Synthesis and stability of deerite,Fe 12 2+ Fe 6 3+ [Si12O40](OH)10, and Fe3+⇋Al3+ substitutions at 15–28 kb

Deerite, Fe ₁₂ ²⁺ Fe ₆ ³⁺ [Si₁₂O₄₀](OH)₁₀, thus far known from ten localities in glaucophane schist terranes, was synthesized at water pressures of 20–25 kb and temperatures of 550–600 °C under the of the Ni/NiO buffer. The X-ray powder diagram, lattice constants and infrared spectrum of the synthetic phase are closely similar to those of the natural mineral. A solid solution series extends from this ferri-deerite end member to some 20 mole % of a hypothetical alumino-deerite, Fe ₁₂ ²⁺ Al ₆ ³⁺ [Si₁₂O₄₀](OH)₁₀. The upper temperature breakdown of ferri-deerite to the assemblage ferrosilite +magnetite+quartz+water occurs at about 490 °C at 15 kb, and 610 °C at 25 kb fluid pressure for the of the Ni/NiO buffer. Extrapolation of these data to lower water pressures indicates that deerite can be a stable mineral only in very low-temperature, high-pressure environments.     

Phase relations between Ca3Fe 2 3 +Si3O12-Fe 3 2 +Fe 2 3 +Si3O12 garnet and CaFeSi2O6-Fe2Si2O6 pyroxene solid solutions

Editorial responsibility: J. Hoefs     

Synthesis, properties and stability of end member boromuscovite, KAl2[BSi3O10](OH)2

End member boromuscovite, with nearly the ideal composition, was synthesized as a single phase from mixtures of its own composition, or with excess boron and water, at high pressures of between 15 and 30 kbar at 700 °C. The mica synthesized consists of a mixture of 2M₁ and 1M polytypes with the cell dimensions of 2M₁: a=5.071(4), b=8.786(4), c=19.830(89) Å, #=95.84(12)°, V=878.5(1.4) ų; and 1M: a=5.059(5), b=8.819(6), c=10.025(17) Å, #=101.39(57)°, V=438.4(1.3) Å. The IR spectrum shows characteristic differences relative to that of muscovite. DTA registers an endothermic peak due to dehydration breakdown above 680 °C. Seeded experiments indicate that boromuscovite is a high-pressure phase requiring minimum pressures of 3 to 10 kbar at temperatures that concomitantly increase from 300 to 750 °C. At lower pressures, the anhydrous solid assemblage K-feldspar + Al-borate (probably Al₄B₂O₉) coexists with a vapor rich in boric acid. The conversion of this assemblage to boromuscovite is connected with increases in the coordination number of B from [3] to [4], and of Al from [4] to [6]. Above 10 kbar, the boromuscovite stability field is limited along its high-temperature side by congruent (or incongruent?) melting of the mica, starting near 750 °C and 10 kbar and increasing to about 900 °C at 50 kbar, although, at such very high pressures a supercritical fluid may be present. Because, even in the presence of excess-boron fluid, the synthetic mica shows invariable X-ray properties, it is concluded that one B atom per formula unit represents the maximum, and - contrary to olenitic tourmalines - no further substitution of Si by B linked with addition of hydrogen takes place. In contrast to muscovite, KAl₂[AlSi₃O₁₀](OH)₂, end member boromuscovite is not stable under normal P-T conditions of the Continental Crust along a 30 °C/km geotherm, and especially not during the intrusion of acidic igneous rocks including their pegmatites, which may explain its scarcity in nature. However, it may form in the waning hydrothermal stages of deep-seated (>10.5 km) pegmatites and - providing sufficient boron is available - in HP/LT subduction zone environments, where it may carry boron to considerable depths.     

Ab initio quantum-mechanical modeling of pyrophyllite [Al2Si4O10(OH)2] and talc [Mg3Si4O10(OH)2] surfaces

Bulk and slab geometry optimizations and calculations of the electrostatic potential at the surface of both pyrophyllite [Al₂Si₄O₁₀(OH)₂] and talc [Mg₃Si₄O₁₀(OH)₂] were performed at Hartree–Fock and DFT level. In both pyrophyllite and talc cases, a modest (001) surface relaxation was observed, and the surface preserves the structural features of the crystal: in the case of pyrophyllite the tetrahedral and octahedral sheets are strongly distorted with respect to the ideal hexagonal symmetry (and basal oxygen are located at different heights along the direction normal to the basal plane), whereas the structure of talc deviates slightly from the ideal hexagonal symmetry (almost co-planar basal oxygen). The calculated distortions are fully consistent with those experimentally observed. Although the potentials at the surface of pyrophyllite and talc are of the same order of magnitude, large topological differences were observed, which could possibly be ascribed to the differences between the surface structures of the two minerals. Negative values of the potential are located above the basal oxygen and at the center of the tetrahedral ring; above silicon the potential is always positive. The value of the potential minimum above the center of the tetrahedral ring of pyrophyllite is ?0.05 V (at 2 Å from the surface), whereas in the case of talc the minimum is ?0.01 V, at 2.7 Å. In the case of pyrophyllite the minimum of potential above the higher basal oxygen is located at 1.1 Å and it has a value of ?1.25 V, whereas above the lower oxygen the value of the potential at the minimum is ?0.2 V, at 1.25 Å; the talc exhibits a minimum of ?0.75 V at 1.2 Å, above the basal oxygen.     

The low-pressure stability of phase A,Mg7Si2O8(OH)6

 Phase A, Mg₇Si₂O₈(OH)₆, is a dense hydrous magnesium silicate whose importance as a host of H₂O in the Earth’s mantle is a subject of debate. We have investigated the low-pressure stability of phase A in experiments on the reaction phase A=brucite+forsterite. Experiments were conducted in piston-cylinder and multi-anvil apparatus, using mixtures of synthetic phase A, brucite and forsterite. The reaction was bracketed between 2.60 and 2.75 GPa at 500° C, between 3.25 and 3.48 GPa at 600° C and between 3.75 and 3.95 GPa at 650^° C. These pressures are much lower than observed in the synthesis experiments of Yamamoto and Akimoto (1977). At 750^° C the stability field of brucite + chondrodite was entered. The enthalpy of formation and entropy of phase A at 1 bar (10⁵ Pa), 298 K, were derived from the experimental brackets on the reaction phase A=brucite+forsterite using a modified version of the thermodynamic dataset THERMOCALC of Holland and Powell (1990), which includes a new equation of state of H₂O derived from the molecular dynamics simulations of Brodholt and Wood (1993). The data for phase A are: ΔH ^o _f =−7126±8 kJ mol^-1, S ^o=351 J K^-1 mol^-1. Incorporating these data into THERMOCALC allows the positions of other reactions involving phase A to be calculated, for example the reaction phase A + enstatite=forsterite+vapour, which limits the stability of phase A in equilibrium with enstatite. The calculated position of this reaction (753° C at 7 GPa to 937° C at 10 GPa) is in excellent agreement with the experimental brackets of Luth (1995) between 7 and 10 GPa, supporting the choice of equation of state of H₂O used in THERMOCALC. Comparison of our results with calculated P-T paths of subducting slabs (Peacock et al. 1994) suggests that, in the system MgO–SiO₂–H₂O, phase A could crystallise in compositions with Mg/Si>2 at pressures as low as 3 GPa. In less Mg rich compositions phase A could crystallise at pressures above approximately 6 GPa. Received: 3 July 1995/Accepted: 14 December 1995     

The crystal structure of sonoraite,Fe3+Te4+O3(OH)·H2O

Summary Sonoraite, FeTeO₃(OH)·H₂O, is monoclinic,P 2₁/c, witha=10.984(2),b=10.268(2),c=7.917(2) Å, =108.49(2)°. For 8 formula units per cell the calculated density is 4.179(2) g/cm³; the observed value is 3.95(1) g/cm³. The Supper-Pace automated diffractometer was used to collect 1884 independent reflections which were corrected for absorption. The structure was determined by an automated symbolic addition procedure. It was refined to a residualR of 6.2% using anisotropic temperature factors for the cations and isotropic temperature factors for the oxygen atoms. Chains of octahedra about Fe extend along [101]; edge-sharing pairs of these octahedra are joined by corner sharing. The Fe–Fe distances across the shared edges are 3.05 and 3.20 Å, short enough to suggest magnetic interactions. All but one H₂O are involved in the chains. The Te⁴⁺ ions have a pseudotetrahedral coordination, with three oxygen ions forming one face of the tetrahedron and the lone electron pair of Te occupying the fourth corner. The O–Te–O average bond angle is 95°. The Fe chains are tied together by Te–O bonds in all three dimensions.

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Die Kristallstruktur von Sonorait, Fe³⁺Te⁴⁺O₃(OH).H₂O

Zusammenfassung Sonorait, FeTeO₃(OH)·H₂O, ist monoklin, P 2₁/c, mit den folgenden Zelldimensionen:a=10,984(2),b=10,268(2),c=7,917(2) Å, =108,49(2)°. Mit 8 Formel-Einheiten errechnet man eine Dichte von 4,179(2) g/cm³; die gemessene Dichte beträgt 3,95(1) g/cm³. Das Supper-Pace automatische Diffraktometer wurde zur Sammlung von 1884 unabhängigen Reflexen benutzt, welche für Absorption korrigiert wurden. Die Struktur wurde mit Hilfe eines vollständig automatischen Programms für symbolische Addition bestimmt. Mit anisotropen Temperaturfaktoren für die Kationen und mit isotropen Temperaturfaktoren für die Sauerstoff-Atome wurde ein Residuum von 6,2% erreicht. Ketten von Eisen-Oktaedern erstrecken sich entlang [101]; Oktaeder-Paare mit gemeinsamen Kanten sind über Eckenverknüpfung verbunden. Die Fe–Fe-Abstände über die gemeinsamen Kanten betragen 3,05 und 3,20 Å, kurz genug, um zu magnetischer Wechselwirkung führen zu können. Nur ein H₂O-Molekül ist nicht Teil einer Kette. Die Te⁴⁺-Ionen befinden sich in pseudotetraedrischer Koordination; drei Sauerstoff-Ionen bilden eine Fläche des Tetraeders, die vierte Ecke wird durch das einsame Elektronenpaar von Te besetzt. Der Mittelwert des O–Te–O-Bindungswinkels beträgt 95° Die Fe-Ketten werden durch Te–O-Bindungen dreidimensional verbunden.

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With 3 Figures     

Characterisation of tobelite (NH4)Al2[AlSi3O10](OH)2 and ND4-tobelite (ND4)Al2[AlSi3O10](OD)2 using IR spectroscopy and Rietveld refinement of XRD spectra

Tobelite (NH₄) Al₂ [AlSi₃O₁₀] (OH)₂, the ammonium analogue of muscovite, and its deuterated form ND₄-tobelite (ND₄) Al₂ [AlSi₃O₁₀] (OD)₂ have been synthesised at 600?°C and 200 and 500 Mpa using a well homogenised, stoichiometric SiO₂-Al₂O₃ oxide mix with Al₂O₃ in excess of 5 mol% and a 25% NH₃ solution whose relative abundance was such that the amount of NH₄ ⁺ stoichiometrically available was in excess of 50%. Characterisation of both tobelite and ND₄-tobelite using IR-spectroscopy, Rietveld refinement of X-ray powder diffraction data, and electron microprobe analysis indicate that, similar to K⁺ in muscovite, the NH₄ ⁺ or ND₄ ⁺ molecule occupies the interlayer site. IR absorption bands caused by NH₄ ⁺ and ND₄ ⁺ can be explained, to a very good approximation, on the basis of T_d symmetry. Nevertheless, substantial line broadening and the occurrence of shoulders indicate a deviation from ideal T_d symmetry. However, even at 77?K, no discrete splitting of the degenerate states could be confirmed. The OH stretching frequencies observed for synthetic tobelite are quite similar to those for muscovite, indicating that the replacement of K⁺ by NH₄ ⁺ has no effect. The low FWHH of the OH bands indicate that the hydroxyl groups are well ordered within the structure. Rietveld refinement of tobelite and ND₄-tobelite indicates that all samples synthesised consist of the 3 different mica polytypes which are typical of muscovite – namely 1M (C2/m), 2M ₁ (C2/c) and 2M ₂ (C2/c). Tobelite and ND₄-tobelite synthesised at 500 Mpa principally contain the 1M polytype, whereas the principle polytype for ND₄-tobelite synthesised at 200 Mpa, is 2M ₂. Rietveld refinement of X-ray diffraction spectra for tobelite synthesised at 200 Mpa was problematic due to the very broad FWHH of the X-ray peaks indicating poor crystallinity. In comparision to synthetic muscovite, the cell dimensions observed for tobelite and its deuterated analogue are quite similar except for the lattice constant c. Due to the larger radius of NH₄ ⁺ or ND₄ ⁺ compared to K⁺ cation, the c-direction is expanded form 10.275 Å in muscovite to approximately 10.540 Å in tobelite and ND₄-tobelite.     

High-pressure synthesis and properties of magnesiocarpholite,MgAl2[Si2O6] (OH)4

Magnesiocarpholite has been synthesized on its own composition between 15 and 25 kb water pressure and 415°–600° C. Best conditions are 25 kb-550° C, starting from a mixture of oxides and synthetic cordierite. Within the MgO-Al₂O₃-SiO₂-H₂O system, possible substitutions appear to be very limited in magnesiocarpholite. Cell-parameters are a=13.706(3), b= 20.075(3), c=5.107(l) Å, space group Ccca. The larger cell, as compared with the most magnesian natural carpholites, is tentatively ascribed to structural disorder. Preliminary stability data confirm the low-temperature character of this mineral which is shown to be a high-pressure equivalent of sudoite+quartz.     

Die Kristallstruktur des Voltaits,K2Fe 5 2+ Fe 3 3+ Al[SO4]12·18H2O

Zusammenfassung Die Kristallstruktur von künstlichem Voltait, K₂Fe₅ ²⁺Fe₃ ³⁺Al[SO₄]₁₂· ·18 H₂O, kubisch hexakisoktaedrisch,Fd3c–O _h ⁸,a ₀=27,254 ,Z-16, wurde mittels photographischer Röntgendaten bestimmt. Die Aufklärung der Struktur erfolgte mit Patterson- und Fouriermethoden unter Zuhilfenahme des multiplen isomorphen Ersatzes. Die Verfeinerung nach der Methode der kleinsten Quadrate ergab mit anisotropen Temperaturfaktoren für 726 beobachteteF _hkl R=0,033. Das Hauptmerkmal der Struktur ist ein 3dimensionales Gerüst aus [Fe³⁺O₆]-Oktaedern, [Fe ₅ ^6/2+ Fe ₁ ^6/3+ O₄(H₂O)₂]-Oktaedern und [K⁺O₁₂]-Polyedern, die durch SO₄-Tetraeder verknüpft werden. Hohlräume dieses Gerüstes werden von ungeordnet orientierten [Al(H₂O)₆]-Oktaedern eingenommen. Es wird gezeigt, daß Al als wesentlicher Bestandteil dieses Voltaits angesehen werden muß.

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The crystal structure of voltaite, K₂Fe₅ ²⁺Fe₃ ³⁺Al[SO₄]₁₂·18H₂O

Summary The crystal structure of synthetic voltaite, K₂Fe₅ ²⁺Fe₃ ³⁺Al[SO₄]₁₂· · 18 H₂O, cubic hexakis-octahedral, space groupFd3c–O _h ⁸,a ₀=27.254 ,Z=16, was determined from photographic X-ray data. The structure was solved by Patterson and Fourier-methods with the aid of multiple isomorphic substitution. Least squares refinement with anisotropic temperature factors resulted inR=0.033 for 726 observedF _hkl . The dominant structural feature is a continous framework composed of [Fe³⁺O₆]-octahedra, [Fe ₅ ^6/2+ Fe ₁ ^6/3+ O₄(H₂O)₂]-octahedra and [K⁺O₁₂]-polyhedra linked by SO₄-tetrahedra. The arrangement gives rise to cages occupied by disordered [Al(H₂O)₆]-octahedra. It is shown that Al must be considered to be a essential constituent of such voltaites.

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Mit 2 Abbildungen     

High pressure X-ray diffraction study of ilvaite CaFe 2 2+ Fe3+[Si2O7/O/(OH)]

An in situ high pressure X-ray diffraction study on synthetic pure ilvaite powder has been performed using a diamond anvil cell. A phase transition from monoclinic to orthorhombic (Pbnm) has been observed at 2.25 Gpa, which can be described as a λ-transition.     

Ephesite,Na(LiAl2) [Al2Si2O10] (OH)2: I. thermal stability and standard state thermodynamic properties

Ephesite, Na(LiAl₂) [Al₂Si₂O₁₀] (OH)₂, has been synthesized for the first time by hydrothermal treatment of a gel of requisite composition at 300≦T(° C)≦700 and \(P_{H_2 O}\) upto 35 kbar. At \(P_{H_2 O}\) between 7 and 35 kbar and above 500° C, only the 2M₁ polytype is obtained. At lower temperatures and pressures, the 1M polytype crystallizes first, which then inverts to the 2M₁ polytype with increasing run duration. The X-ray diffraction patterns of the 1M and 2M₁ poly types can be indexed unambiguously on the basis of the space groups C2 and Cc, respectively. At its upper thermal stability limit, 2M₁ ephesite decomposes according to the reaction (1) $$\begin{gathered} {\text{Na(LiAl}}_{\text{2}} {\text{) [Al}}_{\text{2}} {\text{Si}}_{\text{2}} {\text{O}}_{{\text{10}}} {\text{] (OH)}}_{\text{2}} \hfill \\ {\text{ephesite}} \hfill \\ {\text{ = Na[AlSiO}}_{\text{4}} {\text{] + LiAl[SiO}}_{\text{4}} {\text{] + }}\alpha {\text{ - Al}}_{\text{2}} {\text{O}}_{\text{3}} {\text{ + H}}_{\text{2}} {\text{O}} \hfill \\ {\text{nepheline }}\alpha {\text{ - eucryptite corundum}} \hfill \\ \end{gathered}$$ Five reversal brackets for (1) have been established experimentally in the temperature range 590–750° C, at \(P_{H_2 O}\) between 400 and 2500 bars. The equilibrium constant, K, for this reaction may be expressed as (2) $$log K{\text{ = }}log f_{{\text{H}}_{\text{2}} O}^* = 7.5217 - 4388/T + 0.0234 (P - 1)T$$ where \(f_{H_2 O}^* = f_{H_2 O} (P,T)/f_{H_2 O}^0\) (1,T), with T given in degrees K, and P in bars. Combining these experimental data with known thermodynamic properties of the decomposition products in (1), the following standard state (1 bar, 298.15 K) thermodynamic data for ephesite were calculated: H _f,298.15 ⁰ =-6237372 J/mol, S _298.15 ⁰ =300.455 J/K·mol, G _298.15 ⁰ =-5851994 J/mol, and V _298.15 ⁰ =13.1468 J/bar·mol.     

我国发现的一种含锂新矿物——纤钡锂石 BaMg2LiAl3[Si2O6]2（OH）8

纤钡锂石产于湖南临武香花岭地区一水晶矿锂云母石英脉晶洞中,与锂云母、石英等矿物共生。矿物为浅黄白色,丝绢光泽,呈针状、纤维状、放射状或平行束状集合体,纤维长达1厘米。经X射线单晶及粉晶衍射测定:该矿物属斜方晶系,空间群Ccca,晶胞参数:a=13.60(?),b=20.24(?),e=5.16(?)。最强衍射线为:10.12(?)(100) 4.05(?)(78) 3.39(?)(91) 2.605(?)(31)2.390(?)(28)。     

Synthesis and characterization of Mn-bearing ilvaite CaFe 2−x 2+ MnxFe3+ [Si2O7/O/(OH)]

Summary Synthesis of Mn-bearing ilvaites, CaFe _2–x ²⁺ Mn_xFe³⁺ [Si₂O₇/O/(OH)], with 0 x 0.19, have been performed under hydrothermal conditions at 2 and 3 kbars, T = 300 -400°C and at oxygen fugacities defined by the Fe₂O₃/Fe₃O₄ - and the Ni/NiO -buffer. As shown by X-ray diffraction, the substitution of Fe²⁺ by Mn²⁺ decreases the monoclinic angle and causes a phase transition from monoclinic to orthorhombic at x = 0.19. The Fe-distribution has been determined by Mössbauer spectroscopy.

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Synthese und Charakterisierung von Mn-haltigem Ilvait CaFe _2–x ²⁺ Mn_xFe³⁺ [Si₂O₇/O/(OH)]

Zusammenfassung Mn-haltiger Ilvait CaFe _2–x ²⁺ Mn_x Fe³⁺ [Si₂O₇/O/(OH)] wurde unter hydrothermalen Bedingungen bei Drucken von 2 und 3 kbar, Temperaturen zwischen 300 und 400°C und bei Sauerstoff Fugazitäten, die durch Festkörperpuffer (Fe₂O₃/Fe₃O₄ und Ni/NiO) kontrolliert wurden, hergestellt. Röntgenbeugungsuntersuchungen zeigen, daß mit steigendem Mn-Einbau der monokline Winkel kleiner wird, und daß bei x = 0.19 ein Phasenübergang von der monoklinen zur orthorhombischen Struktur erfolgt. Die Fe-Verteilung wurde mit Mössbauer-Spektroskopie bestimmt.

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With 4 Figures     

Heat capacity of ferrosilite, Fe2Si2O6

The heat capacity of synthetic ferrosilite, Fe₂Si₂O₆, was measured between 2 and 820 K. The physical properties measurement system (PPMS, Quantum Design^®) was used in the low-temperature region between 2 and 303 K. In the temperature region between 340 and 820 K measurements were performed using differential scanning calorimetry (DSC). The C _p data show two transitions, a sharp λ-type at 38.7 K and a small shoulder near 9 K. The λ-type transition can be related to collinear antiferromagnetic ordering of the Fe²⁺ spin moments and the shoulder at 10 K to a change from a collinear to a canted-spin structure or to a Schottky anomaly related to an electronic transition. The C _p data in the temperature region between 145 and 830 K are described by the polynomial $C_{p} {\left[ {\hbox{J\,mol}^{{ - 1}}\,{\hbox{K}}^{{ - 1}} } \right]} = 371.75 - 3219.2T^{{ - 1/2}} - 15.199 \times 10^{5} T^{{ - 2}} + 2.070 \times 10^{7} T^{{ - 3}} $ The heat content [H ₂₉₈ – H ₀] and the standard molar entropy [S ₂₉₈ – S ₀] are 28.6 ± 0.1 kJ mol^?1 and 186.5 ± 0.5 J mol^?1 K^?1, respectively. The vibrational part of the heat capacitiy was calculated using an elastic Debye temperature of 541 K. The results of the calculations are in good agreement with the maximum theoretical magnetic entropy of 26.8 J mol^?1 K^?1 as calculated from the relationship 2*Rln5.     

The upper thermal stability of clinochlore,Mg5Al[AlSi3O10](OH)8, at 10–35 kb P_{{\text{H}}_{\text{2}} {\text{O}}}

Clinochlore, which is, within the limits of error, the thermally most stable member of the Mg-chlorites, breaks down at = P _tot to the assemblage enstatite+forsterite+spinel+H₂O along a univariant curve located at 11 kb, 838 ° C; 15kb, 862 ° C; and 18 kb, 880 ° C (±1 kb ±10 ° C). At water pressures above that of an invariant point at 20.3 kb and 894 ° C involving the phases clinochlore, enstatite, forsterite, spinel, pyrope, and hydrous vapor, clinochlore disintegrates to pyrope+forsterite+spinel+H₂O. The resulting univariant curve has a steep, negative dP/dT slope of –930 bar/ °C at least up to 35 kb.Thus, given the proper chemical environment, Mg-chlorites have the potential of appearing as stable phases within the earth's upper mantle to maximum depths between about 60 and 100 km depending on the prevailing undisturbed geotherm, and to still greater depths in subduction zones. However, unequivocal criteria for mantle derived Mg-chlorites are difficult to find in ultrabasic rocks.     

The distribution of Fe3+ and Ga3+ between two tetrahedral sites in melilites,Ca2(Mg,Fe3+, Ga,Si)3O7

The distribution of Fe³⁺ and Ga³⁺ between the two tetrahedral sites in three synthetic melilites has been studied by using ⁵⁷Fe Mössbauer spectroscopy. In the melilite, (Ca₂Ga₂SiO₇)₅₀ (Ca₂Fe³⁺GaSiO₇)₅₀ (mol %), the distribution of Fe³⁺ and Ga³⁺ in T1 and T2 sites is apparently random, which can be explained in terms of the electrostatic valence rule. However in the melilites, (Ca₂MgSi₂O₇)₅₂ (Ca₂Fe³⁺GaSiO₇)₄₂ (Ca₂Ga₂SiO₇)₆ and (Ca₂MgSi₂O₇)₆₂ (Ca₂Fe³⁺GaSiO₇)₃₆ (Ca₂Ga₂SiO₇)₂ (mol %), Fe³⁺ shows preference for the more ionic T1 site and Ga³⁺ for the more covalent T2 site. If the electronegativity of Ga³⁺ is assumed to be larger than that of Fe³⁺, the mode of distribution of Fe³⁺ and Ga³⁺ can be explained in terms of our previous hypothesis that a large electronegativity induces a stronger preference for the more covalent T2 site.     

A New Compound of the (K,Na)3Ca8Si6[Si4O12]3 Charoite Substrate

Doklady Earth Sciences - As a result of experimental studies on the interaction between a charoite substrate and host lamprophyres of microcline-arfvedsonitic composition, a new...