Li-bearing, disordered Mg-rich tourmaline from a pegmatite-marble contact in the Austroalpine basement units (Styria, Austria)

Pale-blue to pale-green tourmalines from the contact zone of Permian pegmatites to mica schists and marbles from different localities of the Austroalpine basement units (Rappold Complex) in Styria, Austria, are characterized. All these Mg-rich tourmalines have small but significant Li contents, up to 0.29 wt% Li₂O, and can be characterized as dravite, with FeO contents of ?~?0.9–2.7 wt%. Their chemical composition varies from ^X (Na_0.67Ca_0.19?K_0.02?_0.12) ^Y (Mg_1.26Al_0.97Fe²⁺ _0.36Li_0.19Ti⁴⁺ _0.06Zn_0.01?_0.15) ^Z (Al_5.31?Mg_0.69) (BO₃)₃ Si₆O₁₈ ^V (OH)₃? ^W [F_0.66(OH)_0.34], with a?=?15.9220(3), c?=?7.1732(2) Å to ^X (Na_0.67Ca_0.24?K_0.02?_0.07) ^Y (Mg_1.83Al_0.88Fe²⁺ _0.20Li_0.08Zn_0.01Ti⁴⁺ _0.01?_0.09) ^Z (Al_5.25?Mg_0.75) (BO₃)₃ Si₆O₁₈ ^V (OH)₃? ^W [F_0.87(OH)_0.13], with a?=?15.9354(4), c?=?7.1934(4) Å, and they show a significant Al-Mg disorder between the Y and the Z sites (R1?=?0.013–0.015). There is a positive correlation between the Ca content and?<?Y-O?>?distance for all investigated tourmalines (r?≈?1.00), which may reflect short-range order configurations including Ca and Fe²⁺, Mg, and Li. The tourmalines have X_Mg (X_Mg?=?Mg/Mg?+?Fe_total) values in the range 0.84–0.95. The REE patterns show more or less pronounced negative Eu and positive Yb anomalies. In comparison to tourmalines from highly-evolved pegmatites, the tourmaline samples from the border zone of the pegmatites of the Rappold Complex contain relatively low amounts of total REE (~8–36 ppm) and Th (0.1–1.8 ppm) and have low La_N/Yb_N ratios. There is a positive correlation (r?≈?0.91) between MgO of the tourmalines and the MgO contents of the surrounding mica schists. We conclude that the pegmatites formed by anatectic melting of mica schists and paragneisses in Permian time. The tourmalines crystallized from the pegmatitic melt, influenced by the metacarbonate and metapelitic host rocks.     

The crystal structure of an aluminum-rich schorl overgrown by boron-rich olenite from Koralpe, Styria, Austria

Summary The first natural tourmaline (because tourmaline with ^[4]B has also been synthesized, we distinguish here between natural and synthetic tourmaline) that has been unequivocally demonstrated to contain B as a substituent at the T sites was described from Koralpe, Styria, Austria. This colourless B-rich olenite occurs as rims overgrowing schorl (black crystals up to a few cm) that has not yet been structurally characterized. A crystal structure refinement (R = 0.019) of this Al-rich schorl shows that ^[4]B occurs in the overgrown schorl; the optimized occupants of the atomic positions yield ^X (Na_0.64Ca_0.10K_0.06□_0.20) ^Y (Fe²⁺ _1.72Al_1.08Ti_0.11Zn_0.03□_0.06) ^Z (Al_5.70Mg_0.20Fe_0.08 ²⁺Mn_0.02) (^[3]BO₃)_3.00 ^T (Si_5.76 ^[4]B_0.24)O₁₈ [F_0.11(OH)_3.31O_0.58]. This is the first known (Al-rich) schorl where a structure refinement has detected ^[4]B. Comparing the structure refinements and the chemical composition of the Koralpe schorl and other ^[4]B-bearing tourmalines with tourmalines which contain no ^[4]B, it is of interest that only structure refinements of tourmalines which are low in magnesium and with a higher component of olenite show substantial amounts of ^[4]B; the role of Mg in controlling the amount of ^[4]B is not known, but it seems that an Al-component on the Y site (olenite-component), a boron-enriched environment and special P-T-t conditions are necessary to get tourmaline with substantial amounts of ^[4]B. Received July 7, 2000; revised version accepted June 6, 2001     

Tourmaline from the rare-element Pinilla pegmatite, (Central Iberian Zone, Zamora, Spain): chemical variation and implications for pegmatitic evolution

Summary Tourmaline is an ubiquitous constituent in the Pinilla de Fermoselle rare-element pegmatite (Zamora, Spain), as well as in barren pegmatitic and quartz–tourmaline veins inside the associated leucogranite. The rare-element pegmatite shows internal zoning, evolving from a barren facies, in the lower border zone, in contact with the leucogranite, to a Li-rich facies in the upper border zone, close to the host-rocks.Tourmalines from the veins within the leucogranite have highest Mg contents, and belong to the schorl–dravite series. The tourmalines from the rare-element pegmatite mostly belong to the schorl–elbaite series, with chemical compositions within the range of the end-members, whereas the tourmalines associated with the most evolved zone in the pegmatite belong to the elbaite–rossmanite series. The broad compositional range shown by the tourmalines correlates quite well with the pegmatite zoning. The most plausible substitution mechanism for the chemical evolution of tourmalines during crystallization seems to be Mg_–1Fe²⁺₁, [X]_–1^YAl_–1^XNa_–1^YFe²⁺₁, for the foitite–schorl series; ^YFe²⁺_–3^YAl_1.5^YLi_1.5, for the schorl–elbaite vector; ^XNa_–1^YLi_–0.5[X]₁^YAl_0.5, for the elbaite–rossmanite series; and, (OH)₁F₁ for all the tourmalines except the pink elbaites. This chemical variation in tourmaline is consistent with a crystal fractionation model for the evolution of the Pinilla pegmatite.     

Synthesis of Ca-tourmaline in the system CaO-MgO-Al2O3-SiO2-B2O3-H2O-HCl

Summary. ?Ca-tourmaline has been synthesized hydrothermally in the presence of Ca(OH)₂ and CaCl₂-bearing solutions of different concentration at T = 300–700 °C at a constant fluid pressure of 200 MPa in the system CaO-MgO-Al₂O₃-SiO₂-B₂O₃-H₂O-HCl. Synthesis of tourmaline was possible at 400 °C, but only above 500 °C considerable amounts of tourmaline formed. Electron microprobe analysis and X-ray powder data indicate that the synthetic tourmalines are essentially solid solutions between oxy-uvite, CaMg₃- Al₆(Si₆O₁₈)(BO₃)₃(OH)₃O, and oxy-Mg-foitite, □(MgAl₂)Al₆(Si₆O₁₈)(BO₃)₃(OH)₃O. The amount of Ca ranges from 0.36 to 0.88 Ca pfu and increases with synthesis temperature as well as with bulk Ca-concentration in the starting mixture. No hydroxy-uvite, CaMg₃(MgAl₅)(Si₆O₁₈)(BO₃)₃(OH)₃(OH), could be synthesized. All tourmalines have < 3 Mg and > 6 Al pfu. The Al/(Al + Mg)-ratio decreases from 0.80 to 0.70 with increasing Ca content. Al is coupled with Mg and Ca via the substitutions Al₂□Mg₋₂Ca₋₁ and AlMg₋₁H₋₁. No single phase tourmaline could be synthesized. Anorthite ( + quartz in most runs) has been found coexisting with tourmaline. Other phases are chlorite, tremolite, enstatite or cordierite. Between solid and fluid, Ca is strongly fractionated into tourmaline ( + anorthite). The concentration ratio D = Ca(fluid)/Ca(tur) increases from 0.20 at 500 °C up to 0.31 at 700 °C. For the assemblage turmaline + anorthite + quartz + chlorite or tremolite or cordierite, the relationship between Ca content in tourmaline and in fluid with temperature can be described by the equation (whereby T = temperature in °C, Ca(tur) = amount of Ca on the X-site in tourmaline, Ca( fluid) = concentration of Ca²⁺ in the fluid in mol/l). The investigations may serve as a first guideline to evaluate the possibility to use tourmaline as an indicator for the fluid composition.

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Zusammenfassung. ?Synthese von Ca-Turmelin im System CaO-MgO-Al ₂ O ₃ -SiO ₂ -B ₂ O ₃ -H ₂ O-HCl Im System CaO-MgO-Al₂O₃-SiO₂-B₂O₃-H₂O-HCl wurde Ca-Turmalin hydrothermal aus Ca(OH)₂ and CaCl₂-haltigen L?sungen bei T = 300–700 °C und einem konstanten Fluiddruck von 200 MPa synthetisiert. Die Synthese von Turmalin war m?glich ab 400 °C, aber nur oberhalb von 500 °C bildeten sich deutliche Mengen an Turmalin. Elektronenstrahl-Mikrosondenanalysen und R?ntgenpulveraufnahmen zeigen, da? Mischkristalle der Reihe Oxy-Uvit, CaMg₃Al₆(Si₆O₁₈)(BO₃)₃(OH)₃O, und Oxy-Mg-Foitit, □(MgAl₂)Al₆(Si₆O₁₈)(BO₃)₃(OH)₃O gebildet wurden. Der Anteil an Ca variiert zwischen 0.36 und 0.88 Ca pfu und nimmt mit zunehmender Synthesetemperatur und zunehmender Ca-Konzentration im System zu. Hydroxy-Uvit, CaMg₃(MgAl₅) (Si₆O₁₈)(BO₃)₃(OH)₃(OH), konnte nicht synthetisiert werden. Alle Turmaline haben < 3 Mg und > 6 Al pfu. Dabei nimmt das Al/(Al + Mg)- Verh?ltnis mit zunehmendem Ca-Gehalt von 0.80 auf 0.70 ab. Al ist gekoppelt mit Mg und Ca über die Substitutionen Al₂□Mg₋₂Ca₋₁ und AlMg₋₁H₋₁. Einphasiger Turmalin konnte nicht synthetisiert werden. Anorthit (+ Quarz in den meisten F?llen) koexistiert mit Turmalin. Andere Phasen sind Chlorit, Tremolit, Enstatit oder Cordierit. Ca zeigt eine deutliche Fraktionierung in den Festk?rpern Turmalin (+ Anorthit). Das Konzentrationsverh?ltnis D = Ca(fluid)/Ca(tur) nimmt von 0.20 bei 500 °C auf 0.31 bei 700 °C zu. Für die Paragenese Turmalin + Anorthit + Quarz mit Chlorit oder Tremolit oder Cordierit gilt folgende Beziehung zwischen Ca-Gehalt in Turmalin und Fluid und der Temperatur: (wobei T = Temperatur in °C, Ca(tur) = Anteil an Ca auf der X-Position in Turmalin, Ca(fluid) = Konzentration von Ca²⁺ im Fluid in mol/l). Die Untersuchungen dienen zur ersten Absch?tzung, ob Turmalin als Fluidindikator petrologisch nutzbar ist.

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Received July 24, 1998;/revised version accepted October 21, 1999     

Synthesis of tourmaline solid solutions in the system Na2O–MgO–Al2O3–SiO2–B2O3–H2O–HCl and the distribution of Na between tourmaline and fluid at 300 to 700 °C and 200 MPa

Tourmaline has been synthesized hydrothermally at 200 MPa between 300 and 700 °C from oxide mixtures with Mg-Al ratios for the end members dravite NaMg₃Al₆(Si₆O₁₈)(BO₃)₃(OH)₃(OH) and Mg-foitite &ding6F;(Mg₂Al)Al₆ (Si₆O₁₈)(BO₃)₃(OH)₃(OH). Six different Na concentrations were investigated to determine the distribution of Na between tourmaline and fluid in the SiO₂-saturated system Na₂O-MgO-Al₂O₃-SiO₂-B₂O₃-H₂O-HCl. Synthetic tourmaline ranges from X-site vacant (&ding6F;) tourmaline (Mg-foitite) to nearly ideal dravite with Na=0.95 apfu. There are small, but significant, amounts of proton deficiency and negligible tetrahedral Al. Chemical variation is primarily caused by the substitutions Al&ding6F;Mg_-1Na_-1 and minor AlMg_-1H_-1. Varying amounts of Na and &ding6F; determine the Mg/Al ratios. Besides tourmaline and quartz, additional Mg-Al phases are chlorite and, at 700 °C, cordierite. Albite is also present at high Na concentrations in the bulk composition. The c dimension of the tourmaline crystals increases with Na in tourmaline. The amount of Na in the X-site depends strongly on the bulk concentration of Na in the system as well as on the temperature. These factors in turn control the phase assemblage and the composition of the fluid phase. For the assemblage tourmaline + quartz + chlorite/cordierite + fluid, a linear relationship exists between Na concentration in the fluid (quenched after the run) and tourmaline with temperature: T °C [ᆭ °C]=(Na_fluid/Na_tur)앾.878-14.692 (r²=0.96). For the assemblage tourmaline + albite + quartz + fluid, it is: T °C [ᆣ °C]=(Na_fluid/Na_tur)욝.813-6.231 (r²=0.95), where Na_fluid is the concentration of Na⁺ in the final fluid (mol/l) and Na_tur is the number of Na cations in the X-site of tourmaline. The equations are valid in the temperature range of 500-715 °C. Our experiments demonstrate that the occupancy of the X-site in combination with the changing concentrations of Al and Mg can be used to monitor changes in the fluid composition in equilibrium with a growing tourmaline crystal. Currently, this relation can be applied qualitatively to natural tourmaline to explain zoning in Na- and Al/(Al+Mg).     

Coupled substitutions in the tourmaline group

Statistical analysis of 136 natural tourmaline compositions from the literature reveals the presence and extent of coupled substitutions involving several cations and structural sites. In schorls and dravites these are a dehydroxylation type substitution (1) (OH)^–+R²⁺ = R³⁺+O^2– and an alkali-defect type substitution (2) R⁺+R²⁺ = R³⁺+, Al³⁺ being the predominant R³⁺ action. Substitution (1) which represents solid solution towards a proton-deficient end-member, R⁺ R ₃ ³⁺ R ₆ ³⁺ (BO₃)₃ Si₆O₁₈O₃(OH), accounts for three times as much of the observed compositional variability as does (2) which represents substitution toward a hypothetical alkali-free end-member, (R ₂ ²⁺ R³⁺) R ₆ ³⁺ (BO₃)₃Si₆O₁₈(OH)₄. The occurrence of both of these substituions produces intermediates between end-member schorl/ dravite, R⁺ R ₃ ²⁺ R ₆ ³⁺ (BO₃)₃Si₆O₁₈(OH)₄, and a new series within the tourmaline group, R _1–x ⁺ R ₃ ³⁺ R ₆ ³⁺ (BO₃)₃Si₆O₁₈O_3–x (OH)_1+x.In addition to dehydroxylation type, 2(OH)^–+Li⁺ = R³⁺+20^2–, and possibly alkali-defect type, 2R⁺+Li⁺ = R³⁺+2, substitutions, a third type Li⁺+O^2– = (OH)^–+, occurs in the elbaites giving rise to Li-poor, proton-rich species. All three substitutions serve to reduce the Li-content of natural elbaite which, as a result, does not attain the composition of the ideal end-member, Na(Li_1.5Al_1.5)Al₆(BO₃)₃Si₆O₁₈(OH)₄. Substitution from elbaite and schorl/dravite toward R _1–x ⁺ R ₃ ³⁺ R ₆ ³⁺ (BO₃)₃Si₆O₁₈O_3–x(OH)_1+x is very extensive and may be complete.Substitution toward R _1–x ⁺ R ₃ ³⁺ R ₆ ³⁺ (BO₃)₃Si₆O₁₈O_3–x(OH)_1+x results in improved local charge balance. The mean deviation from oxygen charge saturation is at a maximum in end-member schorl, dravite and elbaite. Substitutions (1) and (2) progressively decrease but substitution (1) does so more effectively, which may explain its predominance in nature. However, alkali-defective end-members appear to be unstable regardless of . Substitution (3) in the elbaites cannot be discussed on the basis of charge balance considerations at present due to the lack of structural information on proton-rich species.     

Adsorption of Lead (II), Copper (II) on tourmaline from Altai mine in China's Sinkiang

Tourmaline from Altai mine in China's Sinkiang was used to remove lead (II), copper (II) from aqueous solution. The results demonstrate that tourmaline contains Na(Mg,V)3Al6(BO3)3Si6O18(OH)4, NaFe3Al6(BO3)3Si6O18(OH)4. The data show that Tourmaline from Altai mine in China's Sinkiang can be used natural adsorbent for lead (II), copper (II).It is observed that the adsorption data fitted to the Langmuir isotherm. Furthermore, both Pb (II) and Cu (II) absorbed by tourmaline and tourmaline were characterized by X-ray diffraction, Laser Raman Spectrum, Fourier transform infrared spectroscopy, X-ray energy dispersive spectrometer, Transmission electron microscopy and Zeta potential.     

Fe-rich Li-bearing magnesionigerite-6N6S from Xianghualing tin-polymetallic orefield, Hunan Province, P.R. China

The Fe-rich Li-bearing magnesionigerite-6N6S occurs in the Xianghualing tin-polymetallic ore field, Linwu County, Hunan Province, Peoples Republic of China. It was found near the outer contact zone of the Laizhiling granite body and in the Middle-Upper Devonian carbonate rocks of Qiziqiao Formation. The mineral formed during the skarn stage. Its empirical formula is Sn_1.81Li_0.67(Fe_1.43Zn_1.19 Mn_0.41)_Σ3.03(Al_14.89Mg_1.46 Ti_0.11Si_0.01)_Σ16.47O₃₀(OH)₂. The structure for magnesionigerite-6N6S was solved and refined in space group R-3?m, with a?=?5.7144(8), c?=?55.446(11) Å, V?=?1568.0(4) ų, to R1?=?0.0528. Based on the structural refinement of single crystal diffraction data the formula of magnesionigerite-6N6S is Sn_1.80Li_0.97(Fe_1.89Zn_0.91) _Σ2.80 (Al_14.60Mg_1.63 Ti_0.20)_Σ16.43O₃₀(OH)₂ with Z?=?3. Fe-rich Li-bearing magnesionigerite-6N6S contains 0.74 wt.% Li₂O. The idealized charge-balanced composition of magnesionigerite-6N6S may be expressed by bivalent and trivalent cations: (Mg²⁺)₄(Al³⁺)₁₈O₃₀(OH)₂. The simplified general formula for the 6N6S polysomes in the nigerite and högbomite groups can be given as A _x B_18-x O₃₀(OH)₂, x?=?~4, where A?=?Mg²⁺, Fe²⁺, Zn²⁺; B?=?Al³⁺, Sn⁴⁺, Ti⁴⁺, Li⁺, □.     

Comparative compressibilities of majorite-type garnets

Relative compressibilities of five silicate garnets were determined by single-crystal x-ray diffraction on crystals grouped in the same high-pressure mount. The specimens include a natural pyrope [(Mg_2.84Fe_0.10Ca_0,06) Al₂Si₃O₁₂], and four synthetic specimens with octahedrally-coordinated silicon: majorite [Mg₃(MgSi)Si₃O₁₂], calcium-bearing majorite [(Ca_0.49Mg_2.51)(MgSi)Si₃0₁₂], sodium majorite [(Na_1.88Mgp_0.12)(Mg_0.06Si_1.94)Si₃O₁₂], and an intermediate composition [(Na_0.37Mg_2.48)(Mg_0.13Al_1.07 Si₀₈₀) Si₃O₁₂]. Small differences in the compressibilities of these crystals are revealed because they are subjected simultaneously to the same pressure. Bulk-moduli of the garnets range from 164.8 ± 2.3 GPa for calcium majorite to 191.5 ± 2.5 GPa for sodium majorite, assuming K′=4. Two factors, molar volume and octahedral cation valence, appear to control garnet compression.     

A high pressure-high temperature study of TiO2 solubility in Mg-rich phlogopite: implications to phlogopite chemistry

The solubility of Tio₂ in phlogopites has been experimentally determined in the system K₂Mg₆Al₂Si₆O₂₀(OH)₄-K₂Mg₄TiAl₂Si₆O₂₀(OH)₄-K₂Mg₅TiAl₄Si₄O₂₀(OH)₄ between 825–1300°C and 10–30 kbar under vapour absent conditions. Starting compositions lie along the join K₂Mg₆Al₂Si₆O₂₀(OH)₄-K₂Mg_4.5TiAl₃Si₅O₂₀(OH)₄ which represents a combination of the Mg^[VI]2Si^[IV] = Ti^[VI]2Al^[VI] and $\text{2Mg}{}^{\mathrm{[VI]}}\text{=}\text{Ti}{}^{\mathrm{[VI]}}\text{□}{}^{\mathrm{[VI]}}$ substitution mechanisms for Ti in phlogopites. The results of the experiments indicate a systematic increase in solubility of Ti with increasing temperature and decreasing pressure for given bulk Tio₂ content. Under isobaric conditions high temperature Ti-saturated phlogopite breaks down to Ti-deficient phlogopite + rutile + vapour. Mass balance calculations suggest that the vapour phase may contain K₂O dissolved in H₂O and that the reaction is controlled by the vapour phase. Analyses of phlogopites coexisting with rutile and vapour can be represented in terms of the end-member components phlogopite [K₂Mg₆Al₂Si₆O₂₀(OH)₄], eastonite [K₂Mg₅Al₄Si₅O₂₀(OH)₄], an octahedral site deficient Ti-phlogopite (Ti-OSD) of composition K₂(Mg₄Ti□)Al₂Si₆)O₂₀(OH)₄, and Ti-eastonite [K₂Mg₅TiAl₄Si₄O₂₀(OH)₄]. With decreasing amounts of Ti in these phlogopites there is a decrease in the Ti-eastonite component and increase in the eastonite component.The general equation for the breakdown of Ti-phlogopite solid solution to Ti-free phlogopite + rutile + vapour is: 14 Ti-eastonite + 7 Ti-OSD ? 16 eastonite + 3 phlogopite + 21 rutile + 4 H₂O + 2 K₂O. Lack of knowledge of H₂O and K₂O activities in the vapour phase does not permit evaluation of thermodynamic constants for this reaction. The Ti solubility in phlogopites and hence its potential as a geothermobarometer under lower crustal to upper mantle conditions is likely controlled by common mantle minerals such as forsterite.     

Aluminum substitution mechanisms in perovskite-type MgSiO<Subscript>3</Subscript>: an investigation by Rietveld analysis

Al-containing MgSiO₃ perovskites of four different compositions were synthesized at 27 GPa and 1,873 K using a Kawai-type high-pressure apparatus: stoichiometric compositions of Mg_0.975Si_0.975Al_0.05O₃ and Mg_0.95Si_0.95Al_0.10O₃ considering only coupled substitution Mg²⁺ + Si⁴⁺ = 2Al³⁺, and nonstoichiometric compositions of Mg_0.99Si_0.96Al_0.05O_2.985 and Mg_0.97Si_0.93Al_0.10O_2.98 taking account of not only the coupled substitution but also oxygen vacancy substitution 2Si⁴⁺ = 2Al³⁺ + V_O¨. Using the X-ray diffraction profiles, Rietveld analyses were performed, and the results were compared between the stoichiometric and nonstoichiometric perovskites. Lattice parameter–composition relations, in space group Pbnm, were obtained as follows. The a parameters of both of the stoichiometric and nonstoichiometric perovskites are almost constant in the X _Al range of 0–0.05, where X _Al is Al number on the basis of total cation of two (X _Al = 2Al/(Mg + Si + Al)), and decrease with further increasing X _Al. The b and c parameters of the stoichiometric perovskites increase linearly with increasing Al content. The change in the b parameter of the nonstoichiometric perovskites with Al content is the same as that of the stoichiometric perovskites within the uncertainties. The c parameter of the nonstoichiometric perovskites is slightly smaller than that of the stoichiometric perovskites at X _Al of 0.10, though they are the same as each other at X _Al of 0.05. The Si(Al)–O1 distance, Si(Al)–O1–Si(Al) angle and minimum Mg(Al)–O distance of the nonstoichiometric perovskites keep almost constant up to X _Al of 0.05, and then the Si(Al)–O1 increases and both of the Si(Al)–O1–Si(Al) angle and minimum Mg(Al)–O decrease with further Al substitution. These results suggest that the oxygen vacancy substitution may be superior to the coupled substitution up to X _Al of about 0.05 and that more Al could be substituted only by the coupled substitution at 27 GPa. The Si(Al)–O1 distance and one of two independent Si(Al)–O2 distances in Si(Al)O₆ octahedra in the nonstoichiometric perovskites are always shorter than those in the stoichiometric perovskite at the same Al content. These results imply that oxygen defects may exist in the nonstoichiometric perovskites and distribute randomly.     

Compositional limits and analogs of monoclinic triple-chain silicates

Growing recognition of triple-chain silicates in nature has prompted experimental research into the conditions under which they can form and the extent of solid solution that is feasible for some key chemical substitutions. Experiments were done primarily in the range of 0.1–0.5 GPa and 200–850 °C for durations of 18–1,034 h. A wide range of bulk compositions were explored in this study that can be classified broadly into two groups: those that are Na free and involve various possible chemical substitutions into jimthompsonite (Mg₁₀Si₁₂O₃₂(OH)₄), and those that are Na bearing and involve chemical substitutions into the ideal end-member Na₄Mg₈Si₁₂O₃₂(OH)₄. Numerous attempts to synthesize jimthompsonite or clinojimthompsonite were unsuccessful despite the type of starting material used (reagent oxides, magnesite + SiO₂, talc + enstatite, or anthophyllite). Similarly, the chemical substitutions of F⁻ for OH⁻, Mn²⁺, Ca²⁺, or Fe²⁺ for Mg²⁺, and 2Li⁺ for Mg²⁺ and a vacancy were unsuccessful at nucleating triple-chain silicates. Conversely, nearly pure yields of monoclinic triple-chain silicate could be made at temperatures of 440–630 °C and 0.2 GPa from the composition Na₄Mg₈Si₁₂O₃₂(OH)₄, as found in previous studies, though its composition is most likely depleted in Na as evidenced by electron microprobe and FTIR analysis. Pure yields of triple-chain silicate were also obtained for the F-analog composition Na₄Mg₈Si₁₂O₃₂F₄ at 550–750 °C and 0.2–0.5 GPa if a flux consisting of Na-halide salt and water in a 2:1 ratio by weight was used. In addition, limited chemical substitution could be documented for the substitutions of 2 Na⁺ for Na⁺ + H⁺ and of Mg²⁺ + vacancy for 2Na⁺. For the former, the Na content appears to be limited to 2.5 cations giving the ideal composition of Na_2.5Mg₈Si₁₂O_30.5(OH)_5.5, while for the latter substitution the Na content may go as low as 1.1 cations giving the composition Na_1.1Mg_9.4Si₁₂O_31.9(OH)_4.1 based on a fixed number of Si cations. Further investigation involving Mg for Na cation exchange may provide a pathway for the synthesis of Na-free clinojimthompsonite. Fairly extensive solid solution was also observed for triple-chain silicates made along the compositional join Na₄Mg₈Si₁₂O₃₂(OH)₄–Ca₂Mg₈Si₁₂O₃₂(OH)₄ where the limit of Ca substitution at 450 °C and 0.2 GPa corresponds to Na_0.7Ca_1.8Mg_7.8Si₁₂O_31.9(OH)_4.1 (with the OH content adjusted to achieve charge balance). Aside from the Na content, this composition is similar to that observed as wide-chain lamellae in host actinolite. The relative ease with which Na-rich triple chains can be made experimentally suggests that these phases might exist in nature; this study provides additional insights into the range of compositions and formation conditions at which they might occur.     

Thermodynamic study of calcic amphiboles

The paper reports original thermochemical data on six natural amphibole samples of different composition. The data were obtained by high-temperature melt solution calorimetry in a Tian–Calvet microcalorometer and include the enthalpies of formation from elements for actinolite Ca_1.95(Mg_4.4Fe _0.5 ²⁺ Al₀₁)[Si_8.0O₂₂](OH)₂(–12024 ± 13 kJ/mol) and Ca_2.0(Mg_2.9Fe _1.9 ²⁺ Fe _0.2 ³⁺ )[Si_7.8Al_0.2O₂₂](OH)₂, (–11462 ± 18 kJ/mol), and Na_0.1Ca_2.0(Mg_3.2Fe _1.6 ²⁺ Fe _0.2 ³⁺ )[Si_7.7Al_0.3O₂₂](OH)₂ (–11588 ± 14 kJ/mol); for pargasite Na_0.5K_0.5Ca_2.0-(Mg_3.4Fe _1.8 ²⁺ Al_0.8)[Si_6.2Al_1.8O₂₂](OH)₂ (–12316 ± 10 kJ/mol) and Na_0.8K_0.2Ca_2.0(Mg_2.8Fe _1.3 ³⁺ Al_0.9) [Si_6.1Al_1.9O₂₂](OH)₂ (–12 223 ± 9 kJ/mol); and for hastingsite Na_0.3K_0.2Ca_2.0(Mg_0.4Fe _1.3 ²⁺ Fe _0.9 ³⁺ Al_0.2) [Si_6.4Al_1.6O₂₂](OH)₂ (?10909 ± 11 kJ/mol). The standard entropy, enthalpy, and Gibbs free energy of formation are estimated for amphiboles of theoretical composition: end members and intermediate members of the isomorphic series tremolite–ferroactinolite, edenite–ferroedenite, pargasite–ferropargasite, and hastingsite.     

Synthesis,lattice constants and OH-valence vibrations of an orthorhombic amphibole with excess OH in the system Li2O-MgO-SiO2-H2O

Orthorhombic amphiboles with excess OH, which can be schematically deduced from anthophyllite by the combined substitutions Mg²⁺ + O^2–Li⁺+OH^– and Mg²⁺2 Li⁺, were synthesized at 750–875° C/1 kbar in the system Li₂O-MgO-SiO₂-H₂O. Their phase relations are presented for 800° C/1 kbar . An amphibole with the analytical composition 2.70 wt% Li₂O, 31.1 wt% MgO, 63.0 wt% SiO₂, and 3.29 wt% H₂O has lattice constants a ₀ 18.588 (11), b ₀ 17.966 (10), c ₀ 5.262 (3) Å, V ₀ 1,757.2 (1.5) ų (referred to Space Group Pnma). The OH-valence vibrational spectrum of this amphibole showed v _OH bands at 3,667, 3,708, and 3,725 (shoulder) cm^–1, which are ascribed to OH in the configurations (MgMgMg)-OH, (MgMgMg)-OH-Li (Li in the A-site) of the pseudotrigonal (M1M1M3)-OH arrangement in the amphibole structure, and to Si-OH, respectively. No explanation can at present be offered for an additional shoulder at 3,695 cm^–1. The proposed structural formula is (Li_{0.27 0.73})(Li_1.11 Mg_0.89)· (Mg₅)(Si_8.01O_21.20(OH)_0.80)(OH)_2.00.     

Boron in metapelites controlled by the breakdown of tourmaline and retrograde formation of borosilicates in the Yanai area,Ryoke metamorphic belt,SW Japan

Tourmaline-out isograd formed by the breakdown of tourmaline is defined in the upper amphibolite-facies metapelites in the Yanai area, Ryoke metamorphic belt, SW Japan. The rim composition of tourmaline progressively becomes aluminous with ascending metamorphic grade, and the chemical zoning of tourmaline is controlled by ^X□AlNa_–1Mg_–1 and MgTi^YAl_–2 vectors in low- to medium-grade zones where muscovite is stable, whereas it is controlled by Mg(OH)^YAl_–1O_–1, CaMgO^X□_–1 ^YAl_–1(OH)_–1 and MgTi^YAl_–2 vectors in further higher–grade, muscovite-unstable zones. The size of tourmaline increases drastically where breakdown of muscovite+quartz takes place, probably due to the growth of tourmaline during breakdown of muscovite. On the high-temperature side of the tourmaline-out isograd, depletion of whole-rock boron is observed. Escape of boron-bearing melt or the fluid evolved from the melt during its crystallization probably caused this depletion, although locally trapped, boron-bearing melt or fluid formed irregularly shaped tourmaline and dumortierite during retrograde metamorphism.     

Mg-Fe云母化学成分的解释和分类(Ⅰ)——基本置换和黑云母平面

 中国东部花岗岩类141个Mg-Fe云母的化学成分将近90%的变化属于八面体层内的类质同象置换,置换矢量Mg ₁Fe⁺²和Fe_-3⁺²(R_Ⅵ⁺³)_-2□_Ⅵ组成了天然黑云母平面,大约80%的变化应当解释为基本置换8Mg ₁Fe⁺²+Fe_-3⁺²(R_Ⅵ⁺³)₂□_Ⅵ.这些是Mg-Fe云母在广泛的自然条件下表现出来的最主要的晶体化学关系。文中还提出了置换矢量的长度、分量和以及电价和三个参数,用以识别矿物化学成分变化的类质同象置换特征。     

Magnetic behavior of trioctahedral micas with different octahedral Fe ordering

This contribution is finalized at the discussion of the magnetic structure of two samples, belonging to phlogopite–annite [sample TK, chemical composition ^IV(Si_2.76Al_1.24) ^VI(Al_0.64Mg_0.72 $ {\text{Fe}}_{1.45}^{2 + } $ Mn_0.03Ti_0.15) (K_0.96Na_0.05) O_10.67 (OH)_1.31 Cl_0.02] and polylithionite–siderophyllite joints [sample PPB, chemical composition ^IV(Si_3.14Al_0.86)^VI(Al_0.75Mg_0.01 $ {\text{Fe}}_{1.03}^{2 + } $ $ {\text{Fe}}_{1.03}^{3 + } $ Mn_0.01Ti_0.01Li_1.09) (K_0.99Na_0.01) O_10.00 (OH)_0.65F_1.35]. Samples differ for Fe ordering in octahedral sites, Fe²⁺/(Fe²⁺?+?Fe³⁺) ratio, octahedral composition, defining a different environment around Fe cations, and layer symmetry. Spin-glass behavior was detected for both samples, as evidenced by the dependency of the temperature giving the peak in the susceptibility curve from the frequency of the applied alternating current magnetic field. The crystal chemical features are associated to the different temperature at which the maximum in magnetic susceptibility is observed: 6?K in TK, where Fe is disordered in all octahedral sites, and 8?K in PPB sample, showing a smaller and more regular coordination polyhedron for Fe, which is ordered in the trans-site and in one of the two cis-sites.     

Comprehensive physicochemical study of dioctahedral palygorskite-rich clay from Marrakech High Atlas (Morocco)

This study is devoted to the physicochemical and mineralogical characterizations of palygorskite from Marrakech High Atlas, Morocco. The raw clay and its Na⁺-saturated <2 μm fraction were characterized using chemical, structural, and thermal analytical techniques. Measurements of specific surface area and porous volume are reported. The clay fraction was found to be made up of 95 % of palygorskite and 5 % of sepiolite. An original feature of this palygorskite is its deficiency in zeolitic H₂O. The half-cell structural formula of its dehydrated form was determined on the basis of 21 oxygens to be (Si_7.92Al_0.08)(Mg_2.15Al_1.4Fe_0.4Ti_0.05 $ \square_{1} $ )(Ca_0.03Na_0.08K_0.04)O₂₁, while the hydrated form could be formulated as (Si_7.97Al_0.03)(Mg_2.17Al_1.46Fe_0.40Ti_0.05)(Ca_0.03Na_0.07K_0,03)O_20.18(OH)_1.94(OH₂)_3.88·2.43 H₂O. These formulas show that the (Al³⁺+Fe³⁺)/Mg²⁺ ratio is around 0.84, revealing a pronounced dioctahedral character. Further, inside its octahedral sheet, it was determined that the inner M₁ sites are occupied by vacancies, whereas the M₂ sites are shared between 90 % of trivalent cations (78 % for Al³⁺ and 22 % for Fe³⁺), 7.5 % of Mg²⁺, and 2.5 % of Ti⁴⁺, all of them linked to 1.94 of structural hydroxyls. The two remaining Mg²⁺ by half-cell occupy edge M₃ sites and are coordinated to 3.88 molecules of OH₂. Channels of this palygorskite are deficient in zeolitic H₂O since they contain only 2.43 H₂O molecules. A correlation was found between these results and the observation of very intense and well-resolved FTIR bands arising from dioctahedral domains (mainly Al₂OH, Fe₂OH, and AlFeOH) along with very small responses from a trioctahedral domain (Mg₃OH). Accordingly, a schematic representation of the composition of the octahedral sheet was proposed. The cation exchange capacity, specific surface area, and total pore volume were also assessed to be ca. 21.2 meq/100 g, 116 m²/g, and 0.458 cm³/g, respectively.     

Influence of the <Emphasis Type="Italic">X</Emphasis>-site composition on tourmaline’s crystal structure: investigation of synthetic K-dravite,dravite, oxy-uvite,and magnesio-foitite using SREF and Raman spectroscopy

The crystal structures of synthetic K-dravite [^XK^YMg ₃ ^Z Al ₆ ^T Si₆O₁₈(BO₃) ₃ ^V (OH) ₃ ^W (OH)], dravite [^XNa^YMg ₃ ^Z Al ₆ ^T Si₆O₁₈(BO₃) ₃ ^V (OH) ₃ ^W (OH)], oxy-uvite [^XCa^YMg ₃ ^Z Al ₆ ^T Si₆O₁₈(BO₃) ₃ ^V (OH) ₃ ^W O], and magnesio-foitite [^X?^Y(Mg₂Al)^ZAl ₆ ^T Si₆O₁₈(BO₃) ₃ ^V (OH) ₃ ^W (OH)] are investigated by polarized Raman spectroscopy, single-crystal structure refinement (SREF), and powder X-ray diffraction. The use of compositionally simple tourmalines characterized by electron microprobe analysis facilitates the determination of site occupancy in the SREF and band assignment in the Raman spectra. The synthesized K-dravite, oxy-uvite, and magnesio-foitite have significant Mg–Al disorder between their octahedral sites indicated by their respective average 〈Y–O〉 and 〈Z–O〉 bond lengths. The Y- and Z-site compositions of oxy-uvite (^YMg_1.52Al_1.48(10) and ^ZAl_4.90Mg_1.10(15)) and magnesio-foitite (^YAl_1.62Mg_1.38(18) and ^ZAl_4.92Mg_1.08(24)) are refined from the electron densities at each site. The Mg–Al ratio of the Y and Z sites is also determined from the relative integrated peak intensities of the Raman bands in the O–H stretching vibrational range (3250–3850 cm^?1), producing values in good agreement with the SREF data. The unit cell volume of tourmaline increases from magnesio-foitite (1558.4(3) ų) to dravite (1569.5(4)–1571.7(3) ų) to oxy-uvite (1572.4(2) ų) to K-dravite (1588.1(2) ų), mainly due to lengthening of the crystallographic c-axis. The increase in the size of the X-site coordination polyhedron from dravite (Na) to K-dravite (K) is accommodated locally in the crystal structure, resulting in the shortening of the neighboring O1–H1 bond. In oxy-uvite, Ca²⁺ is locally associated with a deprotonated W (O1) site, whereas vacant X sites are neighbored by protonated W (O1) sites. Increasing the size of the X-site-occupying ion does not detectably affect bonding between the other sites; however, the higher charge of Ca and the deprotonated W (O1) site in oxy-uvite are correlated to changes in the lattice vibration Raman spectrum (100–1200 cm^?1), particularly for bands assigned to the T ₆O₁₈ ring. The Raman spectrum of magnesio-foitite shows significant deviations from those of K-dravite, dravite, and oxy-uvite in both the lattice and O–H stretching vibrational ranges (100–1200 and 3250–3850 cm^?1, respectively). The vacant X site is correlated with long- and short-range changes in the crystal structure, i.e., deformation of the T ₆O₁₈ ring and lengthening of the O1–H1 and O3–H3 bonds. However, X-site vacancies in K-dravite, dravite, and oxy-uvite result only in the lengthening of the neighboring O1–H1 bond and do not result in identifiable changes in the lattice-bonding environment.