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Scapolites are pervasive rock-forming aluminosilicates that are found in metamorphic, igneous, and hydrothermal environments; nonetheless, the stability field of Cl-rich scapolite is not well constrained. This experimental study investigated two reactions involving Cl-rich scapolite. First, the anhydrous reaction 1 of plagioclase + halite + calcite to form scapolite [modeled as: 3 plagioclase (Ab80An20) + 0.8 NaCl + 0.2 CaCO3 = scapolite (Ma80Me20)] was investigated to determine the effect of the Ca-rich meionite (Me = Ca4Al6Si6O24CO3) component on the Na end-member marialite (Ma = Na4Al3Si9O24Cl). Second, the effect of water on this reaction was investigated using the hydrothermally equivalent reaction 2, H2O + scapolite (Ma80Me20) = 3 plagioclase (Ab80An20) + CaCO3 + liquid, where the liquid is assumed to be a saline-rich hydrous-silicate melt. Experiments were conducted with synthetic phases over the range of 500–1030 °C and 0.4–2.0 GPa. For reaction 1, intermediate composition scapolite shows a wide thermal stability and is stable relative to plagioclase + halite + calcite at temperatures above 750 °C at 0.4 GPa and 760 °C at 2.0 GPa. For reaction 2, intermediate scapolite appears to be quite tolerant of water; it forms at a minimum bulk salinity [XNaCl = molar ratio of NaCl/(NaCl+H2O)] of the brine of approximately 0.2 XNaCl at 830 and 680 °C at pressures of 2.0 and 1.5 GPa, respectively. Based on the study done by Almeida and Jenkins (2017), pure marialite is very intolerant of water when compared to intermediate composition scapolite. Compositional changes in the scapolite and plagioclase were characterized by X-ray diffraction and electron microprobe analysis and found to shift from the nominal bulk compositions to the observed compositions of Ma85Me15 for scapolite and to Ab91An09 for plagioclase. These results were used to model the phase equilibria along the marialitemeionite join in temperature-composition space. This study demonstrates that a small change in the scapolite composition from end-member marialite to Ma85Me15 expands the stability field of marialite significantly, presumably due to the high entropy of mixing in scapolite, as well as increases its tolerance to water. This supports the much more common presence of intermediate scapolites in hydrothermal settings than either end-member meionite or marialite as is widely reported in the literature.

Scapolite and analcime are uncommon alteration minerals in syenite. Here we describe a syenite body in a fault zone that hosts these minerals and we elucidate the unusual chemical conditions that led to their formation. The Clarke Head syenite is part of a regional late Devonian–early Carboniferous back-arc A-type granitoid suite in the Cobequid Highlands of Nova Scotia, Canadian Appalachians. The syenite contains magmatic rutile and analcime. Magmatic K-feldspar has been largely replaced by scapolite of marialite composition, which shows a temporal transition from more Cl rich to more carbonate rich varieties. Scapolite veins cut both the syenite and associated gabbro and diorite. Analcime I occurs as a late stage interstitial magmatic mineral. Analcime II forms by alteration of albite lamellae in perthite replaced by scapolite, analcime III vugs that cross-cut scapolite and analcime IV occurs in late veins cross-cutting earlier analcime phases and scapolite. The regional A-type granite plutonism was Na- and halogen rich, with widespread late-magmatic albitization and F-related mobility of REE minerals. However, only at Clarke Head is there extreme Na- and Cl-rich metasomatism, indicated by scapolite and Cl-rich hastingsite (with >1 wt% Cl content). Cl-rich fluids were derived from regional halite evaporites at faulted basin margins. The main circulating fluids were likely of magmatic origin, based on the δ18O of both replacement and vein scapolite. As the NaCl supply was reduced, scapolite became more Ca-rich. Halogen-rich fluids also mobilized Ti and Zr, which were reprecipitated in hydrothermal rutile and zircon as the halogen activity diminished. Scapolite replacement of K-feldspar released large amounts of K+ into the circulating fluids, which converted ferromagnesian silicates in fault zones to secondary biotite.

An Upper Miocene (9.12 ± 0.19 Ma; biotite 40Ar/39Ar) shallow pluton and numerous dykes and sills of felsic–intermediate compositions intruded the Upper Cretaceous, flysch-type sediments in the Mivehrood area, northwest Iran. The intrusions caused extensive thermal metamorphism and metasomatism, leading to the formation of hornfels and skarn. A massive skarn, 1–10 m thick, immediate to the intrusive contact, is bordered by a banded skarn, 100–400 m thick, that grades outward into hornfels and original sediments. The Mivehrood pluton is characterized by steep REE pattern, high Al2O3 (14.64–16.4 wt%) and Sr (380–786 ppm), and low MgO (1.3–3.4 wt%), Y (4.8–10.7 ppm), and Yb (0.35–0.95 ppm), characteristics typical of high-silica adakites. Skarn formation started with thermal metamorphism, followed by anhydrous prograde and hydrous retrograde stages. Prograde and retrograde mineral assemblages are developed in both skarns, represented by garnet–clinopyroxene–wollastonite and epidote–actinolite–scapolite–chlorite, respectively. Granditic F-bearing garnet dominates clinopyroxene in both skarns. The banded skarn contains minor scapolite of marialite composition. The calc-silicate mineral assemblages and the mineral chemistry allow the Mivehrood skarn to be classified as a calcic, oxidized skarn. Mass balance assessments suggest that Fe, Si, and S were significantly enriched, and Na, LILEs, and LREEs were strongly depleted, in the massive skarn. In the banded skarn, Na, K, Si, and S were enriched. Significant dehydration and carbon degassing occurred in both skarns. Stockworks, veins, and replacement bodies of pyrite ± chalcopyrite locally occur in the pluton and the dykes and in the skarns. The δ34SCDT values for the sulfides fall in a narrow range around 0.0 ‰, suggesting a magmatic source for sulfur and possibly the hydrothermal water and solutes involved in the skarn formation.

The crystal structures of marialite (Me6) from Badakhshan, Afghanistan and meionite (Me93) from Mt. Vesuvius, Italy were obtained using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data and Rietveld structure refinements. Their structures were refined in space groupsI4/mandP42/n, and similar results were obtained. TheMe6sample has a formula Ca0.24Na3.37K0.24[Al3.16Si8.84O24]Cl0.84(CO3)0.15, and its unit-cell parameters area=12.047555(7),c=7.563210(6) Å, andV=1097.751(1) Å3. The average ⟨T1-O⟩ distances are 1.599(1) Å inI4/mand 1.600(2) Å inP42/n, indicating that the T1 site contains only Si atoms. InP42/n, the average distances of ⟨T2-O⟩=1.655(2) and ⟨T3-O⟩=1.664(2) Å are distinct and are not equal to each other. However, the mean ⟨T2,3-O⟩=1.659(2) Å inP42/nand is identical to the ⟨T2′-O⟩=1.659(1) Å inI4/m. The ⟨M-O⟩ [7]=2.754(1) Å (M site is coordinated to seven framework O atoms) and M-A=2.914(1) Å; these distances are identical in both space groups. TheMe93sample has a formula of Na0.29Ca3.76[Al5.54Si6.46O24]Cl0.05(SO4)0.02(CO3)0.93, and its unit-cell parameters area=12.19882(1),c=7.576954(8) Å, andV=1127.535(2) Å3. A similar examination of theMe93sample also shows that both space groups give similar results; however, the C–O distance is more reasonable inP42/nthan inI4/m. Refining the scapolite structure nearMe0orMe100inI4/mforces the T2 and T3 sites (both with multiplicity 8 inP42/n) to be equivalent and form the T2′ site (with multiplicity 16 inI4/m), but ⟨T2-O⟩ is not equal to ⟨T3-O⟩ inP42/n. Using different space groups for different regions across the series implies phase transitions, which do not occur in the scapolite series.

Scapolite Me 32.9 (marialite) from Monmouth Township, Ontario, was studied using in situ synchrotron powder X-ray diffraction (XRD) and Rietveld structure refinement of data obtained by heating and cooling from 26 to 902°C. The structure was refined in space group P 4 2 /n . A room-temperature structure, obtained with synchrotron high-resolution powder X-ray-diffraction (HRPXRD) data, has unit-cell parameters a 12.06503(1), c 7.58360(1) A, and V 1103.906(2) A 3 . Moreover, the average bond-distances T 1–O>, T 2–O> and T 3–O> are equal to 1.606(2), 1.726(2) and 1.612(2) A, respectively, and correspond to occupancies T 1 = [Al 0 Si 1 ], T 2 = [Al 0.87 Si 0.13 ], and T 3 = [Al 0.02 Si 0.98 ]. At 902°C, T 1–O>, T 2–O> and T 3–O> are equal to 1.591(5), 1.683(8) and 1.686(7) A, respectively, and correspond to T 1 = [Al 0 Si 1 ], T 2 = [Al 0.55 Si 0.45 ], and T 3 = [Al 0.57 Si 0.43 ]. At 902°C, the T 1 site remains fully occupied by Si atoms, whereas the disorder at the T 2 and T 3 sites is close to being complete. Several structural parameters show a break at 200°C, which corresponds to disorder of the Na and Ca (= M ) atoms. On cooling, the structure does not go back to the initial structural state; some T 2– T 3 and Na–Ca disorder are quenched in. The type- b reflections that give rise to antiphase-domain boundaries (APBs) are present at all temperatures; they arise from Cl–CO 3 order, instead of Al–Si order. At 902°C, except for the anions, there is no difference between the structure of Me 32.9 in space group P 4 2 /n and that of Me 80 in space group I 4/ m .

Scapolite oikocrysts, up to 3 cm in diameter, occur in cognate, cumulate nodules from the 2 Ma rift-related, alkali-basaltic Kula Volcanic Province in Western Turkey. Three scapolite-bearing nodules were found within deposits originating from two adjacent cones. They contain primocrysts of clinopyroxene, biotite, apatite and titanite, together with interstitial kaersutite, biotite, plagioclase and scapolite. Kula scapolites fall mid-way between the meionite (Ca4[Al6Si6O24]CO3) and marialite (Na4[Al3Si9O24]Cl) end-members (∼Me45—71). We believe this to be the first report of interstitial magmatic scapolite.

The scapolite structure consists of two interpenetrating frameworks: [(Si,Al)12O24] and {Na,Ca}{Cl,(CO3)}. The unstrained dimensions of each framework are significantly different (as determined by the valence-matching principle and distance-least-squares refinement); there is an intrinsic steric stress between the two frameworks, the magnitude of which is a function of chemical composition. Evidence of the resulting structural strain is the large U eq values for Cl at the A site across the marialite–meionite series and the large U eq values for (Na,Ca) at the M site in marialite. Local bond-valence considerations indicate extensive SRO (Short-Range Order) about both Cl and (CO3). The short-range bond-valence requirements of the (CO3) group are satisfied by coordination with Ca4 at ~ Me 100, NaCa3 at ~ Me 80, Na2Ca2 at ~ Me 55, Na3Ca at ~ Me 40, and Na4 at ~ Me 20. The driving mechanism for the I 4/ m → P 42/ n phase-transitions is the coupling of SRO between the [(Si,Al)12O24] and {Na,Ca}{Cl,(CO3)} frameworks. At a composition of ~ Me 50, local bond-valence requirements force the local clusters Na2Ca2 to occur in a trans configuration {NaCaNaCa} around the A site. Anion bond-valence requirements also force these clusters to link to each other through Na–O–Na or Ca–O–Ca linkages, and hence the combination of SRO and bond-valence requirements gives rise to LRO (long-range order). In turn, Al and Si are constrained to order at different tetrahedra, an ordering that breaks the topological I 4/ m symmetry and gives rise to two distinct tetrahedra, T (2) and T (3), dominated by Al and Si, respectively. The result is the P 42/ n scapolite structure. As Na or Ca become dominant, the number of Na2Ca2 clusters decreases, the effectiveness of the symmetry-lowering mechanism lessens, and the degree of order of Al and Si over the T (2) and T (3) sites decreases toward both the marialite and meionite ends of the series. At the compositions ~ Me 22 and Me 78, the clusters NaCa3 and Na3Ca dominate, the driving mechanism for lower symmetry vanishes, and the structures revert to maximal I 4/ m symmetry. The nonlinear variations in bulk composition across the marialite–meionite series are also the result of SRO of M -site Na and Ca around the A site [which is occupied by Cl in end-member marialite and (CO3) in end-member meionite]. Local bond-valence requirements indicate extensive SRO about both Cl and (CO3) in scapolite. In particular, Cl cannot occur in I 4/ m meionite except where K is present; in this case, chemical variations indicate that Cl enters the I 4/ m meionite structure as {ClK2Ca2} clusters.

Scapolite-group minerals are a group of rock-forming framework aluminosilicates common in a wide variety of igneous and metamorphic rocks. They have the general formula M 4 [ T 12 O 24 ] A , where M represents Na, K, Ca, Sr, Ba and Fe 2+ , T stands for Si, Al, (Fe 3+ ), and A represents Cl, (CO 3 ), (SO 4 ). They constitute a non-binary solid-solution series involving the end members marialite (Na 4 [Al 3 Si 9 O 24 ] Cl, Ma ) and meionite (Ca 4 [Al 6 Si 6 O 24 ] CO 3 , Me ). The crystal structures of eighteen scapolite-group minerals from Me 4.9 to Me 92.8 [ Me = 100 ∑ M 2+ /∑( M 2+ + M + )] have been refined with Mo K α X-radiation. The crystals, a 12.045–12.208, c 7.564–7.586 A, V 1099.7–1130.1 A 3 , I 4/ m or P 4 2 / n , Z = 2, were analyzed with an electron microprobe subsequently to collection of the X-ray intensity data. There are subtle differences between the behavior of the distinct T tetrahedra in the I 4/ m and P 4 2 / n structures. For Si and Al, determinative curves for the assignment of site populations from T –O> distances were developed for each T site in each structure type, and T -site populations were assigned from these curves. In I 4/ m marialite, there is no Al at the T (1) site. In P 4 2 / n scapolite, Al is strongly ordered at T (2), and Si is strongly ordered at T (3), but this order is never complete. The A -site cation (Na, Ca, K) is [8]-coordinated in end-member marialite and is (on average) [8.5]-coordinated in end-member meionite. Each ( M ϕ n ) polyhedron shares two edges with adjacent ( M ϕ n ) polyhedra, forming a three-dimensional framework that interpenetrates the [ T 12 O 24 ] silicate framework. In (CO 3 )-bearing structures, the model proposed by previous workers, four (CO 3 ) groups arranged about the 4 axis ( I 4/ m ) or 4 axis ( P 4 2 / m ), is consistent across the series from marialite to meionite. Any (SO 4 ) present is disordered by rotation of 90° about the central 4-fold axis; in I 4/ m structures, each arrangement is 50% occupied, whereas in P 4 2 / n structures, the occupancies of the two arrangements are not required to be equal.

Application of an X-ray fluorescence (XRF) microprobe for the analysis of single grains (80 to 1,000 μm in diameter) of Cl-rich minerals for Br has been evaluated for fluorapatite, chlorapatite, scapolite-group minerals (marialite and meonite) and sodalite. A calibration curve based on the Br contents in four international reference materials has been confirmed by measurements on Br-bearing standard solutions and by agreement with the results of Cl-rich minerals from instrumental neutron-activation analyses. Absolute errors associated with individual XRF microprobe analyses ( i.e ., counting statistics alone) are less than 5%, and the calculated limit of detection in the analysis of single mineral grains is ~1 ppm Br. Matrix and grain-size effects are shown to be negligible. Experiments at 1 atmosphere and 800 to 1000°C yield the following distribution coefficients for Br–Cl exchanges between marialite or sodalite and hydrous NaCl–NaBr melts: KDmarialite–melt = 0.97 ± 0.08 and KDsodalite–melt = 0.9 ± 0.1. Therefore, the Cl/Br values in marialite and sodalite closely reflect the halogen proportions of their coexisting melts or fluids. The diffusivity of Br in sodalite follows an Arrhenius relation: DBr = 6.5 × 10−7 exp(−270 ± 10kJ/mol/RT) m2/s, over the temperature range from 800 to 1000°C. DBr in marialite is 1.7 ± 0.3 × 10−19 m2/s at 800°C. The Cl/Br weight ratios of marialite in the Tieshan Fe skarn deposit, China, cluster around 650 ± 40, supporting an origin involving hydrothermal brines from associated evaporites. Scapolite-group minerals in the exoskarns of the Nickel Plate Au skarn deposit, British Columbia, have Cl/Br from 560 to 570, higher than those (130 to 180) of their counterparts in the endoskarns and vuggy cavities. This variation is attributable to an increased involvement of magmatic water from distal to proximal zones. Similarly, scapolite-group minerals in the Grenville U–Th–Mo REE pegmatite–skarn–vein deposits vary widely in Cl/Br, from 80 to 380, indicative of mixed sources of hydrothermal fluids. This study shows the potential of Br analysis of Cl-rich silicate minerals for constraining the sources and evolution of hydrothermal fluids.

Η διείσδυση του πλουτωνίτη της Ξάνθης στην μάζα της Ροδόπης στη Β. Ελλάδα δημιούργησε μια εκτεταμένη μεταμορφική άλω γύρω από τον πλουτωνίτη. Η άλω περιέχει δυο περιοχές έντονης σκαπολιθίωσης στις επαφές μεταξύ γρανοδιορίτη και βιοτιτικού γνεύσιου και μεταξύ μονζονίτη και ψαμμίτη στον οποίο διεισδύουν ανδεσιτικές φλέβες. Η παρούσα εργασία παρουσιάζει τα αποτελέσματα ορυκτολογικής και γεωχημικής έρευνας για το σχηματισμό των σκαπόλιθων και ιδιαίτερα για τη φύση των υδροθερμικών ρευστών προερχόμενων από τον πλουτωνίτη από τα οποία σχηματίστηκαν οι σκαπόλιθοι., Emplacement of the Xanthi Plutonic Complex within the Rhodope Massif of N. Greece created an extensive metamorphic aureole around the plutonite. The aureole contains two areas of intense scapolitization in the contacts between granodiorite and biotitegneiss and between monzonite and sandstone, the latter cross-cut by andesite dykes. This paper reports the results of a mineralogical and geochemical study into the formation of the scapolites and particularly the nature of the plutonite-derived hydrothermal fluids from which scapolites were formed.

The crystal structure of scapolite from the Liitzow-Holm Bay region, East Antarctica with cell formular (Na_Ca_K_Mg_) _ (Al_Si_Fe_) _O_ (Cl_(CO_3) _) _, Me_, was determined in tetragonal, P4_2/n, a=12.122(2); c=7.585(2) A by the single crystal X-ray diffraction method. The structure is the same as that of meionite but with some difference in atomic coordinates and Al percentage in the tetrahedon. The CO_3 groups were mainly disordered on the xy plane and off-centred from the Anion site. The cation site was 8-coordinate rather than 6-coordinate.

Rietveld structure refinement, 29 Si magic angle spinning (MAS) and 27 Al satellite transition (SATRAS) NMR spectroscopic data are given for three natural samples of meionite and for synthetic end-member meionite. Trends in structural parameters and NMR spectra are discussed for the solid-solution series from marialite to meionite. The a cell dimension and 27 Al SATRAS NMR spectra show a continuous change through the series, whereas the space group, c cell dimension and 29 Si MAS NMR spectra show different trends for the three subseries, Si 9.0–8.3, Si8.3–7.4 and Si7.4–6.0. Over the interval Si 9.0–8.3, the space group is I4/m, the c cell dimension increases with Al content, and 29 Si spectra show multiple well-resolved peaks; the anion site is occupied by Cl – , and the T(1) site, by Si. Si–Al order between the T(2) and T(3) sites causes the space group for Si 8.3–7.4 to be P42/n and the 29 Si MAS NMR spectra to be dominated by Si(1) (3Si1Al) and Si(3) (1Si3Al) peaks. Here, there is a negative correlation between c cell dimension and Al content. Over the interval Si 7.4–6.0, the c cell parameter increases with Al content, the space group is I4/m, and multiple 29 Si MAS NMR peaks become broad and poorly resolved. Ca 2+ continues to replace monovalent cations in the M site

Diopside with an unusual violet color from southern Baffin Island, Nunavut, Arctic Canada, is found in calc-silicate lenses, associated with marialite, pargasite, phlogopite, calcite, apatite, titanite, talc, chlorite, plagioclase and quartz. It occurs as massive aggregates of roughly equant grains. Basal parting is evident, and pyroxene cleavage is subtle. Indices of refraction are n_α 1.670(1), n_β 1.675(1), and n_γ 1.695(1), and 2V_Z is equal to 57.6(5)° at 589 nm. Pleochroism is nonexistent. D_(calc) = 3.30 g/cm^3. Cell dimensions determined from powder X-ray diffraction are ɑ 9.730(4), b 8.873(3), c 5.275(2) A, β 105.95(3)°. A single-crystal X-ray structural refinement was performed to determine bond lengths and angles. The empirical formula, based on results of electron-microprobe analyses and absorption spectroscopy indicating 0.30 wt.% H_2O in the structure, is (Ca_(0.96)Na_(0.04))(Mg_(0.86)Al_(0.06) Fe^(2+)_(0.05)Ti^(4+)_(0.02))(Si_(1.89)Al_(0.11))O_(5.93)OH_(0.07). The unusual color is due to intervalence charge-transfer between Fe^(2+) and Ti^(4+) at the M1 site, and is observed owing to the low overall concentration of Fe in the diopside. Cathodoluminescence indicates that Mn^(2+) is present in the M1 site. Absorption spectroscopy demonstrates that Mn^(3+) does not contribute to the violet color.

Two types of zoned scapolite in calc-silicate gneisses from Skallen, East Antarctica, were found. One type varies from Me_ in the core to Me_ at the rim. Associated minerals are mainly diopside, pargasite and spinel. The other varies from Me_ in the core to Me_ at the rim. Associated minerals are mainly diopside, phlogopite and K-feldspar (Or_). Scapolite in this specimen contains characteristically K, and the substitution of K in the M site is 8-12%. The unit cell dimensions of scapolite (Me_) from Skallen are a=12.121 (1), c=7.585 (2) A, and those (Me_) from Sri Lanka are a=12.164 (1), c=7.568 (1) A. These scapolites belong to different isomorphous series. It is likely that the zonation and K-substitution of scapolites from Skallen appear to be formed by a reaction with minerals in direct contact and/or fluids in later stage alteration.

Compositional trends of natural samples of scapolite extend toward the ideal (anhydrous) end-members marialite (Ma) Na4Al3Si9O24Cl and meionite (Me) Ca4Al6Si6O24CO3. Intermediate members do not follow the plagioclase substitution, although Si contents closely correlate with Me% [calculated as 100{Σ(divalent cations)}/4]. The solid solution may be explained by substitutional interactions among the M, T and A sites, as constrained by net charge-balance and maximum site-occupancy requirements of the structural formula M4T12O24A. Two changes in compositional trends located at [(Na3.4Ca0.6)(Al3.6Si8.4)O24]+1 and [(Na1.4Ca2.6)(Al4.7Si7.3)O24]+1.9 divide the series into three portions. Nomenclature consistent with these divisions is marialite (Me 0–15), calcian marialite (15 < Me ≤ 50), sodian meionite (50 < Me < 65) and meionite (Me 65–100). A large number of scapolite occurrences have Si values near 7.3 apfu, corresponding to the 14/m-P42/n symmetry transition and to the stabilization of scapolite in a Cl-poor environment. Trends of Cl with Si or (Na + K) apfu indicate complex substitutional mechanisms, and should not be used to suggest that “mizzonite” or “dipyre” are end-members. Calculation of excess positive charge from measurement of M and T cations indicates that scapolite typically has the maximum content of Cl expected by charge-balance. Anion substitution is complicated by substition of H as hydroxyl or bicarbonate-bisulfate anions. The calculation CO2 = 1−ClFS may apply only to anhydrous scapolites which are rare, otherwise it over-estimates CO2 because it ignores the role of H. CO2 content calculated by charge-balance equations gives better agreement with constraints of the known substitutional mechanism. More H is present than is required by stoichiometry, and molecular H2O may be present in channels along the c-axis.

The heat capacities of five mineral samples from the scapolite solid-solution series, Na 1 Al 3 Si 9 O 24 Cl (marialite) to Ca 4 Al 6 Si 6 O 24 CO 3 (meionite), were measured by the adiabatic method from T = 8 K to T = 350 K and by the differential scanning calorimetry (d.s.c.) method from T = 300 K to T = 1000 K. The meionite (Me) content in per cent {Me = 100 Ca*/(Ca* + Na*)} (where the asterisk indicates that possible substituents are included) and molar heat capacity ( C p.m / R ) at T = 298.15 K for each sample is: Me 28 , 82.07; Me 44 , 82.09; Me 55 , 83.95; Me 69 , 85.80; Me 88 , 84.54. The standard molar entropies, { S m o (298.15 K) - S m o (0K)} R ?1 ( R = 8.31451 J·K -1 ·mol -1 ), at T = 298.15 K for the respective compositions are : 85.05 ± 0.26, 83.78 ± 0.50, 85.22 ± 0.24, 85.76 ± 0.21, and 84.17 ± 0.59. The calculated standard molar entropies (as above) at T = 298.15 K for the end-members marialite and meionite, and for an intermediate composition (mizzonite=Me 75 ) are 84.85, 83.94 and 86.15, respectively. Values of the coefficients in the equation C p.m / R = a + bT + cT 2 + dT ?1/2 + eT -2 (valid from T = 300 K to T = 1000 K) are: (Me x , a , b /K, c /K 2 , d /K ?1/2 , e /K -2 ) Me 88 , 315.580, −0.0795676, 1.52825·10 -5 , −3954·83, 1808460; Me 69 , 261.285, −0.0415017, 8.73053·10 -7 , −3028.28, 1083666; Me 55 , 232.236, −0.0352222, 6.49875·10 -6 , 2505.99, 601750; Me 44 , 276.696, −0.0756614, 2.39722·10 -5 , -3210.40, 1044363; Me 28 , 149.917, 0.0229399, -1.23180·10 -5 , 1208.87; -318470. Smoothed thermodynamic functions for the five samples are also presented. The enthalpies of solution for five natural scapolites were measured in 2PbO·B 2 O 3 melts at T = 973 K by Calvet-type calorimetry.

Scapolites are tetragonal framework aluminosilicates with the general formula M4T 12024A, the major constituents being M = Na + Ca, T = Si + AI, and A = Cl + C03 + S-bearing anions, along with minor Fe, Sr, Ba, K, and H. Substitutions among the M, T, and A sites are independent and constrained only by net charge balance and maximum site occupancy. The cell parameters vary primarily with framework chemistry and Al-Si order and are not a strong function of the M- or A-site substitutions. End-member marialite has a = 12.06(1) and c = 7.551(5) A, and meionite has a = 12.20(1) and c = 7.556(5) A. Changes in cellparameter trends in the series occur at AI3.6Sis.4024 and AI4.7Si7.3024 and are correlated with the locations of P421n-141m phase transitions. The scapolite group can be divided into three isomorphous series: 9.0 > Si > 8.4,8.4 > Si > 7.3, and 7.3 > Si > 6.0, each with its own patterns of Al-Si order. Because scapolites with the same Si content have variable cell parameters, there might be variable degrees of Al-Si order or a minor, secondary influence of the M- or A-site substituents.

¶Post-magmatic garnets occur in volcanic breccias at the base of the Neapolitan Yellow Tuff (NYT) formation in the north-western area of the Phlegraean Fields. We report the results of a comprehensive study of these grandites. Garnet is found on the surfaces of tuffaceous blocks or inside their micropores, and is associated with sodalite, sanidine, marialite and amorphous silica. Garnet samples were examined by scanning electron microscopy (SEM), electron probe microanalysis (EPMA), powder and single-crystal X-ray diffraction (XRD) and infrared spectroscopy (IR). SEM observations on morphology showed typical dodecahedral and icositetrahedral habits. EPM analysis showed that they are close to grossular or andradite end members, with only moderate solid solution between them. X-ray study of single crystals showed cubic cell dimensions ao of 11.86 A (grossular) and 12.04 A (andradite). IR spectroscopy confirmed the presence of hydroxyls in coexisting garnet and sanidine, 0.06 wt% H2O (garnet) and 0.05–0.07 wt% H2O (sanidine), respectively. Well-crystallized sanidine of an earlier generation showed significantly higher water contents, in the range 0.13–0.23 wt% H2O. Type of occurrence and mineralogical features suggest a post-magmatic (pneumatolitic) genesis for these garnets. This is consistent with the physico-chemical processes linked to the eruptive dynamics of the breccias. Experimental studies of garnet synthesis at 550 °C and 2 kbar provide further support for this concept.

Scapolite solid solution has been synthesized at 750°C and 4 kbar and is stable relative to plagioclase + calcite + halite over the range of plagioclase compositions from Ab 85 An 15 to Ab 70 An 30 , although albite + halite is stable relative to marialite, Na 4 Al 3 Si 9 O 24 Cl, and anorthite + calcite is stable relative to meionite, Ca 4 Al 6 Si 6 O 24 CO 3 . A chloride-free scapolite, mizzonite, has been synthesized at the approximate composition NaCa 3 Al 5 Si 7 O 24 CO 3 (Ab. 2An. CaCO 3 ). In the absence of chloride, a three-phase invariant assemblage, sodic plagioclase (~Ab 60 An 40 ) + scapolite + calcite is stable relative to plagioclase + calcite over the approximate range of plagioclase composition Ab 60 An 40 -Ab 35 An 65 and another three-phase invariant assemblage, calcic plagioclase (~Ab 15 An 85 ) + scapolite + calcite is stable over the approximate range Ab 30 An 70 -An 15 An 85 . Unit-cell dimensions and refractive indices have been determined for the scapotite synthesized in these experiments and are compared with values for chemically analyzed natural scapolites. Scapolite must be regarded as a ternary solid solution in which, at a given equivalent An-content, the Cl/CO 3 ratio in the large anion site can vary as a function of NaCl and CaCO 3 activities.