Fe-Li云母化学成分的解释和分类

用置换矢量概念解释了115个天然 Fe-Li 云母化学成分的变化。Fe-Li 云母是三八面体 Li-Fe-Al 云母,其基本置换是四锂云母置换。由于 Al-Li 白云母置换和白云母置换的影响,其化学组成变化的基本趋势呈明显的非线性,因而 Fe-Li 云母不是真正的二元系。作为 Fe-Li 云母,富铁黑云母和铁叶云母都是最富铁的成员,因而建议称 Fe-Li 云母为黑云母-锂云母系列。根据化学成分,晶胞参数和折光率的异常变化还提出了该系列自然分类的方案。     

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.     

Synthetic high-pressure micas compositionally intermediate between the dioctahedral and trioctahedral mica series

Intermediate-composition micas with octahedral occupancy 2.5 have been crystallized experimentally from natural phengite, 50% phengite+50% biotite, and synthetic basalt compositions in the pressure range 20–35 kb and temperatures of 800–1,000° C. Their compositions suggest a complete range of micas with octahedral occupancy between 3.0 and 2.5, but a very restricted range between 2.0 and 2.5. These 2.5-octahedral micas lie close to the new mica series proposed by Seifert and Schreyer (1965, 1971), with one end-member composition of K Mg_2.5 (Si₄O₁₀) (OH₂) which is extended by the present results into alumina-bearing members of the series (e. g. K Mg_1.5 Al_1.0 (Si₃Al_1.0O₁₀) (OH)₂). However, the possibility of interlayering of dioctahedral and trioctahedral micas to give an apparently intermediate composition cannot be ruled out. X-ray powder diffraction data on the critical 060 reflection for the phengite mix suggest a transitional change from a single phengite field, through a 2-phase phengite — 2.5-octahedral mica field to a single phase 2.5-octahedral mica field.Natural micas of similar composition have not so far been identified, due probably to the unlikelihood of obtaining a mineralogical record of an appropriate composition at the restricted pressure and temperature conditions apparently needed to stabilize the 2.5-octahedral mica phase. Nevertheless, such a phase may have an important role in mineral assemblages and melting reactions in the deep continental crust, subducted oceanic crust and in the upper mantle; evidence of its existence may be removed by later, lower-pressure reactions.     

Thermochemical study of natural montmorillonite

The paper reports results of an experimental thermochemical study (in a heat-flux Tian-Calvet microcalorimeter) of montmorillonite from (I) the Taganskoe and (II) Askanskoe deposits and (III) from the caldera of Uzon volcano, Kamchatka. The enthalpy of formation Δ _f H _el ⁰ (298.15 K) of dehydrated hydroxyl-bearing montmorillonite was determined by melt solution calorimetry: ?5677.6 ± 7.6 kJ/mol for Na_0.3Ca_0.1(Mg_0.4Al_1.6)[Si_3.9Al_0.1O₁₀](OH)₂ (I), ?5614.3 ± 7.0 kJ/mol for Na_0.4K_0.1(Ca_0.1Mg_0.3Al_1.5Fe _0.1 ³⁺ )[Si_3.9Al_0.1O₁₀](OH)₂ (II), ?5719 ± 11 kJ/mol for K_0.1Ca_0.2Mg_0.2(Mg_0.6Al_1.3Fe _0.1 ³⁺ ) [Si_3.7Al_0.3O10](OH)₂ (III), and ?6454 ± 11 kJ/mol for water-bearing montmorillonite (I) Na_0.3Ca_0.1(Mg_0.4Al_1.6)[Si_3.9Al_0.1O₁₀](OH)₂ · 2.6H₂O. The paper reports estimated enthalpy of formation for the smectite end members of the theoretical composition of K-, Na-, Mg-, and Ca-montmorillonite and experimental data on the enthalpy of dehydration (14 ± 2 kJ per mole of H₂O) and dehydroxylation (166 ± 10 kJ per mole of H₂O) for Na-montmorillonite.     

Composition of barian mica in multiphase solid inclusions from orogenic garnet peridotites as evidence of mantle metasomatism in a subduction zone setting

Multiphase solid inclusions in minerals formed at ultra-high-pressure (UHP) provide evidence for the presence of fluids during deep subduction. This study focuses on barian mica, which is a common phase in multiphase solid inclusions enclosed in garnet from mantle-derived UHP garnet peridotites in the Saxothuringian basement of the northern Bohemian Massif. The documented compositional variability and substitution trends provide constraints on crystallization medium of the barian mica and allow making inferences on its source. Barian mica in the multiphase solid inclusions belongs to trioctahedral micas and represents a solid solution of phlogopite KMg₃(Si₃Al)O₁₀(OH)₂, kinoshitalite BaMg₃(Al₂Si₂)O₁₀(OH)₂ and ferrokinoshitalite BaFe₃(Al₂Si₂)O₁₀(OH)₂. In addition to Ba (0.24–0.67 apfu), mica is significantly enriched in Mg (X_Mg ~ 0.85 to 0.95), Cr (0.03–0.43 apfu) and Cl (0.04–0.34 apfu). The substitution vector involving Ba in the I-site which describes the observed chemical variability can be expressed as BaFe^IVAlClK_?1Mg_?1Si_?1(OH)_?1. A minor amount of Cr and ^VIAl enters octahedral sites following a substitution vector ^VI(Cr,Al)₂□^VI(Mg,Fe)_?3 towards chromphyllite and muscovite. As demonstrated by variable Ba and Cl contents positively correlating with Fe, barian mica composition is partly controlled by its crystal structure. Textural evidence shows that barian mica, together with other minerals in multiphase solid inclusions, crystallized from fluids trapped during garnet growth. The unusual chemical composition of mica reflects the mixing of two distinct sources: (1) an internal source, i.e. the host peridotite and its garnet, providing Mg, Fe, Al, Cr, and (2) an external source, represented by crustal-derived subduction-zone fluids supplying Ba, K and Cl. At UHP–UHT conditions recorded by the associated diamond-bearing metasediments (c. 1100 °C and 4.5 GPa) located above the second critical point in the pelitic system, the produced subduction-zone fluids transporting the elements into the overlying mantle wedge had a solute-rich composition with properties of a hydrous melt. The occurrence of barian mica with a specific chemistry in barium-poor mantle rocks demonstrates the importance of its thorough chemical characterization.     

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.     

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。     

Natural occurrence of a (Fe,Mn, Mg) tetrasilicic potassium mica

A mica whose structural formula: (K_1.76Na_0.31)(Fe_2.22Mn_1.29Mg_0.99Ti_0.28Al_0.240.98) ·(Si_7.33Al_0.67)O_20.26(F_2.16OH_1.58) closely approximates that of tetrasilicic potassium mica K₂(M ₅ ²⁺ )Si₈O₂₀(OH,F)₄ where M²⁺ represents Mg²⁺, Fe²⁺, Mn²⁺, ..., has been discovered in the matrix of a peralkaline rhyolite (comendite) of the Mont-Dore massif (France). These micas had been obtained previously by synthesis only. In the groundmass of the rock, the micaceous phase is accompanied by a manganoan arfvedsonite, pyrophanite, magnetite, apatite, sphene, zircon and fluorite. The crystallographic properties of the mica are typically that of a tetrasilicic mica, with d ₀₆₀ = 1.533Å and space group C2/m. There is a regular decrease of d ₀₆₀ (parameter b) with the ionic radius of the octahedral cation in synthetic micas containing Fe²⁺, Co²⁺, Mg²⁺, Ni²⁺. The purely Mn²⁺ end-member could not be synthesised; its instability is discussed on the basis of structural considerations. The conditions of crystallization of the micaceous phase are estimated to be 760 ° C, 800 bars with a f _{o 2}=10^–14.7 bar. This mica has crystallized from a residual liquid, with high activity of silica and low activity of alumina, whose origin is discussed. The name MONT-DORITE is proposed for this natural tetrasilicic mica having Fe/Fe+Mg >1/2 and Fe/Fe+Mn >1/2. This name is from the stratovolcano Mont-Dore.     

The upper thermal stability of chlorite + quartz: an experimental study in the system MgO-Al2O3-SiO2-H2O

Experiments up to water pressures of 21 kbar have been undertaken to bracket the reactions chlorite + quartz = talc + kyanite + H₂O, chlorite + quartz = talc + cordierite + H₂O, and talc + kyanite + quartz = cordierite ± H₂O by reversed runs in the system MgO-Al₂O₃-SiO₂-H₂O (MASH). These reaction curves intersect at an invariant point (IP1) at PH₂O = 6.4 ± 0.2 kbar and a temperature of 624 ± 4°C. The curve of the chlorite + quartz breakdown to talc + kyanite + H₂O at water pressures above 6.4 kbar shows a negative dP/dT, with the slope decreasing with rising pressure, whereas the slope of the breakdown curve to talc + cordierite + H₂O at water pressures is clearly positive. The composition of the chlorite solid solution reacting with quartz has been estimated to be approximately Mg_4.85Al_1.15[Al_1.15Si_2.85O₁₀](OH)₈ over the entire pressure range investigated. The composition of the talc solid solution forming by the breakdown of chlorite + quartz appears to be Mg_2.94Al_0.06[Al_0.06Si_3.94O₁₀](OH)₂ at PH₂O = 2kbar. With increasing pressure, the Al content of talc decreases, reaching a value of about 0.06 atoms per formula unit at P,H₂O = 21 kbar. As a consequence of the new experimental data, the existing phase topologies of the MASH-system and K₂O-MASH-system have been revised. For example, the invariant point IP1 and the univariant reaction curve kyanite + talc + H₂O = chlorite + cordierite are stable. For this reason, the development of medium- to high-temperature metamorphic rocks compositionally approximating the MASH-system must be reconsidered. The whiteschists from Sar e Sang, Afghanistan, are treated as an example. The application of the present experimental data to metamorphic rocks of more normal composition requires the examination of the influence of further components. This leads to the conclusion that the introduction of Fe²⁺ into magnesian chlorite extends its stability field in the presence of quartz by 10°-15°C in comparison with pure Mg-chlorite.     

Application of a new approach to the calculation of electrostatic energies of expanded Di- and trioctahedral micas

The electrostatic lattice energies of expanded and unexpanded micas are calculated starting from a “generic” structure the ionic charges of which are varied. The mode of expansion is to move the layers apart perpendicular to (001), the K⁺ ions remaining midway between the layers. The energy required for expansion is a quadratic function of the layer charge. It is larger when the layer charge is in the octahedral sites (K _x Al_2?x Mg _x Si₄O₁₀(OH)₂) than when it is in the tetrahedral sites (K _x Mg₃Si_4?x Al _x O₁₀(OH)₂). Fluormicas have a slightly larger expansion energy than OH-micas. With the tetrahedral layer charge, dioctahedral micas have a slightly larger expansion energy than trioctahedral micas. This mode of expansion is less favourable than the mode usually adopted, viz. an expansion whereby the K ions divide themselves between the layers. The energy difference increases with the separation distance and is about 60 kJ mol^?1 at 2.5 Å expansion. An intercalated water layer would be necessary to stabilize the K ions in positions midway between the layers.     

Si+4 Content of natural phengites

The chemical compositions of white micas separated from adjacent rocks of glaucophane and greenschist facies are compared with respect to their Si⁺⁴ content. The micas are predominantly phengitic, i.e. between muscovite, K[Al₂Si₃AlO₁₀(OH)₂] and celadonite, K[(R⁺²R⁺³)Si₄O₁₀(OH)₂] in composition. Constancy of Si content in micas coming from rocks of different bulk chemical composition but closely similar physical conditions indicates that the silica content of a potassic dioctahedral mica can be used to indicate the pressure and temperature conditions of its formation. This conclusion is in part based upon previous experimental data obtained for synthetic phengites.     

Stability relations of K2Mg5Si12O30, and end member of the merrihueite-roedderite group of meteoritic minerals

The phase K₂Mg₅Si₁₂O₃₀ was synthesized both hydrothermally and dry under a variety of pressures and temperatures, and its stability relations were determined. Under hydrothermal conditions it exhibits a lower stability limit lying at 595°C, 1 kb, and 650°C, 2 kb, due to its breakdown into the hydrous assemblage quartz+KMg_2.5Si₄O₁₀(OH)₂ (a mica phase). Its upper temperature stability under hydrothermal conditions is given by its incongruent melting to MgSiO₃+liquid. Near 820° C at a fluid pressure of approximately 6.5 kb the two univariant curves for these breakdown reactions intersect thus limiting the stability field to lower fluid pressures. — Under anhydrous conditions K₂Mg₅Si₁₂O₃₀ becomes unstable at pressures between approximately 7 and 32.5 kb due to its incongruent melting to the assemblage MgSiO₃+quartz (or coesite)+liquid; this melting curve has a pronounced negative slope. No subsolidus breakdown assemblage was encountered at 32.5 kb down to temperatures as low as 750°C. This behavior is probably due to the instability of other ternary compounds in the system K₂O-MgO-SiO₂ at high pressures and thus to the existence of very low-temperature eutectics involving only binary and unary solid phases plus liquid.It is likely that these stability relations provide a model for those of the natural minerals merrihueite and roedderite which contain Na and Fe₊₂ partly substituting for K and Mg and which were encountered in several meteorites. Therefore, the cosmic events leading to the formation of these minerals must have taken place at relatively low pressures and high temperatures, especially when water was present. The bulk compositions of these minerals appear to be incompatible with average chondritic matter under equilibrium conditions. Hence merrihueite and roedderite are not likely to be found in equilibrated chondrites which contain feldspars instead.     

Thermodynamic properties of Fe-rich smectite-nontronite

The results of thermochemical studies are reported for nontronite samples from the Pinares-de-Majari (Eastern Cuba) (Sample I) and Kempirsai serpentine massif (South Urals, Kazakhstan) (Sample II). The enthalpies of formation of dehydrated hydroxyl-bearing nontronites from elements were determined by melt dissolution calorimetry using high-temperature heat-flux Tiana-Calvet microcalorimeter: Δ _f H _el ^o (298.15 K): ?4958 ± 13 kJ/mol for Mg_0.4(Fe _1.5 ³⁺ Mg_0.4Ni_0.1)[Si_3.7Al_0.3O₁₀](OH)₂ (I) and ?5003.6 ± 8.0 kJ/mol for Mg_0.3Na_0.1Ca_0.1(Fe _1.4 ³⁺ Mg_0.5Ni_0.1)[Si_3.7Al_0.3O₁₀](OH)₂ (II). It was determined experimentally that the enthalpy of dehydration (removal of molecular adsorption and interlayer water) of the studied nontronites is 6 ± 2 kJ per 1 mole H₂O. The enthalpy of formation of nontronite of theoretical composition Mg_0.15Fe ₂ ³⁺ [Si_3.7Al_0.3]O₁₀(OH)₂ was estimated at ?4750 kJ/mol. The Gibbs free energies of formation of the nontronites were calculated.     

On the entropy of glaucophane Na2Mg3Al2Si8O22(OH)2

The heat capacity of glaucophane from the Sesia-Lanza region of Italy having the approximate composition (Na_1.93Ca_0.05Fe_0.02) (Mg_2.60Fe_0.41) (Al_1.83Fe_0.15Cr_0.01) (Si_7.92Al_0.08)O₂₂(OH)₂ was measured by adiabatic calorimetry between 4.6 and 359.4 K. After correcting the C _p ⁰ data to values for ideal glaucophane, Na₂Mg₃Al₂Si₈O₂₂(OH)₂ the third-law entropy S ₂₉₈ ⁰ -S ₀ ⁰ was calculated to be 541.2±3.0 J·mol^-1·K^-1. Our value for S ₂₉₈ ⁰ -S ₀ ⁰ is 12.0 J·mol^-1·K^-1 (2.2%) smaller than the value of Likhoydov et al. (1982), 553.2±3.0, is within 6.2 J·mol^-1·K^-1 of the value estimated by Holland (1988), and agrees remarkably well with the value calculated by Gillet et al. (1989) from spectroscopic data, 539 J·mol^-1·K^-1.     

Celadonite mica: Solid solution and stability

Phase relations for the bulk compositions of the celadonites between the MgAl, MgFe³⁺ and Fe²⁺Fe³⁺ types (celadonite = KR²⁺R³⁺ Si₄O₁₀ (OH)₂) under magnetite-iron and nickel-nickel oxide solid-fluid buffers indicate the extent of solid solution possible in this potassic mica series at temperatures between 300° and 430° C at 2 Kb total pressure. Other possible combinations of Mg, Al, Fe ions in octahedrally coordinated sites did not produce single-phase mica products. The ferrous celadonite micas are stable only under oxygen fugacities where magnetite is the stable oxide—where both Fe²⁺ and Fe³⁺ can coexist. However the celadonite with the highest thermal stability at 2 Kb total pressure, nickel-nickel oxide buffer conditions is the KMgFe³⁺Si₄O₁₀(OH)₂ phase which is stable up to 420°C, well into low grade metamorphic conditions. It is thus apparent that the presence of celadonite or glauconite mica will not be indicative of changing diagenetic conditions.     

Thermochemical study Mg–Fe amphiboles

The paper presents data on the thermochemical study (high-temperature melt calorimetry in a Tian–Calvet microcalorometer) of two natural Mg–Fe amphiboles: anthophyllite Mg_2.0(Mg_4.8Fe_0.2 ²⁺)[Si_8.0O₂₂](OH)₂ from Kukh-i-Lal, southwestern Pamirs, Tajikistan, and gedrite Na_0.4Mg_2.0(Mg_1.7Fe_0.2 ²⁺Al_1.3)[Si_6.3Al_1.7O₂₂](OH)₂ from the Kola Peninsula, Russia. The enthalpy of formation from elements is obtained as–12021 ± 20 kJ/mol for anthophyllite and as–11545 ± 12 kJ/mol for gedrite. The standard entropy, enthalpy, and Gibbs energy of formation are evaluated for Mg–Fe amphiboles of theoretical composition.     

Eifelite,KNa3Mg4Si12O30, a new mineral of the osumilite group with octahedral sodium

Eifelite of variable composition is uniaxial positive withn ₀ near 1.543 andn _e near 1.544, a between 10.14 and 10.15 Å, andc about 14.22 Å, space groupP 6/m 2/c 2/c. There is a complete series of solid solution between the eifelite end member KNa₃Mg₄Si₁₂O₃₀ and roedderite, KNaMg₅Si₁₂O₃₀, following the 2 Na?Mg substitution. Both eifelite and roedderite have milarite-type structures, but Na is always in six-coordinated sites: In roedderite Na occupies solely a newly defined B′^[6]-position which is slightly displaced alongc from the ideal B_[9]-position lying on the (001/2)-mirror plane in K₂Mg₅Si₁₂O₃₀. In eifelite Na is located both inB′^[6] and in theA _[6]-positions, where it partially replaces Mg. Eifelite has the highest cation occupancy of all osumilite group minerals known thus far. Both eifelite and roedderite occur in vesicles of contact metamorphosed basement xenoliths ejected with the leucite tephrite lava of the Quaternary Bellerberg volcano in the Eifel, West Germany. They are considered to be precipitates from highly alkaline, MgSi-rich, but Al-deficient gas phases that originated through interaction of gaseous igneous differentiates with the xenoliths.     

Infrared spectra of high-pressure hydrous silicates in the system MgO-SiO2-H2O

Infrared absorption spectra are presented for four hydrous magnesian silicates which are stable at high pressures and high temperatures characteristic of the earth's mantle: phase A (Mg₇Si₂O₁₄H₆), phase B (approx. Mg₂₃Si₈O₄₂H₆), chondrodite (Mg₅Si₂O₁₀H₂), clinohumite (Mg₉Si₄O₁₈H₂). The results show that phase B as well as phase A, chondrodite and clinohumite contain hydroxyl groups in their structures. It is also suggested that phase B, in addition to phase A, could be of potential importance in the upper mantle and transition zone as a dense host mineral for hydroxyl ions. Chemical composition of phase A was reexamined by microprobe analysis: it agrees well with Mg₇Si₂O₁₄H₆, estimated by Yamamoto and Akimoto (1974, 1977), but disagrees with a newly proposed formula Mg₂SiO₅H₂ by Benimoff and Sclar (1984).     

Cation distribution in amphiboles synthesized along the Ga analogue of the tremolite–aluminotschermakite join

 Amphiboles were synthesized from bulk compositions prepared along the join Ca_1.8Mg_5.2Si₈O₂₂(OH)₂–Ca_1.8Mg₃Ga₄Si₆O₂₂(OH)₂ hydrothermally at 750–850 °C and 1.0–1.8 GPa, and along the join Ca₂Mg₅Si₈O₂₂F₂–Ca₂Mg₃Ga₄Si₆O₂₂F₂, anhydrously at 1000 °C and 0.7 GPa to document how closely the tschermak-type substitution is obeyed in these analogues of aluminous amphiboles. Electron-microprobe analyses and Rietveld X-ray diffraction structure refinements were performed to determine cation site occupancies. The extent of Ga substitution was found to be limited in both joins, but with the fluorine series having about twice the Ga content (0.6 atoms per formula unit, apfu) of the hydroxyl-series amphiboles (0.3 apfu). The tschermak-type substitution was followed very closely in the hydroxyl series with essentially equal partitioning of Ga between tetrahedral and octahedral sites. The fluorine-series amphiboles deviated significantly from the tschermak-type substitution and, instead, appeared to follow a substitution that is close to a Ca-pargasite substitution of the type: ^[6]Ga³⁺+2^[4]Ga³⁺+1/2^[A] Ca²⁺ = ^[6]Mg²⁺+2^[4]Si⁴⁺+1/2^[A]□. Infrared spectroscopy revealed an inverse correlation between the intensity of the OH-stretching bands and the Ga content for the hydroxyl- and fluorine-series amphiboles. The direct correlation between the Ga and F content and inverse relationship between the Ga and OH content may be a general phenomenon present in other minerals and suggests, for example, that high F contents in titanite are controlled by the Al content of the host rock and that there may be similar direct Al–F correlations in tschermakitic amphiboles. Evidence for the possibility that Al (Ga) might substitute onto only a subset of the tetrahedral sites in tschermakitic amphiboles was sought but not observed in this study. Received: 5 March 2001 / Accepted: 31 July 2001     

An experimental study of glaucophanic amphiboles in the system Na2O-MgO-Al2O3-SiO2-SiF4 (NMASF): some implications for glaucophane stability in natural and synthetic systems at high temperatures and pressures

The phase relations of glaucophanic amphiboles have been studied at 18–31 kbar/680–950°C in the synthetic system Na₂O–MgO–Al₂O₃–SiO₂–SiF₄ (NMASF) using the bulk composition of fluor-glaucophane, Na₂Mg₃Al₂Si₈O₂₂F₂. Previous experimental studies of glaucophane in the water-bearing system (NMASH) have been hampered by problems of fine grain size (electron microprobe analyses with low oxide totals and contamination by other phases), and consequently good compositional data are lacking. Fluor-amphiboles, on the other hand, generally have much higher thermal stabilities than their hydrous counterparts. By using the fluorine-analogue system NMASF, amphibole crystals sufficiently coarse for electron microprobe analysis have been obtained. Furthermore, NMASH amphibole phase relations are directly analogous to those of the NMASF system because SiF₄ fills the role of H₂O as the fluid species. High-pressure NMASF amphibole parageneses are comparable to those obtained for NMASH amphiboles under similar pressure-temperature conditions, except that the NMASF solidus was not encountered. In the pressure-temperature range of the NMASF experiments, fluor-glaucophane is unstable relative to glaucophanenyböite-Mg-magnesio-katophorite amphiboles. Variations in synthetic fluor-amphibole composition with P and T are discussed in terms of changes in the thermodynamic activities of the principal amphibole end-members, such as glaucophane (a_Gp) and nyböite (a_Ny) using an ideal-mixing-on-sites model. The most glaucophanic amphiboles analysed have a_Gp=0.50–0.60 and coexist with jadeite and coesite at 30 kbar/800°C. Amphiboles become increasingly nyböitic with decreasing pressure through the NaAlSi_-1 exchange, which is the principal variation observed. The most nyböitic amphiboles have a_Ny =0.65–0.70 and coexist with fluor-sodium-phlogopite and quartz at 21–24 kbar/800–850°C. At 800°C amphiboles are essentially glaucophane-nyböite solid solutions. At 850°C there is some minor displacement along MgMgSi_-1, but Mg-magnesio-katophorite activities are very low (<0.06). Activities of the eight other NMASF amphibole end-members are <0.001, except for eckermannite activity which varies from 0.01–0.11. Our results indicate that: (a) synthetic amphiboles mimic the essential stoichiometries observed in blueschist amphiboles; (b) synthetic studies should be relevant to petrologically important high-pressure parageneses and reactions involving glaucophanicamphiboles, sodic pyroxenes, albite and talc; (c) the high-pressure stability limit of fluorglaucophane lies at pressures higher than those reached in this study (31 kbar); (d) in natural systems an approach to glaucophane stoichiometry should be favoured by high water activities as well as high pressures.Abbreviations and formulae used in this paper Glaucophane (Gp) oNa₂(Mg₃Al₂)Si₈O₂₂(OH,F)₂ - Nyböite (Ny) NaNa₂(Mg₃Al₂)Si₇AlO₂₂(OH,F)₂ - Eckermannite (Ek) NaNa₂(Mg₄Al)Si₈O₂₂(OH,F)₂ - Magnesio-cummingtonite (MC) oMg₂(Mg₅)Si₈O₂₂(OH,F)₂ - Sodium-magnesio-cummingtonite (SMC) NaNaMg(Mg₅)Si₈O₂₂(OH,F)₂ - Sodium-anthophyllite (SAn) NaMg₂(Mg₅)Si₇AlO₂₂(OH,F)₂ - Gedrite (Gd) oMg₂(Mg₃Al₂)Si₆Al₂O₂₂(OH,F)₂ - Sodium-gedrite (SGd) NaMg₂(Mg₄Al)Si₆Al₂O₂₂(OH,F)₂ - Mg-magnesio-aluminotaramite (MAT) NaNaMg(Mg₃Al₂)Si₆Al₂O₂₂(OH,F)₂ - Mg-magnesio-katophorite (MKt) NaNaMg(Mg₄Al)Si₇AlO₂₂(OH,F)₂ - Mg-magnesio-barroisite (MBa) oNaMg(Mg₄Al)Si₇AlO₂₂(OH,F)₂ - Jadeite (Jd) NaAlSi₂O₆ - Enstatite (En) Mg₂Si₂O₆ - Forsterite (Fo) Mg₂SiO₄ - Nepheline (Ne) NaAlSiO₄ - Albite (Ab) NaAlSi₃O₈ - Quartz/Coesite (Qz/Co) SiO₂ - Sodium-phlogopite (Sphl) NaMg₃Si₃AlO₁₀(OH,F)₂ - Talc (Tc) oMg₃Si₄O₁₀(OH,F)₂ - o vacant A-site in amphiboles and interlayer site in talc. Octahedral cations in amphiboles are bracketted