The origin of space group violations in a lunar orthopyroxene

The space group of an orthopyroxene (En₈₆) from a deep crustal lunar rock (sample 76535) that was previously reported as having space group P2₁ ca has been re-examined on an automated X-ray diffractometer. In addition to diffractions violating the b-glide of the conventional space group, Pbca (0kl,k-odd) reported in the earlier study, diffractions violating the a-glide of Pbca are also present. Careful examination of both the a-glide- and b-glide-violations shows them to be sharp, with no evidence of diffuse streaks parallel to a ^*, and with consistent intensities at several rotations about ψ. Diffractions violating the b-glide are in registry with the host, however, those violating the a-glide appear to be out of registry and result from a cell with a slightly longer a of about 18.4 Å, consistent with previous electron diffraction studies. The most reasonable explanation for the observed space group violations is that both the a- and b-glide violations result from ordering of Ca into (100) Guinier-Preston (G-P) zones that possess orthopyroxene topology, but have space group P2₁/c and a cell of a=18.4 Å, b=8.83 Å, c=5.18 Å, and β=90.0°; whereas the Cadepleted host has space group Pbca and a cell of a= 18.230(6) Å, b = 8.828(2) Å, and c=5.1946(9) Å. In addition to the G-P zones which may compose 12% or more of the sample, the crystal contains (100) lamellae of pigeonite, and other samples from the same rock contain lamellae of augite.     

Petrology and P–T path of the Fuping mafic granulites: implications for tectonic evolution of the central zone of the North China craton

The Fuping Complex and the adjoining Wutai and Hengshan Complexes are located in the central zone of the North China craton. The dominant rock types in the Fuping Complex are high‐grade tonalitic–trondhjemitic–granodioritic (TTG) gneisses, with minor amounts of mafic granulites, syntectonic granitic rocks and supracrustal rocks. The petrological evidence from the mafic granulites indicates three stages of metamorphic evolution. The M₁ stage is represented by garnet porphyroblasts and matrix plagioclase, quartz, orthopyroxene, clinopyroxene and hornblende. Orthopyroxene+plagioclase symplectites and clinopyroxene+plagioclase±orthopyroxene coronas formed in response to decompression during M₂ following the peak metamorphism at M₁. Hornblende+plagioclase symplectites formed as a result of further isobaric cooling and retrograde metamorphism during M₃. The P–T estimates using TWQ thermobarometry are: 900–950 °C and 8.0–8.5 kbar for the peak assemblage (M₁), based on the core compositions of garnet, matrix pyroxene and plagioclase; 700–800 °C and 6.0–7.0 kbar for the pyroxene+plagioclase symplectites or coronas (M₂); and 550–650 °C and 5.3–6.3 kbar for the hornblende+plagioclase symplectites (M₃), based on garnet rim and corresponding symplectic mineral compositions. These P–T estimates define a clockwise P–T path involving near‐isothermal decompression for the Fuping Complex, similar to the P–T path estimated for the metapelitic gneisses. The inferred P–T path suggests that the Fuping Complex underwent initial crustal thickening, subsequent exhumation, and finally cooling and retrogression. This tectonothermal path is similar to P–T paths inferred for the Wutai and Hengshan Complexes and other tectonic units in the central zone of the North China craton, but different from anti‐clockwise P–T paths estimated for the basement rocks in the eastern and western zones of the craton. Based on lithological, structural, metamorphic and geochronological data, the eastern and western zones of the craton are considered to represent two different Archean to Paleoproterozoic continental blocks that amalgamated along the central zone at the end of Paleoproterozoic. The P–T paths of the Fuping Complex and other tectonic units in the central zone record the collision between the eastern and western zones that led to the final assembly of the North China craton at c. 1800 Ma.     

Observation of reactions in synthetic Ca-poor pyroxene single crystals at elevated temperatures by X-ray diffraction

Diffuse streaks in diffraction patterns of synthetic pyroxene single crystals at elevated temperatures are used to determine which reactions are initiated and how they proceed. The samples investigated are a) a host orthopyroxene (Wo₄En₈₃Fs₁₃) containing oriented pigeonite (Wo₆En₇₈Fs₁₆) parallel to (100) and b) a pigeonite (Wo₈En₇₅Fs₁₇). The maximum temperatures were 820° C and 1,015° C, respectively. No partial melting occurs at these temperatures, all reactions are in the subsolidus. In case a) augite is formed parallel to the (001) plane of pigeonite, but the augite is not exsolved by the pigeonite. This is proved by the absence of the obligatory streaks between corresponding reflections in highly resolved precession photographs. Instead, there are streaks from augite to the corresponding reflections of the host orthopyroxene. Example b) demonstrates that the temperature of the high-low transformation of pigeonite is very sensitive to the Ca content and clearly depends on the exsolution of augite. This augite is oriented parallel to (100) of pigeonite, not to (001). Both the high and the low pigeonite are present over a range of ～150° C, while the exsolution of augite continues. Simultaneously, orthopyroxene is also formed sharing (100) of pigeonite. There seems to be an indication that only low pigeonite inverts to orthopyroxene.     

The influence of multiple diffraction on the space group determination of orthopyroxene,spodumene, low omphacite and pigeonite

Forbidden reflections of some pyroxenes described as having a lower symmetry than Pbca or C2/c have been examined, mainly using the detailed ψ-scanning method of the four-circle automated diffractometer. All reflections violating the systematic absences of Pbca orthopyroxene, C2/c spodumene, P2/n omphacite and P2₁/c pigeonite were found to be due to the Umweganregung process of multiple diffraction; the Umweganregung peaks observed for the ψ azimuth were indexed in terms of a four-circle geometry. Thus, the space groups of orthopyroxene, α spodumene, low omphacite and low pigeonite were confirmed to be Pbca, C2/c, P2/n and P2₁/c, respectively.     

Structure of Cu-bearing orthopyroxene, Mg(Cu.56,Mg.44)Si2O6, and behavior of Cu2+ in the orthopyroxene structure

Cu-bearing pyroxene, Mg(Cu.₅₆,Mg.₄₄)Si₂O₆, has been synthesized by a flux method and crystal structure refinement has been performed by single crystal X-ray diffraction. It is found that the crystal structure is orthorhombic (space group Pbca) with unit cell dimensions of a=18.221(4), b=8.890(1), c=5.2260(7)Å and the cell volume of 846.5( )3ų. In the M2-site one of the M-O bonds(M-O3B) is extremely expanded from 2.444(2) in enstatite to 2.732(2), thus the coordination polyhedron around M2-site is regarded as square pyramidal rather than square planar or octahedral. It is also found that the M1-site in the pyroxene structure is occupied almost exclusively by Mg, while the M2-site is almost evenly occupied by Mg and Cu. The observed extreme site preference shown by Cu²⁺ is unusual among the divalent cations with similar ionic sizes.     

Pyroxenes in the Shaw (L-7) chondrite

The Shaw L-group chondrite differs from orthodox type 6 ordinary chondrites in ways which suggest that it experienced unusually high metamorphic temperatures and anatexis. Electron microprobe and single crystal X-ray diffraction data indicate that Shaw contains three pyroxenes: the augite (Fs_11.3Wo_38.2) and calcic orthopyroxene (Fs_19.4WO_4·5) reported by other workers and a second, Ca- and Al-poor orthopyroxene (Fs_16·8Wo_1·2) which we interpret as inverted protobronzite. Comparison of the Shaw assemblage with experimental data suggests that a two-phase (augite-protobronzite) assemblage developed at peak metamorphic temperatures of ~1250–1300°C, that partial reaction of augite and protobronzite produced calcic orthopyroxene and by-product spinel at temperatures approximately 150°C lower and that protobronzite inverted to bronzite free of stacking disorder during subsequent slow cooling. The intracrystalline distribution of Fe and Mg in the Ca-poor bronzite (K_E + 0·07; determined by crystal structure analysis) indicates an equilibration temperature of ~500°C.Shaw differs sufficiently in texture and mineralogy from type 6 ordinary chondrites to justify its assignment to a separate petrologic type: L-7.     

Structural and compositional characterization of synthetic (Ca,Sr)-tremolite and (Ca,Sr)-diopside solid solutions

Tremolite (Ca_xSr_1–x)₂Mg₅[Si₈O₂₂/(OH)₂] and diopside (Ca_xSr_1–x)Mg[Si₂O₆] solid solutions have been synthesized hydrothermally in equilibrium with a 1 molar (Ca,Sr)Cl₂ aqueous solution at 750°C and 200 MPa. The solid run products have been investigated by optical, electron scanning and high resolution transmission electron microscopy, electron microprobe, X-ray-powder diffraction and Fourier-transform infrared spectroscopy. The synthesized (Ca,Sr)-tremolites are up to 2000 µm long and 30 µm wide, the (Ca,Sr)-diopsides are up to 150 µm long and 20 µm wide. In most runs the tremolites and diopsides are well ordered and chain multiplicity faults are rare. Nearly pure Sr-tremolite (tr_0.02Sr-tr_0.98) and Sr-diopside (di_0.01Sr-di_0.99) have been synthesized. A continuous solid solution series, i.e. complete substitution of Sr²⁺ for Ca²⁺ on M₄-sites exists for (Ca,Sr)-tremolite. Total substitution of Sr²⁺ for Ca²⁺ on M₂-sites can be assumed for (Ca,Sr)-diopsides. For (Ca,Sr)-tremolites the lattice parameters a, b and β are linear functions of composition and increase with Sr-content whereas c is constant. For the diopside series all 4 lattice parameters are a linear function of composition; a, b, c increase and β decreases with rising Sr-content. The unit cell volume for tremolite increases 3.47% from 906.68 ų for tremolite to 938.21 ų for Sr-tremolite. For diopside the unit cell volume increases 4.87 % from 439.91 ų for diopside to 461.30 ų for Sr-diopside. The observed splitting of the OH stretching band in tremolite is caused by different configurations of the next nearest neighbors (multi mode behavior). Resolved single bands can be attributed to the following configurations on the M₄-sites: SrSr, SrCa, CaCa and CaMg. The peak positions of these 4 absorption bands are a linear function of composition. They are shifted to lower wavenumbers with increasing Sr-content. No absorption band due to the SrMg configuration on the M₄-site is observed. This indicates a very low or negligible cummingtonite component in Sr-rich tremolites, which is also supported by electron microprobe analysis.     

Thermochemistry of (Fe2+, Mg)SiO3 orthopyroxene

Enthalpies of solution in eutectic (Li, Na)₂B₂O₄ melts at 1023 K were measured for five synthetic orthopyroxenes on the join MgSiO₃-FeSiO₃. The pyroxenes were synthesized at 1120°C and 20 kbar and thus were presumed to be highly disordered. The measurements indicate a small positive enthalpy of mixing, with W_H = 950 cal/MSiO₃.Enthalpy of solution measurements were made on a natural, well-ordered orthopyroxene near the composition En_52.5Fs_47.5 and on this material after heat-treatment at 1150°C and 20 kbar. Irreversible expansion of the unit-cell constants of the natural pyroxene after heat-treatment at various temperatures was used to characterize the degree of M-site disorder. The observed enthalpy of solution decrement of 0.85 kcal/MSiO₃ between the natural En_52.5 and the same material heated at 1150° corresponds to about half of the maximum possible disordering, or ΔX_Fe^M1? 0.25, which leads to a ΔH of 7.5 kcal/M₂Si₂O₆, for the exchange reaction: Fe(M2) + Mg(Ml) = Fe(Ml) + Mg(M2) if M-site interaction energy terms are ignored. This ΔH is larger than inferred from any of the analyses of site-occupancy data except that of Besancon (1981), who gave a very similar value. The measured ΔH of disorder and the W_H of mixing together indicate a large ΔH as great as 3.2 kcal for the reciprocal reaction: Fe₂Si₂O₆ + Mg₂Si₂O₆ = Fe(M2)Mg(M1)Si₂O₆ + Fe(M1)Mg(M2)Si₂O₆ as anticipated by Sack (1980).As a consequence of the inferred magnitudes of ΔHof the exchange and reciprocal reactions, departures from ideality of Gibbs energy of mixing of orthopyroxene are very small at 700°–1000°C. Activities of MgSiO₃ and FeSiO₃ may be replaced by their mol fractions at all temperatures in most petrologic calculations.     

The crystal chemistry of lithium and Fe3+ in synthetic orthopyroxene

Lithian ferrian enstatite with Li₂O = 1.39 wt% and Fe₂O₃ 7.54 wt% was synthesised in the (MgO–Li₂O–FeO–SiO₂–H₂O) system at P = 0.3 GPa, T = 1,000°C, fO₂ = +2 Pbca, and a = 18.2113(7), b = 8.8172(3), c = 5.2050(2) Å, V = 835.79(9) Å³. The composition of the orthopyroxene was determined combining EMP, LA-ICP-MS and single-crystal XRD analysis, yielding the unit formula ^M2(Mg_0.59Fe _0.21 ²⁺ Li_0.20) ^M1(Mg_0.74Fe _0.20 ³⁺ Fe _0.06 ²⁺ ) Si₂O₆. Structure refinements done on crystals obtained from synthesis runs with variable Mg-content show that the orthopyroxene is virtually constant in composition and hence in structure, whereas coexisting clinopyroxenes occurring both as individual grains or thin rims around the orthopyroxene crystals have variable amounts of Li, Fe³⁺ and Mg contents. Structure refinement shows that Li is ordered at the M2 site and Fe³⁺ is ordered at the M1 site of the orthopyroxene, whereas Mg (and Fe²⁺) distributes over both octahedral sites. The main geometrical variations observed for Li-rich samples are actually due to the presence of Fe³⁺, which affects significantly the geometry of the M1 site; changes in the geometry of the M2 site due to the lower coordination of Li are likely to affect both the degree and the kinetics of the non-convergent Fe²⁺-Mg ordering process in octahedral sites.     

Non-convergent ordering and displacive phase transition in pigeonite: in situ HT XRD study

A natural Ca-rich pigeonite (En₄₇Fs₄₃Wo₁₀), free of augite exsolution products, was studied by in situ high-temperature single-crystal X-ray diffraction. The sample, monoclinic P2 ₁ /c (a=9.719(7) Å, b=8.947(9) Å, c=5.251(3) Å, β=108.49(5), V=433.0(6) ų), was annealed up to 1000 °C to induce a phase transition from P2 ₁ /c to C2/c symmetry. Complete single-crystal X-ray diffraction data collections were carried out in situ at 650, 750, 850 and 950 °C after the crystal had reached equilibrium for the Fe–Mg intracrystalline exchange reaction at each temperature. The variation, with increasing temperature, of lattice parameters, of intensity of hkl reflections with h + k=2n + 1 (which vanish at high temperature) and of some geometrical parameters from structure refinement, showed that the displacive phase transition P2 ₁ /c?C2/c was continuous in character. This contrasts with the first-order character for the HT phase transition in pigeonite containing significantly less calcium.     

Mineralogy and petrology of the diabasic rocks in a differentiated olivine diabase sill complex,Sierra Ancha,Arizona

The Precambrian Sierra Ancha sill complex, more than 700 feet thick, is a multiple intrusion with a central layer of feldspathic olivine-rich diabase, and upper and lower layers of olivine diabase derived from a high-alumina basalt magma. Minor rock types include albite diabase and albite-diabase pegmatite. Deuteric alteration was extensive. Principal primary minerals are plagioclase (An₇₂ to An₁₆), augite (Wo₄₃En₄₄Fs₁₃ to Wo₄₀En₃₈Fs₂₂), olivine (Fo₇₄ to Fo₅₄), orthopyroxene (En₇₇ to En₄₄), magnetite (Mgt₆₆Usp₃₄ to Mgt₈₉Usp₁₁), and ilmenite (Ilm₈₆Hem₁₄ to Ilm₉₆Hem₄). Ilmenite formed by reaction-exsolution from magnetite_ss is consistently different in compositon from primary ilmenite. Primary ilmenite became enriched in Mn and depleted in Mg as crystallization proceded. A systematic Fe-Mg partition between contacting olivine and orthopyroxene suggests that equilibrium prevailed on an extremely local scale during crystallization. Albite-diabase pegmatite contains a mineral assemblage including augite, ferrosalite (Wo₄₉En₂₈Fs₂₃ to Wo₄₉En₁₄Fs₃₇), albite (An₂ to An₀), and iron-rich chlorite. Altered diabase and albite diabase also have unusually calcium-rich pyroxenes. The calcium-rich pyroxenes, which occur in assemblages like those characterizing some spilites, are richer in calcium and lower in aluminum and titanium than basaltic augite.Contribution No. 1712 of the Division of Geological Sciences, California Institute of Technology, Pasadena, California.     

Northwest Africa 773: lunar mare breccia with a shallow-formed olivine-cumulate component, inferred very-low-Ti (VLT) heritage, and a KREEP connection

Lunar meteorite Northwest Africa 773 (herein referred to as NWA773) is a breccia composed predominantly of mafic volcanic components, including a prominent igneous clast lithology. The clast lithology is an olivine-gabbro cumulate, which, on the basis of mineral and bulk compositions, is a hypabyssal igneous rock related compositionally to volcanic components in the meteorite. The olivine-gabbro lithology exhibits cumulus textures and, in our largest section of it, includes some 48% olivine (Fo₆₄ to Fo₇₀, average Fo₆₇), 27% pigeonite (En₆₀Fs₂₄Wo₁₆ to En₆₇Fs₂₇Wo₆), 11% augite (En₅₀Fs₁₇Wo₃₃ to En₄₇Fs₁₃Wo₄₀), 2% orthopyroxene (En₇₀Fs₂₆Wo₄), 11% plagioclase (An₈₀ to An₉₄), and trace barian K-feldspar, ilmenite, Cr-spinel, RE-merrillite, troilite, and Fe-Ni metal. The Mg/Fe ratios of the mafic silicates indicate equilibration of Fe and Mg; however, the silicates retain compositional variations in minor and trace elements that are consistent with intercumulus crystallization. Accessory mineralogy reflects crystallization of late-stage residual melt. Both lithologies (breccia and olivine cumulate) of the meteorite have very-low-Ti (VLT) major-element compositions, but with an unusual trace-element signature compared to most lunar VLT volcanic compositions, i.e., relative enrichment in light REE and large-ion-lithophile elements, and greater depletion in Eu than almost all other known lunar volcanic rocks. The calculated composition of the melt that was in equilibrium with pyroxene and plagioclase of the cumulate lithology exhibits a KREEP-like REE pattern, but at lower concentrations. Melt of a composition calculated to have been in equilibrium with the cumulate assemblage, plus excess olivine, yields a major-element composition that is similar to known green volcanic glasses. One volcanic glass type from Apollo 14 in particular, green glass B, type 1, has a very low Ti concentration and REE characteristics, including extremely low Eu concentration, that make it a candidate parent melt for the olivine-gabbro cumulate. We infer an origin for the parent melt of NWA773 volcanic components by assimilation of a trace-element-rich partial or residual melt by a magnesian, VLT magma deep in the lunar crust or in the mantle prior to transportation to the near-surface, accumulation of olivine and pyroxene in a shallow chamber, eruption onto a volcanic surface, and incorporation of components into local, predominantly volcanic regolith, prior to impact mixing of the volcanic terrain and related hypabyssal setting, and ejection from the surface of the Moon. Volcanic components such as these probably occur in the Oceanus Procellarum region near the site of origin of the green volcanic glasses found in the Apollo 14 regolith.     

X-ray and HRTEM study of sursassite: Crystal structure,stacking disorder,and sursassite-pumpellyite intergrowth

Sursassite is monoclinic, space group P2 ₁/m, a=8.70, b=5.79, c=9.78 Å, β=108.9°. The crystal structure was determined with X-rays and refined to R=0.065, obtaining Mn₂Al₃[(OH)₃(SiO₄)(Si₂O₇)] as ideal crystal chemical formula. Sursassite, isostructural with macfallite Ca₂Mn₃[(OH)₃(SiO₄)(Si₂O₇)], is closely related to pumpellyite Ca₂Al₃[(OH)₃(SiO₄)(Si₂O₇)]. In fact both sursassite and pumpellyite, apart from the different chemical composition, are built up by common structural layers, which are repeated by different stacking vectors. As a result, faulted stacking sequences are energetically possible. Examination by high resolution transmission electron microscopy (HRTEM) shows that frequent (001) pumpellyite-like lamellae are intergrown with thicker (001) sursassite lamellae. Usually, the guest lamellae are a few unit cells thick along [001] and continuous along the (001) plane, although also rare interrupted lamellae are found.     

Synthesis,crystallographic data,solubility and electrokinetic properties of meta-zeunerite,meta-kirchheimerite and nickel-uranylarsenate

{M[UO₂¦AsO₄]₂ · nH₂O} with M=Cu²⁺, Co²⁺, Ni²⁺ has been synthesized from reagent grade chemicals and by ion exchange of trögerite {HUO₂AsO₄ · 4 H₂O}. Synthetic meta-zeunerite (M=Cu²⁺), meta-kirchheimerite (M=Co²⁺) and nickel-uranylarsenate are all tetragonal. The cell parameters determined from Guinier-Hägg diffraction data for {Cu[UO₂¦AsO₄]₂ · 8 H₂O} are a=b=7.10 Å and c=17.42 Å, with Z=2 and the measured density 3.70 g cm^?3. The cell parameters for {Co[UO₂¦AsO₄]₂ · 7 H₂O} and {Ni[UO²¦AsO⁴]² · 7 H₂O} are a=b=20.25 Å and c=17.20 Å, with Z=16 and the measured density 3.82 and 3.74 g cm^?3, respectively. The solubility products for synthetic Cu-, Co- and Ni-uranylarsenate at 25° C are 10^?49.20, 10^?45.34 and 10^?45.10, respectively. The zeta-potential remains negative between pH=2 and pH=9 and is strongly affected by the presence of different cations.     

Clinopyroxenes from mantle-related xenocrysts in alkaline basalts from Hannuoba (China): augite–pigeonite exsolutions and their thermal significance

Optically homogeneous augite xenocrysts, closely associated with spinel–peridotite nodules, occur in alkali basalts from Hannuoba (Hebei province, China). They were studied by electron and X-ray diffraction to define the occurrence and significance of pigeonite exsolution microtextures. Sub-calcic augite (Wo₃₄) exsolved into En_62–62Fs_25–21Wo_13–17 pigeonite and En_46–45Fs_14–14Wo_40–42 augite, as revealed by TEM through diffuse coarser (001) lamellae (100–300 Å) and only incipient (100) thinner ones (<70 Å). C2/c augite and P2₁/c pigeonite lattices, measured by CCD-XRD, relate through a(Aug)?a(Pgt), b(Aug)?b(Pgt), c(Aug)≠c(Pgt) [5.278(1) vs 5.189(1)Å] and β(Aug)≠β(Pgt) [106.55(1) vs 108.55(2)°]. Cell and site volumes strongly support the hypothesis that the augite xenocrysts crystallised at mantle depth from alkaline melts. After the augite xenocrysts entered the magma, (001) lamellae first formed by spinodal decomposition at a T_min of about 1,100 °C, and coarsened during very rapid transport to the surface; in a later phase, possibly on cooling, incipient (100) lamellae then formed.     

Thermal equation of state of synthetic orthoferrosilite at lunar pressures and temperatures

Iron-rich orthopyroxene plays an important role in models of the thermal and magmatic evolution of the Moon, but its density at high pressure and high temperature is not well-constrained. We present in situ measurements of the unit-cell volume of a synthetic polycrystalline end-member orthoferrosilite (FeSiO₃, fs) at simultaneous high pressures (3.4–4.8 GPa) and high temperatures (1,148–1,448 K), to improve constraints on the density of orthopyroxene in the lunar interior. Unit-cell volumes were determined through in situ energy-dispersive synchrotron X-ray diffraction in a multi-anvil press, using MgO as a pressure marker. Our volume data were fitted to a high-temperature Birch–Murnaghan equation of state (EoS). Experimental data are reproduced accurately, with a  $\varDelta P$ Δ P  standard deviation of 0.20 GPa. The resulting thermoelastic parameters of fs are: V ₀ = 875.8 ± 1.4 Å³, K ₀ = 74.4 ± 5.3 GPa, and $\frac{{\text d}K}{{\text d}T} = -0.032 \pm 0.005\,\hbox{GPa K}^{-1}$ d K d T = - 0.032 ± 0.005 GPa K - 1 , assuming ${K}^{\prime}_{0} = 10 $ K 0 ′ = 10 . We also determined the thermal equation of state of a natural Fe-rich orthopyroxene from Hidra (Norway) to assess the effect of magnesium on the EoS of iron-rich orthopyroxene. Comparison between our two data sets and literature studies shows good agreement for room-temperature, room-pressure unit-cell volumes. Preliminary thermodynamic analyses of orthoferrosilite, FeSiO₃, and orthopyroxene solid solutions, (Mg_1?x Fe _x ) SiO₃, using vibrational models show that our volume measurements in pressure–temperature space are consistent with previous heat capacity and one-bar volume–temperature measurements. The isothermal bulk modulus at ambient conditions derived from our measurements is smaller than values presented in the literature. This new simultaneous high-pressure, high-temperature data are specifically useful for calculations of the orthopyroxene density in the Moon.     

Mazzite,a new mineral,the natural counterpart of the synthetic zeolite Ω

Mont Semiol (also called Mont Semiouse), Montbrison, Loire, France, is the only place where the zeolite offrettite is known to occur. Now, a new mineral, Mazzite, has been found in the same locality. It occurs in the form of tiny needles, and its properties closely match those of the synthetic “molecular sieve” Ω. The chemical formula is Na_0.03K_1.91Ca_1.35Mg_1.99[Al_9.77Si_26.54O₇₂]·28.03 H₂O. Lattice dimensions are a=18.392 Å, c=7.646 Å, with P 6₃/mmc as most probable space group; D_calc.=2.108. Offrettite and mazzite occur in association with phillipsite, chabazite, calcite and siderite. The mineral has been so named in honour of Prof. Fiorenzo Mazzi.     

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.     

Structural systematics of Ge substitution in primitive pyroxenes

Pyroxenes of general stoichiometry Mg(Ge _x Si_1?x )O₃ were encountered in attempts to synthesise Ge-substituted talcs at 0.2 GPa, 650–700 °C. Orthopyroxenes (Pbca) of compositions x = 0.21, 0.30, and 0.34 were identified, and also a P2₁/c clinopyroxene of composition x = 0.63, and C2/c clinopyroxenes of compositions x = 0.91 and 1. End-member clinoenstatite MgSiO₃-P2₁/c synthesised at 16 GPa, 1300 °C and transformed from C2/c was also included in the study. Crystal structure refinements using single-crystal XRD data showed that unit-cell parameters vary linearly with Si–Ge for the Pbca and P2₁/c pyroxenes, both of which have two symmetrically non-equivalent tetrahedral chains. Refinement of Si–Ge occupancies at tetrahedral sites showed that the two chains of all primitive pyroxenes have very different compositions, with X_Ge(TB) ? X_Ge(TA). This difference arises from the greater flexibility of the B-chain to rotate in response to tetrahedral expansion due to increasing Ge content. The TA-M2 shared polyhedral edge imposes significant constraints on the flexibility of the A-chain, which can accommodate much less Ge than the B-chain. Linear trends of cell parameters, site occupancies, and structural parameters for the primitive pyroxenes, when extrapolated to published data for MgGeO₃–Pbca, extend across the entire Si–Ge join.