Crystal structure and refined formula of garyansellite Mg<Subscript>2</Subscript>Fe<Superscript>3+</Superscript>(PO<Subscript>4</Subscript>)<Subscript>2</Subscript>(OH) · 2H<Subscript>2</Subscript>O

Single-crystal study of the structure (R = 0.0268) was performed for garyansellite from Rapid Creek, Yukon, Canada. The mineral is orthorhombic, Pbna, a = 9.44738(18), b = 9.85976(19), c = 8.14154(18) Å, V = 758.38(3) ų, Z = 4. An idealized formula of garyansellite is Mg₂Fe³⁺(PO₄)₂(OH) · 2H₂O. Structurally the mineral is close to other members of the phosphoferrite–reddingite group. The structure contains layers of chains of M(2)O₄(OH)(H₂O) octahedra which share edges to form dimers and connected by common edges with isolated from each other M(1)O₄(H₂O)₂ octahedra. The neighboring chains are connected to the layer through the common vertices of M(2) octahedra and octaahedral layers are linked through PO₄ tetrahedra.     

X-ray single-crystal and Raman study of knorringite,Mg3(Cr1.58Mg0.21Si0.21)Si3O12, synthesized at 16 GPa and 1,600 °C

The crystal structure of a knorringite-type compound, Mg₃(Cr_1.58Mg_0.21Si_0.21)Si₃O₁₂, synthesized in a multi-anvil press at P = 16 GPa and T = 1,600 °C, was refined from single-crystal X-ray diffraction data up to R = 2.36 % for 314 independent reflections. Garnet was found to be cubic and have space group Ia $\overline{3}$ d, with the unit cell parameters a = 11.5718 (1) Å, V = 1,549.54 (2) Å³. The knorringite crystal studied contains 21 mol% of majorite end-member. The structural characterization of knorringitic garnet is important because the study of its thermodynamic constants provides new constraints on thermobarometry of peridotitic garnet assemblages of the lowermost upper mantle. The Raman spectra of synthetic knorringite have been obtained for the first time.     

Crystal chemical characteristics of α-CaSi2O5, a new high pressure calcium silicate with five-coordinated silicon synthesized at 1500°C and 10 GPa

The crystal structure of α-CaSi₂O₅ synthesized at conditions of 1500°C and 10 GPa, has been solved and refined in centrosymmetric space group P , using single crystal X-ray diffraction data. The composition (Z=4) and unit cell are Ca_1.02Si_1.99O₅ by EPMA analysis and a=7.243(2) Å, b=7.546(4) Å, c=6.501(4) Å, α=81.43(5)°, β=84.82(4)°, γ=69.60(3)°, V=329.5(3) ų, yielding the density value, 3.55 g/cm³. The structure is closely related to that of titanite, CaTiSiO₅ and features the square-pyramid five-fold coordination of silicon by oxygen. The ionic radius for five-coordinated Si calculated from the bond distances is 0.33 Å. The substantial deviation of valence sum for Ca indicates the existence of local strain and the instability of α-CaSi₂O₅ at room pressure.     

Sodium trisilicate: A new high-pressure silicate structure (Na2Si[Si2O7])

Crystals of sodium trisilicate (Na₂Si₃O₇) have been grown in the presence of melt at 9 GPa, 1200 °C using the MA6/8 superpress at Edmonton, and the X-ray structure determined at room pressure (R=2.0%). Na₂Si₃O₇ is monoclinic with a=8.922(2) Å, b= 4.8490(5) Å, c=11.567(1) Å, β=102.64(1)° (C2/c), D _x = 3.295 g·cm^-3. Silicon occurs in both tetrahedral and octahedral coordination (^[6]Si∶^[4]Si = l∶2). The SiO₄ tetrahedra form a diorthosilicate [Si₂O₇] group and are linked by the isolated SiO₆ octahedra via shared corners into a framework of 6-membered (^[4]Si-^[4]Si-^[6]Si^[4]Si-^[4] Si-^[6]Si) and 4-membered (^[4]Si-^[6]Si-^[4]Sr-^[6]Si) rings: 〈^[6]Si-O〉=1.789 Å, 〈^[4]Si-O〉= 1.625 Å, ^[4]Si-O-^[4]Si=132.9° and the bridging oxygen is overbonded (s = 2.22). Channels parallel to b-axis and [110] accommodate Na in irregular 6-fold coordination: 〈Na-O〉 = 2.511 Å.     

Perrierite-(La), (La,Ce,Ca)4(Fe2+,Mn)(Ti,Fe3+,Al)4(Si2O7)2O8, a new mineral species from the Eifel volcanic district, Germany

Non-metamict perrierite-(La) discovered in the Dellen pumice quarry, near Mendig, in the Eifel volcanic district, Rheinland-Pfalz, Germany has been approved as a new mineral species (IMA no. 2010-089). The mineral was found in the late assemblage of sanidine, phlogopite, pyrophanite, zirconolite, members of the jacobsite-magnetite series, fluorcalciopyrochlore, and zircon. Perrierite-(La) occurs as isolated prismatic crystals up to 0.5 × 1 mm in size within cavities in sanidinite. The new mineral is black with brown streak; it is brittle, with the Mohs hardness of 6 and distinct cleavage parallel to (001). The calculated density is 4.791 g/cm³. The IR spectrum does not contain absorption bands that correspond to H₂O and OH groups. Perrierite-(La) is biaxial (-), α = 1.94(1), β = 2.020(15), γ = 2.040(15), 2V _meas = 50(10)°, 2V _calc = 51°. The chemical composition (electron microprobe, average of seven point analyses, the Fe²⁺/Fe³⁺ ratio determined from the X-ray structural data, wt %) is as follows: 3.26 CaO, 22.92 La₂O₃, 19.64 Ce₂O₃, 0.83 Pr₂O₂, 2.09 Nd₂O₃, 0.25 MgO, 2.25 MnO, 3.16 FeO, 5.28 Fe₂O₃, 2.59 Al₂O₃, 16.13 TiO₂, 0.75 Nb₂O₅, and 20.06 SiO₂, total is 99.21. The empirical formula is (La_1.70Ce_1.45Nd_0.15Pr_0.06Ca_0.70)_Σ4.06(Fe _0.53 ²⁺ Mn_0.38Mg_0.08)_Σ0.99(Ti_2.44Fe _0.80 ³⁺ Al_0.62Nb_0.07)_Σ3.93Si_4.04O₂₂. The simplified formula is (La,Ce,Ca)₄(Fe²⁺,Mn)(Ti,Fe³⁺,Al)₄(Si₂O₇)₂O₈. The crystal structure was determined by a single crystal. Perrierite-(La) is monoclinic, space group P2₁/a, and the unit-cell dimensions are as follows: a =13.668(1), b = 5.6601(6), c = 11.743(1) Å, β = 113.64(1)°; V = 832.2(2) ų, Z = 2. The strong reflections in the X-ray powder diffraction pattern are [d, Å (I, %) (hkl)]: 5.19 (40) (110), 3.53 (40) ( $\overline 3 $ 11), 2.96 (100) ( $\overline 3 $ 13, 311), 2.80 (50) (020), 2.14 (50) ( $\overline 4 $ 22, $\overline 3 $ 15, 313), 1.947 (50) (024, 223), 1.657 (40) ( $\overline 4 $ 07, $\overline 4 $ 33, 331). The holotype specimen of perrierite-(La) is deposited at the Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow, Russia, with the registration number 4059/1.     

Vigrishinite, Zn2Ti4 − x Si4O14(OH,H2O,□)8, a new mineral from the Lovozero alkaline complex, Kola Peninsula, Russia

A new mineral vigrishinite, epistolite-group member and first layer titanosilicate with species-defining Zn, was found at Mt. Malyi Punkaruaiv, in the Lovozero alkaline complex, Kola Peninsula, Russia. It occurs in a hydrothermally altered peralkaline pegmatite and is associated with microcline, ussingite, aegirine, analcime, gmelinite-Na, and chabazite-Ca. Vigrishinite forms rectangular or irregularly shaped lamellae up to 0.05 × 2 × 3 cm flattened on [001]. They are typically slightly split and show blocky character. The mineral is translucent to transparent and pale pink, yellowish-pinkish or colorless. The luster is vitreous. The Mohs’ hardness is 2.5–3. Vigrishinite is brittle. Cleavage is {001} perfect. D _meas = 3.03(2), D _calc = 2.97 g/cm³. The mineral is optically biaxial (?), α = 1.755(5), β = 1.82(1), γ = 1.835(8), 2V _meas = 45(10)°, 2V _calc = 50°. IR spectrum is given. The chemical composition (wt %; average of 9 point analyses, H₂O is determined by modified Penfield method) is as follows: 0.98 Na₂O, 0.30 K₂O, 0.56 CaO, 0.05 SrO, 0.44 BaO, 0.36 MgO, 2.09 MnO, 14.39 ZnO, 2.00 Fe₂O₃, 0.36 Al₂O₃, 32.29 SiO₂, 29.14 TiO₂, 2.08 ZrO₂, 7.34 Nb₂O₅, 0.46 F, 9.1 H₂O, ?0.19 O=F₂, total is 101.75. The empirical formula calculated on the basis of Si + Al = 4 is: H_7.42(Zn_1.30Na_0.23Mn_0.22Ca_0.07Mg_0.07K_0.05Ba_0.02)_Σ1.96(Ti_2.68Nb_0.41Fe _0.18 ³⁺ Zr_0.12)_Σ3.39(Si_3.95Al_0.05)_{Σ4 20.31}F_0.18. The simplified formula is: Zn₂Ti_4?x Si₄O₁₄(OH,H₂O,□)₈ (x < 1). Vigrishinite is triclinic, space group P $\bar 1$ , a = 8.743(9), b = 8.698(9), c = 11.581(11)Å, α = 91.54(8)°, β = 98.29(8)°, γ = 105.65(8)°, V = 837.2(1.5) ų, Z = 2. The strongest reflections in the X-ray powder pattern (d, Å, ?I[hkl]) are: 11.7-67[001], 8.27-50[100], 6.94-43[0 $\bar 1$ 1, $\bar 1$ 10], 5.73–54[1 $\bar 1$ 1, 002], 4.17-65[020, $\bar 1$ $\bar 1$ 2, 200], and 2.861-100[3 $\bar 1$ 0, 2 $\bar 2$ 2, 004, 1 $\bar 3$ 1]. The crystal structure model was obtained on a single crystal, R = 0.171. Vigrishinite and murmanite are close in the structure of the TiSiO motif, but strongly differ from each other in part of large cations and H-bearing groups. Vigrishinite is named in honor of Viktor G. Grishin (b. 1953), a Russian amateur mineralogist and mineral collector, to pay tribute to his contribution to the mineralogy of the Lovozero Complex. The type specimen is deposited in the Fersman Mineralogical Museum of Russian Academy of Sciences, Moscow.     

Sapphirine II

The crystal structure of aP2₁/a polymorph of sapphirine (a=11.286(3),b=14.438(2),c=9.957(2) Å, β=125.4(2) °) of composition [Mg_3.7Fe _0.1 ²⁺ Al_4.1- Fe _0.1 ³⁺ ]^IV[Si_1.8Al_4.2]^IVO₂₀ was refined using structure factors determined by both neutron and x-ray diffraction methods to conventionalR factors of 0.067 and 0.031. respectively, forF _obs>2σ. The results of the two refinements agree reasonably well, but a half-normal probability plot (Abrahams, 1974) comparing the two data sets indicates that the pooled standard deviations of the atomic coordinates have been underestimated by a factor of two. The structure of sapphirine, solved initially by Moore (1969), consists of cubic closest packed oxygens with octahedral and predominantly tetrahedral layers alternately stacked along [100]. The layer in which 70% of the octahedral sites are occupied has an Mg-Al distribution characterized by Mg-rich octahedra sharing edges mainly with Al-rich octahedra. Mean octahedral bond lengths correlate well with Al occupancy determined by neutron site refinement if the relative number of shared octahedral edges is taken into account (see Table 1). The predominantly tetrahedral layer has 10% of the octahedral sites occupied by Al and 30% of the tetrahedral sites occupied by Al-Si in the ratio 2.33∶1. There are single chains of Al-Si tetrahedra parallel toz with corner-sharing wing tetrahedra (T5 andT6) on either side in the (100) plane. The meanT-O distance is highly correlated with Al occupancy, X_Al, as determined from the neutron site refinement: $$\langle T - O\rangle = 1.656 + 0.105X_{Al} (r^2 = 0.995).$$ Details of the neutron refinement are summarized below.     

Chromium solubility in anhydrous Phase B

The crystal structure and chemical composition of a crystal of (Mg_14?x Cr _x )(Si_5?x Cr _x )O₂₄ (x ≈ 0.30) anhydrous Phase B (Anh-B) synthesized in the model system MgCr₂O₄–Mg₂SiO₄ at 12 GPa and 1600 °C have been investigated. The compound was found to be orthorhombic, space group Pmcb, with lattice parameters a = 5.900(1), b = 14.218(2), c = 10.029(2) Å, V = 841.3(2) Å³ and Z = 2. The structure was refined to R ₁ = 0.065 using 1492 independent reflections. Chromium was found to substitute for both Mg at the M3 site (with a mean bond distance of 2.145 Å) and Si at the octahedral Si1 site (mean bond distance: 1.856 Å), according to the reaction Mg²⁺ + Si⁴⁺ = 2Cr³⁺. Such substitutions cause a reduction in the volume of the M3 site and an increase in the volume of the Si-dominant octahedron with respect to the values typically observed for pure Anh-B and Fe²⁺-bearing Anh-B. Taking into account that Cr³⁺ is not expected to be Jahn–Teller active, it appears that both the Cr³⁺–for–Mg and Cr³⁺–for–Si substitutions in the Anh-B structure decrease the distortion of the octahedra. Electron microprobe analysis gave the Mg_13.66(8)Si_4.70(6)Cr_0.62(4)O₂₄ stoichiometry for the studied phase. The successful synthesis of this phase provides new information for the possible mineral assemblages occurring in the Earth’s deep upper mantle and shed new light on the so-called X discontinuity that has been observed at 275–345 km depth in several subcontinental and subduction zone environments.     

Phase F,a new hydrous magnesium silicate synthesized at 1000 °C and 17 GPa: Crystal structure and estimated bulk modulus

The structure of phase F, a non-stoichiometric hydrous silicate synthesized in a uniaxial, split-sphere, multi-anvil apparatus at conditions of 17 GPa and 1000° C, has been solved and refined in space group P6₃cm, using synchrotron X-ray data for a single crystal of a size 18 × 24 × 30 μm. The composition and unit cell for phase F are Mg_3.35Si_5.51H_7.26O₁₈, a=5.073(3) Å, c=14.013(9) Å, 7= 312.3(5) ų. The structure contains layers with many similarities to superhydrous phase B. The layers of oxygen atoms are stacked in the ABCBAC-type double cubic closest packing arrangement. The bulk modulus of phase F was estimated from the structural and compositional relationship to superhydrous phase B and periclase.     

Aklimaite, Ca4[Si2O5(OH)2](OH)4 · 5H2O, a new natural hydrosilicate from Mount Lakargi, the Northern Caucasus, Russia

A new mineral aklimaite, Ca₄[Si₂O₅(OH)₂](OH)₄ · 5H₂O, has been found near Mount Lakargi, Upper Chegem caldera, Kabardino-Balkaria, the Northern Caucasus, Russia, in the skarnified limestone xenolith in ignimbrite. This hydrothermal mineral occurs in a cavity of altered larnite skarn and is associated with larnite, calcium humite-group members, hydrogarnets, bultfonteinite, afwillite, and ettringite. Aklimaite forms transparent, colorless (or occasionally with pinkish tint) columnar or lath-shaped crystals up 3 × 0.1 × 0.01 mm in size, flattened on {001} and elongated along {010}; they are combined in spherulites. The luster is vitreous; the cleavage parallel to the {001} is perfect. D _calc = 2.274 g/cm³. The Mohs’ hardness is 3–4. Aklimaite is optically biaxial, negative, 2V _meas > 70°, 2V _calc = 78°, α = 1.548(2), β = 1.551(3), γ = 1.553(2). The IR and Raman spectra are given. The chemical composition (wt %, electron microprobe) is as follows: 0.06 Na₂O, 0.02 K₂O, 45.39 CaO, 0.01 MnO, 0.02 FeO, 24.23 SiO₂, 0.04 SO₃, 3.22 F, 27.40 H₂O_(calc.), ?1.36 -O=F₂; the total is 99.03. The empirical formula calculated on the basis of 2Si apfu with O + OH + F = 16 is as follows: (Ca_4.02Na_0.01)_Σ4.03[Si_2.00O_5.07(OH)_1.93][(OH)_3.16F_0.84] _Σ4.00 · 5H₂O. The mineral is monoclinic, space group C2/m, a = 16.907(5), b = 3.6528(8), c = 13.068(4) Å, β = 117.25(4)·, V= 717.5(4) ų, Z = 2. Aklimaite is representative of the new structural type, the sorosilicate with disilicate groups [Si₂O₅(OH)₂]. The strongest reflections in the X-ray powder patterns [d, Å (hkl)] are: 11.64(100)(001), 2.948(32)(310, 203), 3.073(20) ( $\bar 404$ , $\bar 311$ ), 2.320(12)(005, 510), 2.901 (11)(004), 8.30(10) $\left( {\bar 201} \right)$ . The type specimen is deposited in the Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow.     

Crystal chemistry, surface morphology and X-ray photoelectron spectroscopy of Fe-rich osumilite from Mt. Arci, Sardinia (Italy)

Microprobe analysis, single crystal X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy, and X-ray absorption spectroscopy were applied on Fe-rich osumilite from the volcanic massif of Mt. Arci, Sardinia, Italy. Osumilite belongs to the space group P6/mcc with unit cell parameters a = 10.1550(6), c = 14.306(1) Å and chemical formula (K_0.729)_C (Na_0.029)_B (Si_10.498 Al_1.502)_T1 (Al_2.706 Fe _0.294 ²⁺ )_T2 (Mg_0.735 Mn_0.091 Fe _1.184 ²⁺ )_AO₃₀. Structure refinement converged at R = 0.0201. Unit cell parameter a is related to octahedral edge length as well as to Fe²⁺ content, unlike the c parameter which does not seem to be affected by chemical composition. The determination of the amount of each element on the mineral surface, obtained through X-ray photoelectron spectroscopy high-resolution spectra in the region of the Si_2p, Al_2p, Mg_1s and Fe_2p core levels, suggests that Fe presents Fe²⁺ oxidation state and octahedral coordination. Two peaks at 103.1 and 100.6 eV can be related to Si⁴⁺ and Si¹⁺ components, respectively, both in tetrahedral coordination. The binding energy of Al_2p, at 74.5 eV, indicates that Al is mostly present in the distorted T2 site, whereas the Mg peak at 1,305.2 eV suggests that this cation is located at the octahedral site. X-ray absorption at the Fe L_2,3-edges confirms that iron is present in the mineral structure, prevalently in the divalent state and at the A octahedral site.     

Crystal chemistry of Fe3+-bearing (Mg,Fe)SiO3 perovskite: a single-crystal X-ray diffraction study

Magnesium silicate perovskite is the predominant phase in the Earth’s lower mantle, and it is well known that incorporation of iron has a strong effect on its crystal structure and physical properties. To constrain the crystal chemistry of (Mg, Fe)SiO₃ perovskite more accurately, we synthesized single crystals of Mg_0.946(17)Fe_0.056(12)Si_0.997(16)O₃ perovskite at 26 GPa and 2,073 K using a multianvil press and investigated its crystal structure, oxidation state and iron-site occupancy using single-crystal X-ray diffraction and energy-domain Synchrotron Mössbauer Source spectroscopy. Single-crystal refinements indicate that all iron (Fe²⁺ and Fe³⁺) substitutes on the A-site only, where \( {\text{Fe}}^{ 3+ } /\Upsigma {\text{Fe}}\sim 20\,\% \) based on Mössbauer spectroscopy. Charge balance likely occurs through a small number of cation vacancies on either the A- or the B-site. The octahedral tilt angle (Φ) calculated for our sample from the refined atomic coordinates is 20.3°, which is 2° higher than the value calculated from the unit-cell parameters (a = 4.7877 Å, b = 4.9480 Å, c = 6.915 Å) which assumes undistorted octahedra. A compilation of all available single-crystal data (atomic coordinates) for (Mg, Fe)(Si, Al)O₃ perovskite from the literature shows a smooth increase of Φ with composition that is independent of the nature of cation substitution (e.g., \( {\text{Mg}}^{ 2+ } - {\text{Fe}}^{ 2+ } \) or \( {\text{Mg}}^{ 2+ } {\text{Si}}^{ 4+ } - {\text{Fe}}^{ 3+ } {\text{Al}}^{ 3+ } \) substitution mechanism), contrary to previous observations based on unit-cell parameter calculations.     

Structural characterization of high-pressure C–Na<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>5</Subscript> by single-crystal diffraction and <Superscript>29</Superscript>Si MAS NMR

Single crystals of C–Na₂Si₂O₅ have been synthesized from the hydrothermal recrystallization of a glass. The title compound is monoclinic, space group P2₁/c with Z= 8 and unit-cell parameters a= 4.8521 (4)Å, b=23.9793(16)Å, c=8.1410(6)Å, β=90.15(1)° and V=947.2(2)ų. The structure has been determined by direct methods and belongs to the group of phyllosilicates. It is based on layers of tetrahedra with elliptically six-membered rings in chair conformation. The sequence of directedness within a single ring is UDUDUD. The sheets are parallel to (010) with linking sodium cations in five- and sixfold coordination. Concerning the shape and the conformation of the rings, C–Na₂Si₂O₅ is closely related to β-Na₂Si₂O₅. However, both structures differ in the stacking sequences of the layers. A possible explanation for the frequently observed polysynthetic twinning of phase C is presented. In the ²⁹Si MAS-NMR spectrum of C–Na₂Si₂O₅ four well-resolved lines of equal intensity are observed at ?86.0, ?86.3, ?87.4, and ?88.2?ppm. The narrow range of isotropic chemical shifts reflects the great similarity of the environments of the different Si sites. This lack of pronounced differences in geometry renders a reliable assignment of the resonance lines to the individual sites on the basis of known empiric correlations and geometrical features impossible.     

The crystal structure of waylandite from Wheal Remfry, Cornwall, United Kingdom

Sr- and Ca-rich waylandite, $ {\left( {{\hbox{B}}{{\hbox{i}}_{0.{54}}}{\hbox{S}}{{\hbox{r}}_{0.{31}}}{\hbox{C}}{{\hbox{a}}_{0.{25}}}{{\hbox{K}}_{0.0{1}}}{\hbox{B}}{{\hbox{a}}_{0.0{1}}}} \right)_{\Sigma 1.12}}{{\hbox{H}}_{0.{18}}}{\left( {{\hbox{A}}{{\hbox{l}}_{{2}.{96}}}{\hbox{C}}{{\hbox{u}}_{0.0{2}}}} \right)_{\Sigma 2.98}}{\left[ {{{\left( {{{\hbox{P}}_{0.{97}}}{{\hbox{S}}_{0.0{3}}}{\hbox{S}}{{\hbox{i}}_{0.0{1}}}} \right)}_{\Sigma 1.00}}{{\hbox{O}}_4}} \right]_2}{\left( {\hbox{OH}} \right)_6} $ , from Wheal Remfry, Cornwall, United Kingdom has been investigated by single-crystal X-ray diffraction and electron microprobe analyses. Waylandite crystallises in space group R $ \overline 3 $ ? m, with the cell parameters: a?=?7.0059(7) Å, c?=?16.3431(12) Å and V?=?694.69(11) ų. The crystal structure has been refined to R ₁?=?3.76%. Waylandite has an alunite-type structure comprised of a rhombohedral stacking of (001) composite layers of corner-shared AlO₆ octahedra and PO₄ tetrahedra, with (Bi,Sr,Ca) atoms occupying icosahedrally coordinated sites between the layers.     

The lattice energy of forsterite. Charge distribution and formation enthalpy of the SiO 4 4− ion

In the lattice energy expression of forsterite, based on a Born-Mayer (electrostatic+repulsive+dispersive) potential, the oxygen charge z _o, the hardness parameter ρ and the repulsive radii r _Mg and r _Si appear as unknown parameters. These were determined by calculating the first and second partial derivatives of the energy with respect to the cell edges, and equalizing them to quantities related to the crystal elastic constants; the overdetermined system of equations was solved numerically, minimizing the root-mean-square deviation. To test the results obtained, the SiO ₄ ^4? ion was assumed to move in the unit-cell, and the least-energy configuration was sought and compared with the experimental one. By combining the two methods, the optimum set of parameters was: z _o=?1.34, ρ=0.27 Å, r _Mg=0.72 Å, r _Si=0.64 Å. The values ?8565.12 and ?8927.28 kJ mol^?1 were obtained, respectively, for the lattice energy E _Land for its ionic component E _L ⁰ ,which accounts for interactions between Mg²⁺ and SiO ₄ ^4? ions only. The charge distribution calculated on the SiO ₄ ^4? ion was discussed and compared with other results. Using appropriate thermochemical cycles, the formation enthalpy and the binding energy of SiO ₄ ^4? were estimated to be: ΔH _f(SiO ₄ ^4? )=2117.6 and E(SiO ₄ ^4? )=708.6 kJ mol^?1, respectively.     

Magnesiochloritoid: Compressibility and high pressure structure refinement

The response of magnesiochloritoid to pressure has been studied by single crystal X-ray diffraction in a diamond anvil cell, using crystals with composition Mg_1.3Fe_0.7Al₄Si₂O₁₀(OH)₄. The unit cell parameters decrease from a = 9.434 (3), b = 5.452 (2), c = 18.136 (5) Å, β = 101.42° (2) (1 bar pressure) to a = 9.370 (7), b = 5.419 (5), c = 17.88 (1) Å, β = 101.5° (1) (42 kbar pressure), following a slightly anisotropic compression pattern (linear compressibilities parallel to unit cell edges: β _a = 1.85, β _b = 1.74, β_c = 3.05 × 10^?4 kbar^?1) with a bulk modulus of 1480 kbar. Perpendicular to c, the most compressible direction, the crystal structure (space group C2/c) consists of two kinds of alternating octahedral layers connected via isolated SiO₄ tetrahedra. With increasing pressure the slightly wavy layer [Mg_1.3Fe_0.7AlO₂(OH)₄] tends to flatten. Furthermore, the octahedra in this layer, with all cations underbonded, are more compressible than the octahedra in the (A1₃O₈) layer with slightly overbonded aluminum. Comparison between high-pressure and high-temperature data yields the following equations: $$\begin{gathered} a_{P,T} = 9.434{\text{ }}{\AA} - 174 \cdot 10^{ - 5} {\text{ }}{\AA}{\text{kb}}^{{\text{ - 1}}} \cdot P \hfill \\ {\text{ }} + 9 \cdot 10^{ - 5} {\text{ }}{\AA}^\circ C^{ - 1} \cdot (T - 25^\circ C) \hfill \\ b_{P,T} = 5.452{\text{ }}{\AA} - 95 \cdot 10^{ - 5} {\text{ }}{\AA}{\text{kb}}^{{\text{ - 1}}} \cdot P \hfill \\ {\text{ }} + 5 \cdot 65 \cdot 10^{ - 5} {\text{ }}{\AA}^\circ C^{ - 1} \cdot (T - 25^\circ C) \hfill \\ c_{P,T} = 18.136{\text{ }}{\AA} - 549 \cdot 10^{ - 5} {\text{ }}{\AA}{\text{kb}}^{{\text{ - 1}}} \cdot P \hfill \\ {\text{ }} + 16 \cdot 2^{ - 5} {\text{ }}{\AA}^\circ C^{ - 1} \cdot (T - 25^\circ C) \hfill \\ \end{gathered} $$ with P in kbar and T in °C. These equations indicate that the unit cell and bond geometry of magnesiochloritoid at formation conditions do not differ greatly from those at the outcrop conditions, e.g. the calculated unitcell volume is 917.3 ų at P = 16 kbar and T=500 °C, whereas the observed volume at room conditions is 914.4 ų. In addition, they show that the specific gravity increases from formation at depth to outcrop at surface conditions.     

Ferrotochilinite, 6FeS · 5Fe(OH)2, a new mineral from the Oktyabr’sky deposit, Noril’sk district, Siberia, Russia

A new mineral, ferrotochilinite, ideally 6FeS · 5Fe(OH)₂, was found at the Oktyabr’sky Mine, Oktyabr’skoe Cu-Ni deposit, Noril’sk, Krasnoyarsk krai, Siberia, Russia. It is associated with ferrovalleriite, magnetite and Fe-rich, chlorite-like phyllosilicate in the cavities of pentlandite-mooihoekite-cubanite ore with subordinate magnetite and chalcopyrite. Ferrotochilinite occurs as flattened on [001], prismatic to elongated lamellar crystals up to 0.1 × 0.5 × 3.2 mm, typically split and curved. Aggregates (up to 6.5 mm in size) are fanlike, rosette-like, or chaotic. Ferrotochilinite is dark bronze. The streak is black. The luster is moderately metallic. The Mohs’ hardness is ca. 1; VHN is 13 kg/mm². Cleavage is {001} perfect, micalike. Individuals are flexible, inelastic. D(calc) = 3.467 g/cm³. In reflected light, ferrotochilinite is gray, with the hue changing from pale beige to bluish; bireflectance is distinct. Anisotropy is distinct, with gray bluish to yellowish beige rotation colors. No internal reflections. Reflectance values [R _min-R _max, % (λ, nm)] are: 11.6–11.4 (470), 11.2–12.4 (546), 11.1–13.6 (589), 11.0–15.5 (650). The IR spectrum shows the presence of (OH) groups bonded with Fe cations and the absence of H₂O molecules. Chemical composition (wt %; electron probe; H content is calculated) is as follows: 0.02 Mg, 61.92 Fe, 0.03 Ni, 0.09 Cu, 19.45 S, 16.3 O, 1.03 H _calc; the total is 98.84. The empirical formula calculated on the basis of 6 S atoms is: Mg_0.01Fe_10.96Ni_0.005Cu_0.015S₆(OH)_10.07 = (Fe_5.98Cu_0.0015Ni_0.005)_Σ6S₆(OH)_9.80(Fe _4.89 ²⁺ Mg_0.01)_Σ4.90(OH)_9.80Fe _0.09 ³⁺ (OH)_0.27. Ferrotochilinite is monoclinic, space group is C2/m, Cm or C2, the unit-cell dimensions are: a = 5.463(5), b = 15.865(17), c = 10.825(12) Å, β = 93.7(1)°, V = 936(3) ų, Z = 2. The strongest reflections in the X-ray powder diffraction pattern (d, Å-I[hkl]) are: 10.83-13[001], 5.392-100[002], 3.281-7[023], 2.777-7[150], 2.696-12[004, $20\bar 1$ ], 2.524-12[ $22\bar 1$ , $20\bar 2$ ], 2.152-8[134, 153], 1.837-11[135, $17\bar 3$ ]. Ferrotochilinite is a structural analog of tochilinite, with Fe²⁺ instead of Mg in the hydroxide part. The type specimen is deposited in Fersman Mineralogical Museum of Russian Academy of Sciences, Moscow.     

High-pressure perovskites on the join CaTiO3-FeTiO3

An exploratory high-pressure study of the join CaTiO₃-FeTiO₃ has uncovered two intermediate perovskites with the compositions CaFe₃Ti₄O₁₂ and CaFeTi₂O₆. These perovskites have ordering of Ca²⁺ and Fe²⁺ on the A sites. Both of these perovskites are unusual in that the A sites containing Fe²⁺ are either square planar or tetrahedral, due to the particular tilt geometries of the octahedral frameworks. For CaFe₃Ti₄O₁₂, the structure has been refined from a powder using the Rietveld technique. This compound is a cubic double perovskite (SG Im $\bar 3$ , a = 7.4672 Å), isostructural with NaMn₇O₁₂. Fe²⁺ is in a square-planar A site (similar to Mn³⁺ in NaMn₇O₁₂) with Fe-O = 2.042(3) Å, with distant second neighbors in a rectangle at Fe-O = 2.780(6) Å. Calcium is in a distorted icosahedron with Ca-O =2.635(5) Å. CaFeTi₂O₆ crystallizes in a unique tetragonal double perovskite structure (SG P4₂/nmc, a = 7.5157(2), c = 7.5548(2)), with A-site iron in square-planar (Fe-O = 2.097(2) Å) and tetrahedral (Fe-O = 2.084(2) Å) coordination, again with distant second neighbor oxygens near 2.8 Å. Rietveld refinement was also performed for the previously known perovskite-related form of FeTiO₃ recovered from high pressure (lithium niobate type). This compound is trigonal R3c, with a = 5.1233(1) and c = 13.7602(2). The ordered perovskites appear to be stable at 1215 GPa and CaFe₃Ti₄O₁₂ is found as low as 5 GPa. Thus these perovskites may be important to upper mantle mineralogy, particularly in kimberlites. These compounds are the first known quenchable perovskites with large amounts of A-site ferrous iron, and add greatly to the known occurrences of ferrous iron in perovskites.     

Lavoisierite, Mn2+ 8[Al10(Mn3+Mg)][Si11P]O44(OH)12, a new mineral from Piedmont, Italy: the link between “ardennite” and sursassite

The new mineral species lavoisierite, ideally Mn²⁺ ₈[Al₁₀(Mn³⁺Mg)][Si₁₁P]O₄₄(OH)₁₂, has been discovered in piemontite-bearing micaschists belonging to the Piedmontese Nappe from Punta Gensane, Viù Valley, Western Alps, Italy. It occurs as yellow-orange acicular to prismatic-tabular crystals up to a few millimeters in length, with white streak and vitreous luster, elongated along [010] and flattened on {001}. Lavoisierite is associated with quartz, “mica,” sursassite, piemontite, spessartine, braunite, and “tourmaline.” Calculated density is 3.576 g cm^?3. In plane-polarized light, it is transparent, pleochroic, with pale yellow parallel to [010] and yellow-orange normal to this direction; extinction is parallel and elongation is positive. Birefringence is moderate; the calculated average refraction index n is 1.750. Lavoisierite is orthorhombic, space group Pnmm, with a 8.6891(10), b 5.7755(3), c 36.9504(20) Å, V 1854.3(2) Å³, Z = 2. Calculated main diffraction lines of the X-ray powder diffraction pattern are [d in Å, (I), (hkl); relative intensities are visually estimated]: 4.62 (m) (112), 2.931 (vs) (1110), 2.765 (s) (1111), 2.598 (s) (310), 2.448 (ms) (028). Chemical analyses by electron microprobe give (in wt%) P₂O₅ 2.08, V₂O₅ 0.37, SiO₂ 34.81, TiO₂ 0.13, Al₂O₃ 22.92, Cr₂O₃ 0.32, Fe₂O₃ 0.86, Mn₂O₃ 6.92, MnO 19.09, MgO 5.73, CaO 1.94, Na₂O 0.01, H₂O 5.44, sum 100.62 wt%. H₂O content was calculated from structure refinement. The empirical formula, based on 56 anions, is (Mn _5.340 ²⁺ Mg_1.810Ca_0.686Na_0.006)_Σ=7.852(Al_8.921Mn _1.739 ³⁺ Mg_1.010Fe _0.214 ³⁺ Cr_0.084Ti_0.032)_Σ=12.000(Si_11.496P_0.582V_0.081)_Σ=12.159O_43.995(OH)_12.005. The crystal structure of lavoisierite was solved by direct methods and refined on the basis of 1743 observed reflections to R ₁ = 4.6 %. The structure is characterized by columns of edge-sharing octahedra running along [010] and linked to each other by means of [SiO₄], [Si₂O₇], and [Si₃O₁₀] groups. Lavoisierite, named after the French chemist and biologist Antoine-Laurent de Lavoisier (1743–1794), displays an unprecedented kind of structure, related to those of “ardennite” and sursassite.