-site parameters under the assumption that all Ti and other HFSE 4+ order at the M2 site in amphibole, as is currently done in geochemical studies, is strongly biased. In the presence of significant dehydrogenation, amphibole/melt D 0 values obtained from modeling based on the elastic-strain theory starting from the more-accurate site populations for Ti may be only 1/4 of those obtained by using the total Ti content, and the derived site parameters E and r 0 are more consistent with octahedral coordination. This result has important consequences for the prediction of D values under P - T conditions different from those of the experimental work. Applying the above concepts to data from natural assemblages, we obtained a significantly narrower (0.3–2.4 vs. 1.5–8.9) and more reasonable range of variation for amphibole/clinopyroxene D Ti . A relationship between these values for D Ti and pressure is also now apparent.

The crystal structure of a Th-U-rich, Y-poor hellandite-(Ce) occurring in a volcanic ejectum from Capranica (Vico volcanic complex, Latium, Italy) was refined to an R index of 1.5% for 2226 observed reflections [I > 3 sigma(I)]. Hellandite from Capranica is monoclinic P2/a with a = 19.068 (8), b = 4.745 (2), c = 10.289 (3) Angstrom, beta = 111.18 (3)degrees. The extra-framework (distorted) tetrahedral cavity, usually occupied by H in hellandite, is partially (35-40%) occupied by Be and/or Li. When this is the case, the [B4Si4O22] tetrahedral chains of hellandite are locally cross-linked to form a sheet, which is built up by the same types of rings as in semenovite but in a different arrangement. The structure refinement and the electron- and ion-microprobe analyses provide constraints on hellandite-group crystal chemistry. The presence of ([4])(Be, Li) at the Be site is coupled with the substitution of F and/or O for OH at the O5 site; the amount of extra-framework cations thus constrains the OH content in the general formula. X-ray data exclude the presence of other OH sites in the structure, in agreement with direct H determination by ion probe (1.02 H apfu + 0.98 F apfu = 2.0 apfu of monovalent anions at O5 vs. 4.5-8 apfu proposed previously). The complete chemical analysis also eliminates the need for a vacancy at the REE sites. The general formula of hellandite-group minerals is thus (Ca,REE)(8)(Th,U,Y,REE)(4)(Ti,Fe3+,Al)(2)(O,F,OHx)(4)(Be,Li)(4-x)Si8B8O44, where electroneutrality is assured by the appropriate mixing of heterovalent substituents at the M sites.

Single-crystal structure-refinement and electron-microprobe analysis of synthetic amphiboles with nominal fluoro-eckermannite composition and different trivalent cations (Al, Sc, Ti (super 3+) , V (super 3+) , Cr (super 3+) , Ga,) show significant deviations from nominal stoichiometry. Synthetic Sc- and Cr-bearing fluoro-eckermannite are close to nominal stoichiometry, whereas the corresponding nominal Al-, V (super 3+) -, Ga-, and Ti-bearing species contain very few trivalent cations and approximate Na(NaMg)Mg 5 Si 8 O 22 F 2 , a composition that has not been found in natural systems. The presence of a significant cummingtonite component strongly affects unit-cell parameters, coordination geometry around the B-, C-, and T-group sites, and cation ordering at the A-group sites. The high-charge cations are completely ordered at the M2 site, and there is a well-developed linear relationship between "M2-O" and the constituent-cation radius at the M2 site. The synthetic fluoro-eckermannite structure is stabilized by large spherically symmetric trivalent cations at the M2 site; for small spherically symmetric trivalent cations, it is not stable (at least at the synthesis conditions used here). Synthetic chromium-fluoro-eckermannite is stabilized by the non-spherically symmetric 3d 3 electronic arrangement, whereas Ga and V (super 3+) (3d 2 ) do not stabilize the fluoroeckermannite structure.

Electron microprobe analysis of amphiboles from Pargas, Finland, shows up to 2.42 Ca apfu (atoms per formula unit), far in excess of the usual maximum value of 2.00 Ca apfu observed in amphiboles. The most Ca-rich composition is (Ko.12Nao.35Cao.52)(Cal.92Mn6.65 Fe6.63)(Mg3.73 Fe6.t3 Tio.o2Alo.82)(Si5.63AI2.37 )022 [(OH)0.46F 1.54]. Fluor-cannilloite, ideally CaCa2(Mg4AI)(Si5AI3)022F2,is a new amphibole species in which Ca is dominant at the A site. Fluor-cannilloite occurs as isolated anhedral grains and small granular aggregates in a marble together with calcite, muscovite, anorthite, aluminous diopside, pyrope, and fluorite. It is grayish green with a grayish white streak, brittle, H = 6, Dmeas = 3.05 g/cm3, and shows perfect {110} cleavage. In plane-polarized light, it is pale greenish gray to almost colorless, rather cloudy and mottled in appearance, and is not pleochroic. Fluor-cannilloite is biaxial positive, a = 1.611(2), (3= 1.616(2), l" = 1.633(2), 2 V = 49(2)°, dispersion weak r > v. It is monoclinic, space group C21m, a = 9.826(4), b = 17.906(9), c = 5.301(3) A, (3 = 105.41(4)°, V = 899.2 A3, Z = 2. The ten strongest X-ray diffraction lines in the