A study on the effects of drying conditions on the stability of NaNd(CO3)2·6H2O and NaEu(CO3)2·6H2O

The effect of different drying conditions on the stability of NaNd(CO₃)·6H₂O and NaEu(CO₃)·6H₂O and the identity of the decomposition product have been investigated. The rate of decomposition and the nature of the altered phases are dependant on the drying conditions used. When the phases are oven dried at 120 °C, the decomposition is immediate and the phase completely alters to Nd₂(CO₃)₃ or Eu₂(CO₃)₃ respectively. Under less severe drying conditions, the Na rare earth carbonate phases alter to Nd₂(CO₃)₃·8H₂O and Eu₂(CO₃)₃·8H₂O over a period of 24–48 h, but they can be kept indefinitely in a water saturated environment. The implications for using Nd and Eu as actinide analogues are discussed.     

Crystal chemistry of perovskite-type compounds in the tausonite-loparite series, (Sr1−2 x Na x La x )TiO3

Perovskite-type compounds in the series tausonite-loparite, (Sr_{1?2 x} Na _x La _x )TiO₃, were synthesized by solid-state reaction (final heating at 1200–1300?°C), and studied using “conventional” and synchrotron X-ray powder diffractometry. The structures of intermediate compositions were determined using the Rietveld profile refinement method. In the compositional range 0?≤x?≤ 0.1, the series comprises perovskites characterized by an undistorted cubic structure (space group Pm3¯m, a?≈ 3.905–3.902?Å, Z?=?1). Intermediate compounds in the range 0.15?≤?x?≤?0.35 crystallize with tetragonal symmetry (I4/mcm, a?≈? , c?≈? , Z?=?4) derived from the cubic aristotype by antiphase rotation of the TiO₆ octahedra about a fourfold axis. The angle of rotation estimated from the positional parameters of oxygen atoms ranges from 2.5(7)° to 5.5(4)°. The cubic-to-tetragonal transition arises from substitution of Sr²⁺ by the comparatively smaller Na¹⁺ and La³⁺ cations. A further transition from the tetragonal to rhombohedral symmetry (R3¯c, a?≈? , c?≈?2 , Z?=?6) occurs between x?=?0.35 and 0.40, and apparently does not involve formation of perovskite with an intermediate two-tilt structure (Imma). The rhombohedral structure is characterized by a multicomponent octahedral tilt about a threefold axis ranging in magnitude from 6.5(2)° to 7.7(2)°. In the series (Sr_{1?2 x} Na _x La _x )TiO₃, the unit-cell dimensions decrease, and the degree of structural distortion increases with x.     

一种铋复硫盐新矿物——锡林郭勒矿Pb-(3+x)Bi-(2-(2/3)x)S-6

该矿物发现于内蒙蒙锡林郭勒盟东乌旗朝不楞矿区的矽卡岩型铁矿床中,为一主要含Pb、Bi、S的铋复硫盐新矿物。它的化学成分与硫铋铅矿相近,但其晶体结构不同。根据产地命名为锡林郭勒矿(xilingolite)。     

一种铋复硫盐新矿物——锡林郭勒矿Pb_(3+x)Bi_(2-(2/3)x)S_6

该矿物发现于内蒙蒙锡林郭勒盟东乌旗朝不楞矿区的矽卡岩型铁矿床中,为一主要含Pb、Bi、S的铋复硫盐新矿物。它的化学成分与硫铋铅矿相近,但其晶体结构不同。根据产地命名为锡林郭勒矿(xilingolite)。     

High-pressure structural studies of Li x La1/3NbO3 (x = 1/6, 1/3, 1/2, 2/3)

The high-pressure behavior of Li _x La_1/3NbO₃ (x = 1/6, 1/3, 1/2, 2/3) perovskites where Li cations were substituted for the existing vacancies was studied using synchrotron X-ray diffraction. It was shown that all these materials undergo irreversible pressure-induced amorphization around 14.5 GPa regardless of the Li concentration. The Li-inserted materials were found to exhibit a standard pressure response (bulk modulus pressure derivative B ₀′ ~4) when in the crystalline phase, whereas La_1/3NbO₃ shows a linear volume contraction versus pressure, i.e., B ₀′ ~(?1). These results suggest that the structural collapse is not a consequence of cation disorder resulting from the Nb atoms (B-site) migrating to the A-site vacancies. The observed pressure response can be understood by increased occupancy of the A-sites opposing the tilting of the NbO₆ octahedra. The pressure evolution of the Nb oxidation state is discussed.     

Kasatkinite, Ba2Ca8B5Si8O32(OH)3 · 6H2O6, a new mineral from the Bazhenovskoe deposit, the Central Urals, Russia

A new mineral, kasatkinite, Ba₂Ca₈B₅Si₈O₃₂(OH)₃ · 6H₂O, has been found at the Bazhenovskoe chrysotile asbestos deposit, the Central Urals, Russia in the cavities in rhodingite as a member of two assemblages: (l) on prehnite, with pectolite, calcite, and clinochlore; and (2) on grossular, with diopside and pectolite. Kasatkinite occurs as spherulites or bunches up to 3 mm in size, occasionally combined into crusts. Its individuals are acicular to hair-like, typically split, with a polygonal cross section, up to 0.5 mm (rarely, to 6 mm) in length and to 20 μm in thickness. They consist of numerous misoriented needle-shaped subindividuals up to several dozen μm long and no more than 1 μm thick. Kasatkinite individuals are transparent and colorless; its aggregates are snow white. The luster is vitreous or silky. No cleavage was observed; the fracture is uneven or splintery for aggregates. Individuals are flexible and elastic. The Mohs’ hardness is 4–4.5. D _meas = 2.95(5), D _calc = 2.89 g/cm³. Kasatkinite is optically biaxial (+), α = 1.600(5), β = 1.603(2), γ = 1.626(2), 2V _meas = 30(20)°, 2V _calc = 40°. The IR spectrum is given. The ¹¹B MAS NMR spectrum shows the presence of BO₄ in the absence of BO₃ groups. The chemical composition of kasatkinite (wt %; electron microprobe, H₂O by gas chromatography) is as follows: 0.23 Na₂O, 0.57 K₂O, 28.94 CaO, 16.79 BaO, 11.57 B₂O₃, 0.28 Al₂O₃, 31.63 SiO₂, 0.05 F, 9.05 H₂O, ?0.02 ?O=F₂; the total is 99.09. The empirical formula (calculated on the basis of O + F = 41 apfu, taking into account the TGA data) is: Na_0.11K_0.18Ba_1.66Ca_7.84B_5.05Al_0.08Si_8.00O_31.80(OH)_3.06F_0.04 · 6.10H₂O. Kasatkinite is monoclinic, space group P2₁/c, P2/c, or Pc; the unit-cell dimensions are a = 5.745(3), b = 7.238(2), c = 20.79 (1) Å, β = 90.82(5)°, V = 864(1) ų, Z = 1. The strongest reflections (d Å–I[hkl]) in the X-ray powder diffractions pattern are: 5.89–24[012], 3.48–2.1[006], 3.36–24[114]; 3.009–100[ $12\bar 1$ , 121, $10\bar 6$ ], 2.925–65[106, $12\bar 2$ , 122], 2.633–33[211, 124], 2.116–29[ $13\bar 3$ , 133, 028]. Kasatkinite is named in honor of A.V. Kasatkin (b. 1970), a Russian amateur mineralogist and mineral collector who has found this mineral. Type specimen is deposited in the Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow.     

The crystal chemistry of lithium in the alluaudite structure: a study of the (Na1− x Li x )1.5Mn1.5Fe1.5(PO4)3 solid solution (x = 0 to 1)

Summary Phosphates of compositions (Na_1–xLi_x)_1.5Mn_1.5Fe_1.5(PO₄)₃ were synthesized by solid state reactions in air, and pure alluaudite-type compounds were obtained for x=0.00, 0.25, and 0.50. Rietveld refinements of X-ray powder diffraction data indicate the occurrence of Mn²⁺ in the M(1) site, and of Fe³⁺ and Mn²⁺ in the M(2) site. For x=0.25 and 0.50, A(1) is occupied by Li⁺ and Na⁺, whereas A(2) is occupied by Na⁺ and vacancies. A careful examination of the number of electrons occurring in the A sites of the alluaudite-type compounds (Na_1–xLi_x)MnFe₂(PO₄)₃ and (Na_1–xLi_x) CdIn₂(PO₄)₃ confirms that lithium occupies only the A(1) crystallographic site of the alluaudite structure.     

High-Temperature Behavior of the CuMo3O10⋅H2O Compound

Geology of Ore Deposits - CuMo3O10⋅H2O crystals have been obtained by hydrothermal synthesis as a result of reaction between (NH4)6Mo2O24⋅4H2O and Cu(CH3COO)2 at 220°C for 7 days....     

High-pressure behaviour of Ca-rich C 2 /c clinopyroxenes along the join diopside-enstatite (CaMgSi2O6-Mg2Si2O6)

 An in situ high-pressure (HP) X-ray diffraction investigation of synthetic diopside and of the Ca_0.8Mg_1.2Si₂O₆ clinopyroxene (Di₈₀En₂₀) was performed up to respectively P=40.8 and 15.1 GPa, using high brilliance synchrotron radiation. The compression of the cell parameters is markedly anisotropic, with β_b ⋙ β_c > β_a > β_asinβ for any pressure range and for both diopside and Di₈₀En₂₀. The compressibility along the crystallographic axes decreases significantly with pressure and is higher in Di₈₀En₂₀ than in diopside. The β cell parameter decreases as well with pressure, at a higher rate in Di₈₀En₂₀. The cell volume decreases at almost the same rate for the two compositions, since in diopside a higher compression along a* occurs. A change in the mechanism of deformation at P higher than about 5–10 GPa is suggested for both compositions from the analysis of the strain induced by compression. In diopside at lower pressures, the deformation mainly occurs, at a similar rate, along the b axis and at a direction 145° from the c axis on the (0 1 0) plane. At higher pressures, instead, the deformation occurs mostly along the b axis. In Di₈₀En₂₀ the orientation of the strain axes is the same as in diopside. The substitution of Ca with Mg in the M2 site induces at a given pressure a higher deformation on (0 1 0) with respect to diopside, but a similar change in the compressional behaviour is found. Changes in the M2 polyhedron with pressure can explain the above compressional behaviour. A third-order Birch-Murnaghan equation of state was fit to the retrieved volumes, with K=105.1(9) GPa, K′=6.8(1) for diopside and K=107.3(1.4) GPa, K′=5.7(3) for Di₈₀En₂₀; the same equation can be applied for any pressure range. The elasticity of diopside is therefore not significantly affected by Mg substitution into the M2 site, in contrast to the significant stiffening occurring for Ca substitution into Mg-rich orthopyroxenes. Received: 3 January 2000 / Accepted: 21 May 2000     

Solid solubility in the system NaLREETi2O6– ThTi2O6 (LREE, light rare-earth elements): experimental and analytical data

The existence of an incomplete solid solution series between loparite (NaLREETi₂O₆), a member of the perovskite mineral group, and thorutite (ThTi₂O₆) is established on the basis of experimental and mineralogical data. The products of low- and high-pressure synthesis in the system NaLaTi₂O₆– ThTi₂O₆ were studied by energy-dispersive spectrometry, X-ray diffractometry and Rietveld analysis. At atmospheric pressure, Th is incorporated in loparite as both ThTi₂O₆ and Na₂ThTi₃O₉. In synthetic systems, the maximum determined ThTi₂O₆ content of loparite is 18 mol%, with a corresponding A-site cation deficiency of 9%. The structure of such loparite is tetragonal and presumably derived from the cubic aristotype by octahedral rotation [I4/mcm, a=5.4652(1) Å, c=7.7476(2) Å]. At a pressure of 6 GPa, no solubility between loparite and ThTi₂O₆ is observed, and Th is accommodated in the loparite structure entirely as Na₂ThTi₃O₉ (up to 30 mol%). Naturally occurring loparite contains up to 29 mol% ThTi₂O₆, based on the conventional method of analysis recalculation, or 23.5 mol% ThTi₂O₆, assuming the presence of protons at the vacant A-sites. ThTi₂O₆ synthesized by the solid-state reaction, crystallizes with monoclinic symmetry [C2/m, a=9.8140(2) Å, b=3.8228(1) Å, c=7.0313(2) Å,β=118.82(1)°]. Atomic coordinates for ThTi₂O₆ obtained in this study from X-ray powder data, as well as structural parameters derived from the new data, are in a good agreement with those known from single-crystal refinement. ThTi₂O₆ does not crystallize at high pressure, and Th is accommodated in perovskite-type compounds and cubic ThO₂ that provide a twelve- and eight-fold coordination site for Th, respectively.     

Günterblassite, (K,Ca)3 − x Fe[(Si,Al)13O25(OH,O)4] · 7H2O, a new mineral: the first phyllosilicate with triple tetrahedral layer

A new mineral, günterblassite, has been found in the basaltic quarry at Mount Rother Kopf near Gerolstein, Rheinland-Pfalz, Germany as a constituent of the late assemblage of nepheline, leucite, augite, phlogopite, åkermanite, magnetite, perovskite, a lamprophyllite-group mineral, götzenite, chabazite-K, chabazite-Ca, phillipsite-K, and calcite. Günterblassite occurs as colorless lamellar crystals up to 0.2 × 1 × 1.5 mm in size and their clusters. The mineral is brittle, with perfect cleavage parallel to (001) and less perfect cleavage parallel to (100) and (010). The Mohs hardness is 4. The calculated and measured density is 2.17 and 2.18(1) g/cm³, respectively. The IR spectrum is given. The new mineral is optically biaxial and positive as follows: α = 1.488(2), β = 1.490(2), γ = 1.493(2), 2V _meas = 80(5)°. The chemical composition (electron microprobe, average of seven point analyses, H₂O is determined by gas chromatography, wt %) is as follows: 0.40 Na₂O, 5.18 K₂O, 0.58 MgO, 3.58 CaO, 4.08 BaO, 3.06 FeO, 13.98 Al₂O₃, 52.94 SiO₂, 15.2 H₂O, and the total is 98.99. The empirical formula is Na_0.15K_1.24Ba_0.30Ca_0.72Mg_0.16F _0.48 ²⁺ [Si_9.91Al_3.09O_25.25(OH)_3.75] · 7.29H₂O. The crystal structure has been determined from a single crystal, R = 0.049. Günterblassite is orthorhombic, space group Pnm2₁; the unit-cell dimensions are a = 6.528(1), b = 6.970(1), c = 37.216(5) Å, V = 1693.3(4) ų, Z = 2. Günterblassite is a member of a new structural type; its structure is based on three-layer block [Si₁₃O₂₅(OH,O)₄]. The strong reflections in the X-ray powder diffraction pattern [d Å (I, %) are as follows: 6.532 (100), 6.263 (67), 3.244 (49), 3.062 (91), 2.996 (66), 2.955 (63), and 2.763 (60). The mineral was named in honor of Günter Blass (born in 1943), a well-known amateur mineralogist and specialist in electron microprobe and X-ray diffraction. The type specimen of günterblassite is deposited in the collections of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia, with the registration number 4107/1.     

Experimental study of the K2ZrSi3O9 (wadeite)–K2TiSi3O9 and K2(Zr,Ti)Si3O9–phlogopite systems at 2–3 GPa

Experiments ranging from 2 to 3 GPa and 800 to 1300 °C and at 0.15 GPa and 770 °C were performed to investigate the stability and mutual solubility of the K₂ZrSi₃O₉ (wadeite) and K₂TiSi₃O₉ cyclosilicates under upper mantle conditions. The K₂ZrSi₃O₉–K₂TiSi₃O₉ join exhibits complete miscibility in the P–T interval investigated. With increasing degree of melting the solid solution becomes progressively enriched in Zr, indicating that K₂ZrSi₃O₉ is the more refractory end member. At 2 GPa, in the more complex K₂ZrSi₃O₉–K₂TiSi₃O₉–K₂Mg₆Al₂Si₆O₂₀(OH)₄ system, the presence of phlogopite clearly limits the extent of solid solution of the cyclosilicate to more Zr-rich compositions [Zr/(Zr + Ti) > 0.85], comparable to wadeite found in nature, with TiO₂ partitioning strongly into the coexisting mica and/or liquid. However, at 1200 °C, with increasing pressure from 2 to 3 GPa, the partitioning behaviour of TiO₂ changes in favour of the cyclosilicate, with Zr/(Zr + Ti) of the K₂(Zr,Ti)Si₃O₉ phase decreasing from ∼0.9 to ∼0.6. The variation in the Ti content of the coexisting phlogopite is related to its degree of melting to forsterite and liquid, following the major substitution ^VITi+^VI□=2^VIMg. Received: 26 January 1999 / Accepted: 10 January 2000     

Vibrational analysis of dioptase Cu6Si6O18 · 6(H2O) and its puckered six-membered ring

The normal modes of vibration and their frequencies are calculated for dioptase, a mineral whose crystal structure (space group R or C _3i ² ) consists of puckered six-membered silicate rings (Si₆O₁₈) linked by Cu²⁺ ions and H₂O groups. The calculation employs a valence force potential consisting of central interactions between nearest neighbors and bond-bending interactions centered at the Si⁴⁺ and Cu²⁺ ions. The force constants are determined by fitting the calculated frequencies to values obtained by measuring the single-crystal Raman spectra. The calculated frequencies are in reasonable agreement with experiment, permitting assignment of normal modes to the observed spectral frequencies. Considerable mixing of Cu and H₂O motions with those of the ring is found for the Raman-active modes below 430 cm^-1. The normal modes and frequencies of the hypothetical isolated ring with C _3i symmetry are determined by neglecting all interactions between the rings and the surrounding Cu and H₂O. The identification of normal modes characteristic of the puckered six-membered silicate rings and the effect of the environment on these modes may prove useful in the interpretation of the Raman spectra of amorphous silicates.     

Copper-substituted manganese-denningites,Mn(Mn1−x Cu x )(Te2O5)2 (0 ≤x ≤ 1): Synthesis and crystal chemistry

Summary The crystal structures of copper-substituted manganese-denningites, Mn(Mn_1–x Cu _x )(Te₂O₅)₂ (0 x 1), were refined in space groupP4₂/nbc from single-crystal X-ray data. Single crystals with different degree of Cu-substitution suitable for X-ray investigation were synthesized under hydrothermal conditions, varying the Cu/Mn ratio and thepH-value. One main feature of the crystal structure is the distribution of Mn and Cu atoms among an eight and a six-coordinated site, respectively. Bond strength calculations support the site occupancies of the MnO₈-polyhedra and MeO₆-octahedra (Me = Cu., Mn_1–x ). The decrease in length of the four Me^[6]-O bonds clearly correlates with the increase of the Cu-substitution resulting in a distortion of the octahedra according to the Jahn-Teller effect of divalent copper. The stronger decrease of the lattice parameterc as compared toa is probably due to the variations of the bond lengths.

---

Kupfer-substituierte Mangan-Denningite, Mn(Mn_1–x Cu _x )(Te₂O₅)₂ (0 x 1): Synthese und Kristallchemie

Zusammenfassung Die Kristallstrukturen von Kupfer-substituierten Mangan-Denningiten, Mn(Mn_–x Cu _x )(Te₂O₅)₂ (0 x 1) wurden mittels Einkristall-Röntgenbeugungsdaten in der RaumgruppeP4₂/nbc verfeinert. Geeignete Einkristalle mit unterschiedlich starker Cu-Substitution wurden unter hydrothermalen Bedingungen durch Variieren des Cu/Mn-Verhältnisses bzw. despH-Wertes dargestellt. Ein wesentlicher struktureller Gesichtspunkt ist die Verteilung der Mn und Cu-Atome auf eine acht- bzw. sechskoordinierte Punktlage. Die Verringerung von vier Me^[6]-O Bindungslängen ist klar korrelierbar mit zunehmender Cu-Substitution, und führt zu einer Verzerrung der Oktaeder gemäß dem für zweiwertiges Kupfer bekannten Jahn-Teller Effekt. Bindungsstärkenberechnungen belegen die Besetzung der MnO₈-Polyeder und MeO₆-Oktaeder (Me = Cu _x Mn_1–x ). Die bevorzugte Verkleinerung der Gitterkonstantec gegenübera kann auf die Variationen der Bindungslängen zurückgeführt werden.

---

---

Dedicated to Prof Dr. Josef Zemann on the occasion of his 70th birthday

---

With 10 Figures     

High-pressure behaviour along the jadeite NaAlSi2O6–aegirine NaFeSi2O6 solid solution up to 10 GPa

The volume variation as a function of pressure along the jadeite–aegirine solid solution was determined at room temperature up to pressures between 6.5 and 9.7 GPa by single-crystal X-ray diffraction. The unit-cell volumes collected at room pressure for the different compositions indicate a slight deviation from linearity along the join. The pressure–volume data have been fitted using a third-order Birch-Murnaghan equation of state (BM3-EoS). The bulk modulus, K _T0, varies from 134.0(7) GPa for pure jadeite to 116.1(5) GPa for pure aegirine. Its evolution with composition along the join is not linear and can be described by the following second order polynomial:

Mössbauer spectroscopic and x-ray powder diffraction studies of synthetic micas on the join annite KFe3AlSi3O10(OH)2-phlogopite KMg3AlSi3O10(OH)2

Micas of the composition K(Fe_3–x Mg _x )AlSi₃ O₁₀(OH)₂ (x=0.6, 1.2, 1.8, 2.4 and 3.0, corresponding to ann₈₀phl₂₀, ann₆₀phl₄₀, ann₄₀phl₆₀, ann₂₀phl₈₀ and ann₀phl₁₀₀) were synthesized hydrothermally under controlled oxygen fugacity conditions. Lattice parameters a ₀ and b ₀ show a distinct linear decrease with increasing Mg content. With increasing ferric iron content a deviation from this linear trend is observed especially within iron rich samples. The tetrahedral rotation angle increases smoothly from 0° in annite to 9.1° in phlogopite. Mössbauer spectra show Fe²⁺ and Fe³⁺ on the octahedral M1 and M2 sites and partially also Fe³⁺ on the tetrahedral site. There is a smooth increase of the quadrupole splitting on both the M1 and the M2 site going from annite to phlogopite, probably due to changes in the lattice contribution to the electric field gradient, assuming a positive correlation between quadrupole splitting and distortion. Fe³⁺ contents, as determined by Mössbauer spectroscopy, versus oxygen fugacity shows that, depending on the composition of the micas, minimum amounts of Fe³⁺ are present. For ann₈₀phl₂₀ this minimum amount of Fe³⁺ is about 8% decreasing to about 1–2% Fe³⁺ for ann₂₀phl₈₀.The molar volume of each solid solution member has been estimated from the determined relations of the molar volume versus % Fe³⁺ contents, extrapolated back to 0% Fe³⁺. Plotting these volumes as a function of X_phl shows that negative excess volume occur in the annitephlogopite join, with the maximum deviation from ideality around X _phl=0.3. Margules volume parameters have been constrained as: Wv, AnnPhl=0.018±0.016 J/(bar.mol) and Wv, PhlAnn=-0.391±0.025 J(bar.mol) (three site basis).     

Solubility of Ca6[Al(OH)6]2(CrO4)3·26H2O,the chromate analog of ettringite; 5–75°C

Ca₆[Al(OH)₆]₂(CrO₄)₃·26H₂O, the chromate analog of the sulfate mineral ettringite, was synthesized and characterized by X-ray diffraction, Fourier transform infra-red spectroscopy, thermogravimetric analyses, energy dispersive X-ray spectrometry, and bulk chemical analyses. The solubility of the synthesized solid was measured in a series of dissolution and precipitation experiments conducted at 5–75°C and at initial pH values between 10.5 and 12.5. The ion activity product (IAP) for the reaction Ca₆[Al(OH)₆]₂(CrO₄)₃·26H₂O⇌6Ca²⁺+2Al(OH)⁻₄+3CrO²⁻₄+4OH⁻+26H₂O varies with pH unless a CaCrO_4(aq) complex is included in the speciation model. The log K for the formation of this complex by the reaction Ca²⁺+CrO²⁻₄=CaCrO_4(aq) was obtained by minimizing the variance in the IAP for Ca₆[Al(OH)₆]₂(CrO₄)₃·26H₂O. There is no significant trend in the formation constant with temperature and the average log K is 2.77±0.16 over the temperature range 5–75°C. The log solubility product (log K_SP) of Ca₆[Al(OH)₆]₂(CrO₄)₃·26H₂O at 25°C is −41.46±0.30. The temperature dependence of the log K_SP is log K_SP=A−B/T+D log(T) where A=498.94±48.99, B=27,499±2257, and D=−181.11±16.74. The values of ΔG⁰_r,298 and ΔH⁰_r,298 for the dissolution reaction are 236.6±3.9 and 77.5±2.4 kJ mol⁻¹. the values of ΔC⁰_P,r,298 and ΔS⁰_r,298 are −1506±140 and −534±83 J mol⁻¹ K⁻¹. Using these values and published standard state partial molal quantities for constituent ions, ΔG⁰_f,298=−15,131±19 kJ mol⁻¹, ΔH⁰_f,298=−17,330±8.6 kJ mol⁻¹, ΔS⁰₂₉₈=2.19±0.10 kJ mol⁻¹ K⁻¹, and ΔC⁰_Pf,298=2.12±0.53 kJ mol⁻¹ K⁻¹, were calculated.     

The crystal chemistry of synthetic potassium-bearing neighborite, (Na1− x K x )MgF3

A series of fluoride perovskites related to neighborite was investigated using X-ray and neutron diffraction techniques, and Rietveld profile refinement of powder diffraction data. The series (Na_{1? x} K _x )MgF₃ comprises orthorhombic (Pbnm, a?≈? , b?≈? , c?≈?2a _p , Z=4) perovskites in the compositional range 0?≤?x?≤?0.30, tetragonal perovkites (P4/mbm, a?≈? , c?≈?a _p , Z=2) in the range 0.40?≤?x?≤?0.46, and cubic phases (Pm3¯m, Z=1) for x?>?0.50. The structure of the orthorhombic neighborite is derived from the perovskite aristotype by rotation of MgF₆ octahedra about the [110] and [001] axes of the cubic subcell. The degree of rotation, measured as a composite tilt Φ about the triad axis, varies from 18.2° at x=0 to 11.2° at x=0.30 (as determined from the fractional atomic coordinates). Orthorhombic neighborite also shows a significant displacement of Na and K from the “ideal” position (≤0.25?Å). The tetragonal members of the neighborite series exhibit only in-phase tilting about the [001] axis of the cubic subcell (φ) ranging from 4.5° to 4.8° (determined from the atomic coordinates). The solid solution (Na_{1? x} K _x )MgF₃, shows a regular variation of unit-cell dimensions with composition from 3.8347?Å for the end-member NaMgF₃ (reduced to pseudocubic subcell, a _p ) to 3.9897?Å for KMgF₃. This variation is accompanied by increasing volumes of the A-site polyhedra, whereas the volume of MgF₆ octahedra initially decreases (up to x=0.40), and then increases concomitantly with K content. The polyhedral volume ratio, V _A /V _B , gradually increases towards the tetragonal structural range, in agreement with diminishing octahedral rotation in the structure. The P4/mbm-type neighborite has an “anomalous” polyhedral volume ratio (ca. 5.04) owing to the critical compression of MgF₆ polyhedra.     

Lahnsteinite, Zn4(SO4)(OH)6 · 3H2O, a new mineral from the Friedrichssegen Mine, Germany

A new mineral, lahnsteinite, has been found in the dump of the Friedrichssegen Mine, Bad Ems district, Rhineland-Palatinate (Rheinland-Pfalz), Germany. Lahnsteinite, occurring as colorless tabular crystals in the cavities of goethite, is associated with pyromorphite, hydrozincite, quartz, and native copper. The Mohs’ hardness is 1.5; the cleavage is perfect parallel to (001). D _calc = 2.995 g/cm³, D _meas = 2.98(2) g/cm³. The IR spectrum is given. The new mineral is optically biaxial, negative, α = 1.568(2), β = 1.612(2), γ = 1.613(2), 2V _meas = 18(3)°, 2V _calc = 17°. The chemical composition (wt %, electron microprobe data; H₂O was determined by gas chromatography of ignition products) is as follows: 3.87 FeO, 1.68 CuO, 57.85 ZnO, 15.83 SO₃, 22.3 H₂O, total is 101.53. The empirical formula is (Zn_3.3Fe_0.27Cu_0.11)_Σ3.91(S_0.98O₄)(OH)₅ · 3H_2.10O. The crystal structure has been studied on a single crystal. Lahnsteinite is triclinic, space group P1, a = 8.3125(6), b = 14.545(1), c = 18.504(2) Å, α = 89.71(1), β = 90.05(1), γ = 90.13(1)°, V = 2237.2(3) ų, Z = 8. The strong reflections in the X-ray powder diffraction pattern [d, Å (I, %)] are: 9.30 (100), 4.175 (18), 3.476 (19), 3.290 (19), 2.723 (57), 2.624 (36), 2.503 (35), 1.574 (23). The mineral has been named after its type locality near the town of Lahnstein. The type specimen of lahnsteinite is deposited in the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, registration number 4252/1.     

An optical transform and Monte Carlo study of the diffuse X-ray scattering in mullite,Al2(Al2+2xSi2−2x)O10−x

Diffuse X-ray scattering distributions have been recorded for reciprocal layers normal to c ^* of a synthetic mullite with composition Al₂(Al_2+2xSi_2–2x) O_10–xx, where represents an oxygen vacancy and x is close to 0.4. The effect on the diffuse scattering pattern of different schemes for ordering of the oxygen vacancies and the accompanying cation shifts within a single ab-layer of mullite has been investigated using optical diffraction analogue experiments in conjunction with Monte Carlo simulation.A simple scheme in which the only driving force for the ordering is that the O_c oxygens are precluded from being bonded to four T/T^* cations, results in a 2D diffuse intensity distribution which displays many of the features that are observed in the x-ray scattering patterns. An alternative scheme in which bonding to four T/T^* cations was favoured, resulted in a domain structure consisting of islands of the -alumina structure within a matrix of sillimanite. This gave diffraction patterns bearing no resemblance to the observed x-ray patterns and can, we believe, be discounted completely.