Volume 97, Issue 4

Crystal structures of the three polytypes of veatchite, Sr 2 B 11 O 16 (OH) 5 ·H 2 O, are determined by X-ray, single-crystal studies. The polytypes are: veatchite-1A, triclinic space group P1̄, with a = 6.6378(6), b = 6.7387(6), c = 20.982(2) Å, α = 87.860(1), β = 82.696(12), γ = 60.476(1)°, V = 809.7(2) Å 3 ; veatchite-1M, monoclinic space group P2 1 , with a = 6.7127(4), b = 20.704(1), c = 6.6272(4) Å, β = 119.209(1)°, V = 805.4(2) Å 3 ; and veatchite-2M,monoclinic space group Cc with a = 6.6070(3), b = 11.7125(5), c = 20.6848(9) Å, β = 91.998(1)°, V = 1599.7(2) Å 3 . The crystal structures have two layer types with similar fundamental building blocks: A layer FBB = 3Δ2□:<Δ2□>-<2Δ□> and B layer FBB = 3Δ2□,1Δ:<Δ2□>-<2Δ□>,Δ (Grice et al. 1999). Unique in this FBB is the lone polyhedron with triangular coordination, which consists of a neutrally charged [B(OH) 3 ] group. Layering has a directional component and depending on layer sequence the symmetry may be centrosymmetric or non-centrosymmetric. Related layered borate structures, biringuccite, nasinite, gowerite, and volkovskite, are compared. Observations indicate that veatchite-1A is the low-temperature and low-pressure polytype, veatchite-1M the high-temperature polytype, and veatchite-2M the moderate temperature and higher pressure polytype.

Falsterite, ideally Ca 2 MgMn 2+ 2 (Fe 2 0.5 Fe 3+ 0.5 ) 4 Zn 4 (PO 4 ) 8 (OH) 4 (H 2 O) 14 , is a new mineral from the Palermo No. 1 pegmatite in North Groton, Grafton County, New Hampshire, U.S.A., and also occurs at the Estes pegmatite quarry, Baldwin, Cumberland County, Maine, U.S.A. It formed as the result of secondary alteration of primary triphylite and associated sphalerite. The crystals occur as very thin greenish-blue plates and rectangular laths, up to 0.7 mm in length, but no more than a few micrometers thick. Laths are flattened on {010}, elongate along [100], and exhibit lamellar twinning. The mineral is transparent and has a very pale greenish-blue streak, vitreous luster, Mohs hardness of about 2, flexible tenacity, irregular fracture, and perfect cleavage on {010}. The measured and calculated densities are 2.78(3) and 2.837 g/cm 3 , respectively. It is optically biaxial (-), a = 1.575(10), b = 1.600(5), g = 1.610(5) (white light), 2V meas = 60(10), and 2V calc = 63.8. Falsterite exhibits strong dispersion, r > v. The optical orientation is X = b, Y ≈ a, Z ≈ c. Pleochroism is pronounced: X, Z = colorless to very pale yellow, Y = blue green; Y >> X ≈ Z. Electron-microprobe analyses (average of 7), with FeO and Fe 2 O 3 apportioned and H 2 O calculated on structural grounds, provided: CaO 6.36, MgO 2.13, MnO 8.10, ZnO 18.49, FeO 8.02, Fe 2 O 3 8.90, Al 2 O 3 0.02, P 2 O 5 31.81, H 2 O 16.17, total 100.00 wt%. The empirical formula (based on 50 O atoms) is Ca 2.02 Mg 0.94 Mn 2+ 2.04 Fe 2+ 1.99 Fe 2+ 1.99 Zn 4.05 P 7.99 O 32 (OH) 4 (H 2 O)14. The mineral dissolves very easily in cold, dilute HCl. Falsterite is monoclinic, P2 1 /c, with the unit-cell parameters: a = 6.3868(18), b = 21.260(7), c = 15.365(5) Å, b = 90.564(6), V = 2086.2(1.1) Å 3 , and Z = 2. The eight strongest lines in the X-ray powder diffraction pattern are [dobs in Å(I)(hkl)]: 12.86(34)(011); 10.675(100)(020); 4.834(12)(102,112); 4.043(18)(132); 3.220(25)(152); 3.107(14)(044); 2.846(19)(222); 1.596(14)(0·12·4). The structure of falsterite (R 1 = 6.42% for 714 F o > 4σF) contains edge-sharing chains of Fe 2+ /Fe 3+ O 6 octahedra and corner-sharing chains of ZnO 4 tetrahedra along [100]. These chains are linked to one another by PO4 tetrahedra, forming a sheet parallel to {010}. Mn 2+ O 6 octahedra and CaO 7 polyhedra also link to this sheet, resulting in a thick slab. The slabs are bridged in the [010] direction by edge-sharing dimers of MgO 6 octahedra, which link to the slabs by sharing edges with ZnO 4 tetrahedra in adjacent slabs. The structures of falsterite and schoonerite, while topologically quite different, share similar components and structural features.

The new mineral pavlovskyite Ca 8 (SiO 4 ) 2 (Si 3 O 10 ) forms rims together with dellaite Ca 6 (Si 2 O 7 )(SiO 4 )(OH) 2 around galuskinite Ca 7 (SiO 4 ) 3 CO 3 veins cutting calcio-olivine skarns in the Birkhin gabbro massif. In addition, skeletal pavlovskyite occurs in cuspidine zones of altered carbonate xenoliths in the ignimbrites of the Upper Chegem caldera (North Caucasus). The synthetic analog of pavlovskyite has been synthesized before and is known from cement-like materials. Isotypic to pavlovskyite is the synthetic germanate analog Ca 8 (GeO 4 ) 2 (Ge 3 O 10 ). The crystal structure of pavlovskyite, space group Pbcn, a = 5.0851(1), b = 11.4165(3), c = 28.6408(8) Å, V = 1662.71(7) Å 3 , Z = 4, has been refined from X-ray single-crystal data to R1 = 3.87%. The new colorless mineral has a Mohs hardness of 6-6.5, biaxial (-), α = 1.656(2), β = 1.658(2), γ = 1.660(2) (589 nm), 2V (meas) = 80(5)°, 2V (calc) = 89.9°, medium dispersion: r > v, optical orientation: X = b, Y = c, Z = a. For comparison with pavlovskyite, the crystal structure of kilchoanite Ca 6 (SiO 4 )(Si 3 O 10 ) from the Birkhin massif [space group I2cm, a = 11.4525(2), b = 5.0867(1), c = 21.996(3) Å, V = 1281.40(4) Å 3 , Z = 4] has been refined from single-crystal X-ray data to R1 = 2.00%. Pavlovskyite represents a 1:1 member of a polysomatic series with calcio-olivine γ-Ca 2 SiO 4 and kilchoanite Ca 6 (SiO 4 )(Si 3 O 10 ) as end-member modules. The structure is characterized by strongly folded trisilicate units (Si 3 O 10 ) interwoven with a framework of CaO 6 and CaO 8 polyhedra. Olivine-like slices with orthosilicate groups are interstratified with the characteristic trisilicate module of Ca 4 (Si 3 O 10 ) composition. Although the optical properties of pavlovskyite and kilchoanite are similar, both minerals can be distinguished by chemical analyses (different Ca/Si ratio), X-ray diffraction, and Raman spectroscopy. The new mineral is named after V.E. Pavlovsky (1901-1982), an outstanding geologist in the area of Eastern Siberia, in particular of the Baikal region.

We have recently discovered significant amounts of zaccagnaite, a natural Zn-Al-CO 3 hydrotalcite in the El Soplao cave (north Spain). The El Soplao zaccagnaite is speleothemic, i.e., formed in the cave, and therefore it represents a new cave mineral. The origin of El Soplao zaccagnaite is related to the diagenesis of Zn- and Al-rich ferromanganese speleo-stromatolites, where it occurs as a porefilling cement that likely precipitated at low temperature (≤ ~11 °C). In some stromatolite layers, the abundance of zaccagnaite crystals is large enough to enable their physical separation. This has allowed us to obtain its X-ray powder-diffraction pattern, infrared spectrum, and differential thermal/ thermogravimetric profiles. The cell parameters of the El Soplao zaccagnaite, refined from X-ray powder diffraction data are: a = 3.06616(1) and c = 22.6164(1) Å [α = β = 90°, γ = 120°; V = 184.139(1) Å 3 ; Z = 3], consistent with a new trigonal polytype of zaccagnaite: zaccagnaite-3R. Besides, the El Soplao zaccagnaite shows some features previously unknown in natural hydrotalcites, such as octahedral-like morphologies and fluorescence zoning. Electron microprobe analyses revealed that the El Soplao zaccagnaite-3R has an unusual chemistry for natural hydrotalcites, as it is significantly more rich in Al (Zn 2+ /Al 3+ = 1.6) than the hexagonal (2H) polytype (Zn 2+ /Al 3+ = 2.0). The simplified chemical formula deduced from electron microprobe analysis is (Zn 0.6 Al 0.4 )(OH) 2 (CO 3 ) 0.2 ·0.5H 2 O, where C and water were calculated by stoichiometry. The carbon content calculated by stoichiometry (2.2 wt%) is in good agreement with that measured with the electron microprobe on gold-coated samples (2.5 wt%). The presence of interlayer water and CO 3 groups was confirmed by thermogravimetric analysis coupled to mass spectroscopy, and by the analysis of the infrared spectrum.

Silicon incorporation in TiO 2 phases at increasing pressures until 20 GPa at 1300 °C has been studied by XRD and TEM. Rutile is the stable Si-doped TiO 2 phase until at least 7 GPa, transforming into α-PbO 2 structured TiO 2 between 7 and 10 GPa. The further transformation to the TiO 2 polymorph with the baddeleyite structure, akaogiite, has not been observed on the quenched samples. XRD and TEM-EDX data suggest that the Si-doped TiO 2 akaogiite polymorph is non-quenchable and reverts to α-PbO 2 structured TiO 2 when releasing the pressure. This transformation gives rise to α-PbO 2 structured TiO 2 grains decorated with π fringes stacking faults. Silicon solubility in TiO 2 phases increases with increasing the synthesis pressure until 16 GPa, implying the substitutional solid solution to be the mechanism of solubility. The influence of the dopants on the stability of the rutile and the α-PbO 2 structured TiO 2 has also been analyzed.

Zoned phenocrysts in volcanic rocks potentially provide an archive of magmatic processes. As a crystal grows and comes into contact with different melt batches, the chemical and textural signature of this journey is recorded within its crystal lattice. The timescale of some magmatic processes can be investigated through the relaxation of chemical gradients across crystal growth zones through the application of diffusion modeling techniques. One of the current limitations to diffusion modeling is the spatial and analytical resolution of the chemical profile that conventional techniques such as electron probe microanalyzer (EPMA), dynamic secondary ion mass spectrometry (SIMS), and laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) can achieve. Here, for the first time, we present time-of-flight (TOF) SIMS (TOF-SIMS) data for zoning of orthopyroxene crystals from the May 1982 eruption of Mount St. Helens volcano, U.S.A., and cross-calibrate these data between backscattered electron images and EPMA. TOF-SIMS has the advantage of being able to achieve micrometer to nanoscale spatial resolution of major elements as well as analyses of light elements, such as Li, and trace and minor elements (Na, K, and Ni) at concentrations that cannot be achieved by EPMA, provided that convolution (overlap) effects and polyatomic mass inferences are carefully considered. With TOF-SIMS analyses we identified zoning of Li on a spatial scale (ca. 5-10 μm) that would be inaccessible to most other conventional analytical techniques. Preliminary results indicate that Li, a fast-diffusing element, may be introduced to the crystals in the minutes, hours, or days prior to eruption and may provide insights into pre-eruptive magmatic processes. Thus, TOF-SIMS has the potential to be a powerful tool for obtaining minor and trace element profiles across compositional interfaces within crystals at high-spatial resolution.

Reactions involving the VI Ti IV Al- VI Al IV Si exchange in muscovite, biotite, and hornblende were calibrated thermodynamically using a set of experimental and natural data in rutile- plus quartz/ coesite-bearing assemblages. The specific respective reactions are K(Al 2 )(AlSi 3 )O 10 (OH) 2 + TiO 2 = K(AlTi)(Al 2 Si 2 )O 10 (OH) 2 + SiO 2 (R1) K(□MgAl)Si 4 O 10 (OH) 2 + TiO 2 = K(□MgTi)AlSi 3 O 10 (OH) 2 + SiO 2 (R2) Ca 2 Mg 3 Al 2 Al 2 Si 6 O 22 (OH) 2 + 2TiO 2 = Ca 2 Mg 3 Ti 2 Al 4 Si 4 O 22 (OH) 2 + 2SiO 2 . (R3) Ideal mixing on octahedral or octahedral plus tetrahedral sites and a non-ideal van Laar solution model yield the best regression results for thermodynamic fit parameters, with R2 values of 0.98-1.00. Isopleths of the equilibrium constant (K eq ) show minimal pressure dependencies of <1 °C/kbar, implying that the equilibria are poor barometers. Model reproducibility of the ideal portion of the equilibrium constant (K id ) is excellent (ca. ±0.1 to 0.3, 2σ), but the absolute value of the combined term ΔS+K id is quite small (absolute values from 0 to 4), so calibration residuals propagate to temperature errors >±50-100 °C, 1σ. Whereas the consistency of a mica or hornblende composition with a known T can be evaluated precisely, Ti chemistry in these reactions is sensitive to composition and does not resolve T (or P) well. The activity of TiO 2 in rutile [a(rt)] was also evaluated using both the garnetrutile- ilmenite-plagioclase-quartz (GRIPS) equilibrium and our new calibrations in rutile-absent, ilmenite-bearing rocks whose peak P-T conditions are otherwise known. Metapelites have average a(rt) of 0.9 (GRIPS) and 0.8 (R1), whereas amphibolites have a(rt) of 0.95 (GRIPS and R3). A value for a(rt) of 0.80 ± 0.20 (metapelites) and 0.95 +0.05/-0.25 (amphibolites) is recommended for traceelement thermomobarometers in ilmenite-bearing, rutile-absent rocks. The dependence of Ti contents of minerals on a(rt) and the reequilibration of Ti during metamorphic reactions both deserve further exploration, and may affect application of trace-element thermobarometers.

The origin of dolomite is a long-standing enigma in sedimentary geology. It has been proposed that microorganisms, especially anaerobic microorganisms, can overcome kinetic barriers to facilitate dolomite precipitation, although their specific role in dolomite formation is still unclear. Our experimental results demonstrate that disordered dolomite can be synthesized at room temperature abiotically from solutions containing polysaccharides such as carboxymethyl cellulose or agar. We propose that when dissolved in solution, polysaccharides can be strongly adsorbed on Ca-Mg carbonate surfaces through hydrogen bonding. The adsorbed polysaccharides may help weaken the chemical bonding between surface Mg 2+ ions and water molecules, which can lower the energy barrier to the desolvation of surface Mg 2+ -water complexes, enhance Mg 2+ incorporation into the precipitating carbonate, and thereby promote disordered dolomite formation. In natural environments, it is possible that polysaccharides produced by microorganisms, e.g., extracellular polysaccharides, may play a key role in promoting disordered dolomite nucleation and crystallization. In marine sediments, the accumulated dissolved carbohydrates produced from organic matter degradation during early diagenesis may also serve as catalysts for disordered dolomite formation.

High-pressure and high-temperature phase relations of FeTiO 3 were investigated up to a pressure of about 74 GPa and 2600 K by synchrotron X-ray diffraction and analytical transmission electron microscopy. We conclude that FeTiO 3 ilmenite transforms into the following phase(s) with increasing pressure: FeTiO 3 (perovskite) at 20-30 GPa, Fe 2 TiO 4 (Ca 2 TiO 4 -type) + TiO 2 (OI-type) at 30-44 GPa and high temperature, FeO (wüstite) + TiO 2 (OI) at 30-44 GPa and low temperature, and wüstite + FeTi 3 O 7 (orthorhombic phase) above 44 GPa. Among these dense high-pressure polymorphs, FeTi 3 O 7 is a new compound and its structure analysis was tried using particle swarm optimization simulation. This method successfully found a new high-density FeTi 3 O 7 structure, and Rietveld refinement based on this model structure gave an excellent fit with the experimentally obtained X-ray diffraction pattern. This new high-density FeTi 3 O 7 structure consists of polyhedra for monocapped FeO 7 prisms, bicapped TiO 8 prisms, and tricapped TiO 9 prisms, which develop on the b-c plane and stack along the a axis. The dense compound assemblage found in FeTiO 3 is promising for investigating the behavior of ABX 3 compounds under ultrahigh pressures.

Ringwoodite (γ-Mg 2 SiO 4 ) is the stable polymorph of olivine in the transition zone between 525-660 km depth, and can incorporate weight percent amounts of H 2 O as hydroxyl, with charge compensated mainly by Mg vacancies (Mg 2+ = 2H + ), but also possibly as (Si 4+ = 4H + and Mg 2+ + 2H + = Si 4+ ). We synthesized pure Mg ringwoodite containing 2.5(3) wt% H 2 O, measured by secondary ion mass spectrometry (SIMS), and determined its compressibility at 300 K by single-crystal and powder X-ray diffraction (XRD), as well as its thermal expansion behavior between 140 and 740 K at room pressure. A third-order Birch-Murnaghan equation of state (BM3 EOS) fits values of the isothermal bulk modulus K T0 = 159(7) GPa and (dKT/dP) P=0 = K′ = 6.7(7) for single-crystal XRD; K T0 = 161(4) GPa and K′ = 5.4(6) for powder XRD, with K T0 = 160(2) GPa and K′ = 6.2(3) for the combined data sets. At room pressure, hydrous ringwoodite breaks down by an irreversible unit-cell expansion above 586 K, which may be related to dehydration and changes in the disorder mechanisms. Single-crystal intensity data were collected at various temperatures up to 736 K, and show that the cell volume V(cell) has a mean thermal expansion coefficient α V0 of 40(4) ×10 −6 /K (143-736 K), and 29(2) ×10 −6 /K (143-586 K before irreversible expansion). V(Mg) have α 0 values of 41(3) ×10 −6 /K (143-736 K), and V(Si) has α 0 values of 20(3) ×10 −6 /K (143-586 K) and 132(4) ×10 −6 K (586-736 K). Based on the experimental data and previous work from 29 Si NMR, we propose that during the irreversible expansion, a small amount of H + cations in Mg sites transfer to Si sites without changing the cubic spinel structure of ringwoodite, and the substituted Si 4+ cations move to the normally vacant octahedral site at (½, ½, 0). Including new SIMS data on this and several Mg-ringwoodite samples from previous studies, we summarize volume-hydration data and show that the Mg 2+ = 2H + dominates up to about 2 wt% H 2 O, where a discontinuity in the volume vs. H 2 O content trend suggests that other hydration mechanisms become important at very high H 2 O contents.

Ferromagnesite [(Mg,Fe)CO 3 ] has been proposed as a candidate host mineral for carbon in the Earth’s mantle. Studying its physical and chemical properties at relevant pressures and temperatures helps our understanding of deep-carbon storage in the planet’s interior and on its surface. Here we have studied high-pressure vibrational and elastic properties of magnesian siderite [(Mg 0.35 Fe 0.65 )CO 3 ] across the electronic spin transition by Raman and X-ray diffraction spectroscopies in a diamond-anvil cell. Our results show an increase in Raman shift of the observed lattice modes of magnesian siderite across the spin transition at 45 GPa as a result of an ∼8% unit-cell volume collapse and a 10% stiffer lattice (higher bulk modulus). C-O bond lengthening in the strong, rigid (CO 3 ) 2- unit across the spin transition contributes to a competitive decrease in Raman shift, most evident in the Raman shift decrease of the symmetric stretching mode. Combined vibrational and elastic results are used to derive the mode Grüneisen parameter of each mode, which drops significantly across the transition. These results suggest that the low-spin state has distinctive vibrational and elastic properties compared to the high-spin state. Analyses of all recent experimental results on the (Mg,Fe)CO 3 system show no appreciable compositional effect on the transition pressure, indicating weak iron-iron exchange interactions. Our results provide new insight into understanding the effects of the spin transition on the vibrational, elastic, and thermodynamic properties of (Mg,Fe)CO 3 as a candidate carbon-host in the deep mantle.

The electronic spin and valence states of iron in lower-mantle silicate perovskite have been previously investigated at high pressures using various experimental and theoretical techniques. However, experimental results and their interpretation remain highly debated. Here we have studied a wellcharacterized silicate perovskite starting sample [(Mg 0.9 ,Fe 0.1 )SiO 3 ] in a chemically inert Ne pressure medium at pressures up to 120 GPa using synchrotron Mössbauer spectra. Analyses of the Mössbauer spectra explicitly show a high-spin to low-spin transition of the octahedral-site Fe 3+ occurring at ~13-24 GPa, as evidenced from a significant increase in the hyperfine quadrupole splitting. Two quadrupole doublets of the A-site Fe 2+ , with extremely high-QS values of 4.1 and 3.1 mm/s, occur simultaneously with the spin transition of the octahedral-site Fe 3+ and continue to develop to 120 GPa. It is conceivable that the spin-pairing transition of the octahedral-site Fe 3+ causes a volume reduction and a change in the local atomic-site configurations that result in a significant increase of the quadrupole splitting in the dodecahedral-site Fe 2+ at 13-24 GPa. Our results here provide a coherent explanation for recent experimental and theoretical results on the spin and valence states of iron in perovskite, and assist in comprehending the effects of the spin and valence states of iron on the properties of the lower-mantle minerals.

The spectral behavior of gypsum dehydration in the visible to near-infrared (350-2500 nm) wavelength range was investigated by partially dehydrating the four main habits of gypsum (alabaster, satin spar, selenite, and massive) to form bassanite. Powdered samples of gypsum dehydrated at 100-115 °C and hand samples dehydrated at 115-130 °C, coinciding with a peak in mass (water) loss in the samples. As gypsum dehydrates, its characteristic H 2 O absorption bands at 1443 and 1945 nm shift to shorter wavelengths. Band depths and widths of absorptions at ~1200, 1400-1600, 1750, 1945, 2100-2200, and ~2400 nm all decrease with increasing temperature. The samples also underwent visible changes upon dehydration, both at the macroscopic and microscopic level, becoming very friable with an increase in fine grains. No consistent relationship was observed between dehydration temperature and grain size or habit. The samples were monitored for up to 20 months after dehydration, during which time none rehydrated to form gypsum. While the transition from gypsum to bassanite is very abrupt, bassanite does not readily rehydrate to form gypsum again in ambient conditions, and therefore may not be as unstable as previously thought by terrestrial occurrences of bassanite being predominantly restricted to hyperarid climates.

Rehydroxylation of the previously dehydroxylated dioctahedral 2:1 layer clay mineral occurs preferentially in specific sites within the former octahedral sheet. The rehydroxylation of dehydroxylated Al-rich and Al,Mg-rich 2:1 layers occurs as trans-vacant (tv) structural arrangements, regardless of whether the initial structure was tv or cis-vacant (cv). In nontronite (Fe-rich 2:1 layer clay), the dehydroxylate pseudo-cv structure is probably directly reconstructed into the rehydroxylated cv structure without migration of octahedral cations. Rehydroxylation occurs preferentially in the R 3+ -O r -R 3+ former octahedral structural arrangements (Or = residual oxygen) over R 2+ -O r -R (R = R 3+ or R 2+ = Al 3+ , Fe 3+ or Mg 2+ , Fe 2+ ). In the case of the R 2+ octahedral substitution, the interlayer cation is attracted to the electrostatically undersaturated residual oxygen of the R 2+ -O r -R arrangement, which blocks the ability of water molecules to pass through the ditrigonal cavity and rehydroxylate the previously dehydroxylated local arrangement. The pyrophyllite-like type of octahedral R 3+ -O r -R 3+ arrangements, formed due to the lack of tetrahedral substitution and resulting in the absence of interlayer cations, is thus favored for rehydroxylation over the mica-like R 3+ -O r -R 3+ arrangements where Al occurs in the tetrahedral sheet. The valence of the interlayer cation and the charge density of the 2:1 layer clay mineral, which controls the interlayer cation content, also affect the degree of rehydroxylation. Dehydroxylated 2:1 layer minerals with a high-rehydroxylation potential, including beidellite and illite, use all the adsorbed water molecules that persist above 200 °C for rehydroxylation; the water vapor from the ambient environment also becomes a source of H 2 O molecules for rehydroxylation. The high demand for water molecules to use for rehydroxyltion results in a noticeable gain of mass in the temperature interval between 200 and 350 °C even during heating.

Thermal behavior of afghanite, (Na 15 K 5 Ca 11 ) Σ31 [Si 24 Al 24 O 96 ](SO 4 ) 6 Cl 6 , P31c, a = 12.7961(7) Å, c = 21.4094(13) Å, an eight-layer member of the cancrinite group, has been investigated by combined electron microprobe analysis, X-ray single-crystal diffraction, and high-temperature X-ray powder diffraction. Non-ambient X-ray powder diffraction data were collected in the 323-1223 K thermal range on a specimen from Case Collina, Latium, Italy. Structural refinement and site assignment based on the bond-valence analysis, performed on room-temperature single-crystal X-ray diffraction data, provided more accurate site allocation of cations than the available model in the literature. The results show that the cancrinite cages alternating with the liottite cages are more compressed along the c-axis than the remaining ones. As a result the chlorine atom, located at the center of the cages, is driven off-axis to release the steric strain due to the cage compression. Thermal expansion shows a discontinuity at 448 K for both a and c unit-cell parameters, a feature previously reported for other cancrinite-like minerals. Up to 448 K, the c-parameter expands significantly and more than the aparameter. A further discontinuity has been detected at 1073 K for the c-parameter. Mean linear and volume thermal expansion coefficients (×10 -6 K -1 ) in the 323 < T < 448 K thermal range are ᾱ a = 12.9(4), ᾱ c = 17.9(9), and ᾱ V = 43.7(18). Above this discontinuity temperature, the thermal expansion is reverted becoming greater for the a-parameter. Mean linear and volume thermal expansion coefficients in the 448 < T < 1073 K thermal range are ᾱ c = 8.22(3), ᾱ c = 3.52(4), and ᾱ V = 19.68(8). In the 1073 < T < 1223 K thermal range the values are ᾱ a = 6.35(9), ᾱ c = 5.02(14), and ᾱ V = 17.74(9). Afghanite shows a significant microstrain at RT that increases up to ca. 700 K and subsequently decreases as a function of T. Cooling to RT allows a significantly release of ε 0 microstrain, which is coupled with a significant expansion of the c-parameter as compared to the starting RT data. The expansion of the c-parameter has been mainly attributed to the full expansion of the cancrinite cages alternating with the liottite cages. Upon reheating at 1173 K, the microstrain increases back to approximately the same value calculated for the first heating process. Repeated heating-RT-cooling cycles led to the partial afghanite structure disruption and the partial conversion, via an intermediate disordered phase, to haüyne. As repeated heating/cooling cycles did not modify the ε 0 values both at RT and at HT, it can therefore be concluded that the strain release occurs prevalently during the first heating/cooling cycle.

In this study, a natural Th-absent xenotime [(Y+HREE)PO 4 ], is enriched in specific areas with respect to Th + Si utilizing a series of alkali-bearing fluids that included 2 N NaOH, 2 N KOH, Na 2 Si 2 O 5 + H 2 O, and NaF + H 2 O, in addition to ThO 2 and SiO 2 . Charge and fluid were sealed in Au capsules and placed in the piston-cylinder apparatus (CaF 2 assemblies) at 1000 MPa and 900 °C (8 to 25 days) or in cold-seal autoclaves on a hydrothermal line at 500 MPa and 600 °C (23 days). BSE imaging, EMP analysis, and TEM indicate that a fraction of the xenotime grains in the 2 N KOH, Na 2 Si 2 O 5 + H 2 O, and NaF + H 2 O experiments have altered areas enriched in Th + Si. No reaction was observed in the 2 N NaOH experiments. The altered areas occur as a series of curvilinear intergrowths with sharp compositional boundaries that extend from the edge of the xenotime grain into the interior. Formation of these Th + Si enriched areas is interpreted as a consequence of fluid-mediated coupled dissolution-reprecipitation.

Fayalite (Fe 2 SiO 4 ), and other Fe-rich olivine, is often found in the reducing environments of the Moon, Mars, and other extraterrestrial bodies, but the oxidation state of the terrestrial mantle restricts the amount of Fe found in olivine on Earth. For this reason, synthetic fayalite is needed for use in planetary-analog and other studies. Here we present a method for the synthesis of nanocrystalline fayalite (nanofayalite) using a sol-gel technique. Iron(II) chloride, sodium ethoxide, and tetraethyl orthosilicate (TEOS) were reacted to produce a precursor gel, which was subsequently calcined under reducing conditions to crystallize nanofayalite. Powder X-ray diffraction analyses indicate that the produced nanofayalite is nearly pure, with minor amounts (0.5-3%) of metallic Fe in some batches. Scanning electron microscope images of nanofayalite crystals show euhedral to subhedral crystals that range in size between 100 and 150 nm. Estimates of specific surface area were determined by both the Brunauer-Emmett-Teller (BET) and Langmuir adsorption methods and indicate average surface areas of 27.7 and 45.3 m 2 /g, respectively. Regulation of the redox environment was the critical challenge for this synthesis, but careful control of oxygen fugacity during reactant addition and mixing, sol-gel drying, and calcination ensured fayalite crystallization.

The high-temperature heat capacity of fayalite was reinvestigated using drop and differential scanning calorimetry. The resulting data together with drop calorimetry data taken from the literature were analyzed yielding C P J/(mol·K) = -584.388 + 129 440·T −1 - 3.84956·10 7 ·T −2 + 4.10143·10 9 ·T −3 + 98.4368·ln(T). This new C P polynomial is recommended for calculating phase equilibria involving fayalite at mantle conditions. Using thermal expansion coefficient and isothermal bulk modulus data from the literature, the isochoric heat capacity was calculated resulting in C V J/(mol·K) = - 217.137 + 63 023.1·T −1 - 2.15863·10 7 ·T −2 + 2.23513·10 9 ·T −3 + 51.7620·ln(T).

The crystal structures of the isostructural orthorhombic sulfates celestite (SrSO 4 ), anglesite (PbSO 4 ), and barite (BaSO 4 ) were refined by Rietveld methods using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data. Their structural model was refined in space group Pbnm. The unit-cell parameters are a = 6.87032(3), b = 8.36030(5), c = 5.34732(1) Å, and V = 307.139(3) Å 3 for SrSO 4 ; a = 6.95802(1), b = 8.48024(3), c = 5.39754(1) Å, and V = 318.486(1) Å 3 for PbSO 4 ; and a = 7.15505(1), b = 8.88101(3), c = 5.45447(1) Å, and V = 346.599(1) Å 3 for BaSO 4 . The average <M-O> [12] distances are 2.827(1), 2.865(1), and 2.953(1) Å for SrSO 4 , PbSO 4 , and BaSO 4 , respectively, and their corresponding average <S-O> [4] distances are 1.480(1), 1.477(3), and 1.471(1) Å. The geometrical features of the SO 4 and MO 12 polyhedra become more symmetrical from SrSO 4 to BaSO 4 . Across the series, the a, b, and c parameters vary non-linearly with increasing V. The radii of the M 2+ cations, r M , <M-O> [12], and <S-O> [4] distances vary linearly with V. These structural trends arise from the effective size of the M 2+ cation (r M : Sr < Pb < Ba) that is coordinated to 12 O atoms.

The thermal transformation, stability field, and reaction kinetics of serpentine minerals (antigorite, chrysotile, and lizardite) have been studied to draw a comprehensive model for their dehydroxylation and recrystallization reactions. In situ X-ray powder diffraction (XRPD) and kinetic studies were combined with transmission electron microscopy (TEM) observations to describe the mechanisms of dehydroxylation and later high-temperature crystallization. During dehydroxylation, a metastable transition phase with a characteristic peak around 9 Å was observed in antigorite and, to a minor extent, in lizardite. Rietveld refinements confirmed that the 9 Å phase actually possesses a talc-like structure. The appearance of this phase is controlled by structure and kinetic factors. The kinetic parameters and reaction mechanism for lizardite and antigorite dehydroxylation in air at ambient pressure were calculated using the Avrami models and compared to those of chrysotile. For both lizardite and antigorite, the kinetics of dehydroxylation is controlled by diffusion. Apparent activation energy of the reaction in the temperature range 612-708 °C was 221 and 255 kJ/mol for lizardite and antigorite, respectively. The reaction sequences of chrysotile, lizardite, and antigorite leading to the formation of stable high-temperature products (i.e., forsterite and enstatite) are described taking into account previous topotactic and dissolution-recrystallization models.

U-thorite, (Th,U)SiO 4 , from Ambohijatrova Masindray, Madagascar, was investigated to understand the behavior of Th and U during recrystallization of amorphous radiation-damaged, (Th,U)- orthosilicates. Optical microscopy and electron microprobe analyses reveal two types of U-thorite: (i) large (about 1 cm), orange, amorphous grains with composition: (Th 0.88±0.02 U 0.09-0.01 Pb 0.029±0.002 REE 0.01±0.001 ) 1.00±0.01 Si 1.00±0.01 ; and (ii) green, microcrystalline U-thorite with composition: (Th 0.76±0.05 U 0.08±0.01 Ca 0.07±0.01 Pb 0.014±0.005 REE 0.009±0.001 ) 0.92±0.07 Si 1.12±0.06 . Ca-free U-thorite-(i) is enriched in Th, U, and Pb (7.1, 1.2, and 1 wt%, respectively), and depleted in Si (3.0 wt%) compared to U-thorite-(ii). Recrystallization of U-thorite-(i) resulted in fracturing that facilitated migration of mobilized Th and U over a distance of about 300 μm, as evidenced by precipitation of U-thorite-(ii) in the fractures in associated apatite and garnet. Transmission electron microscopy observations and selected-area electron diffraction (SAED) patterns confirm that U-thorite-(i) is amorphous. U-thorite-(ii) forms: (1) single crystals (>1 μm in size) with variable amounts of amorphous material; or (2) randomly oriented, nanocrystalline aggregates (5-10 nm in size). TEM-EDX analyses show that the Th/U ratio in U-thorite-(i) and U-thorite-(ii) is ~6. High-angle annular dark-field scanning TEM (HAADF-STEM) and high-resolution TEM reveal that nanocrystalline Th-coffinite (20-40 nm in size) with Th/U ratio = 0.6, formed during recrystallization of U-thorite-(i). The calculated chemical Th-U-Pb ages of Uthorite-( i) range from 2.1-1.9 Ga and from 1.8-1.6 Ga, whereas U-thorite-(ii) ages range from 1.6-0.5 Ga. The calculated cumulative radiation dose for U-thorite-(i) varies from 1.6-1.8 × 10 18 α-decay events/mg, which is equivalent to 136-152 displacements per atom (dpa), and for U-thorite-(ii) from 3-4.4 × 10 17 (α-decay events/mg) (=27-37 dpa). The cumulative dose for Th-coffinite is 9.8 × 10 17 α-decay events/mg (84 dpa).

The clinopyroxene-type compound FeGeO 3 was synthesized using ceramic sintering techniques at 1273 K in evacuated silica tubes and investigated by powder neutron diffraction between 4 and 300 K, X-ray diffraction, SQUID magnetometry, and 57 Fe Mössbauer spectroscopy. The title compound shows space group C2/c symmetry (high pressure, HP-topology) between 4 and 900 K. No structural phase transition is present within this temperature interval, whereas lattice parameters show discontinuities around 50 and 15 K, which are due to magnetic phase transitions and the associated magneto-elastic coupling of the lattice. The magnetic susceptibility data show two maxima in their temperature dependence, one at ~47 K, the second around 12 K (depending on the external field), indicative of two magnetic transitions in the title compound. Neutron data shows that for T < 45 K, FeGeO 3 orders magnetically, having a simple collinear structure, with space group C2/c, and with the spins aligned parallel to the crystallographic b-axis, both on M1 and M2. The coupling within the M1/M2 band is ferromagnetic, whereas between them it is antiferromagnetic. As the bulk magnetic measurements in the paramagnetic state revealed a dominating ferromagnetic coupling, the intra-chain interactions dominate the inter-chain interaction. At 12 K, additional magnetic reflections appear, revealing a second magnetic phase transition. Spins are rotated away from the b-axis toward the a-c plane. The coupling within the M1 chain is still ferromagnetic and antiferromagnetic between the M1 chains. However, spins on M1 and M2, are no longer collinear. The moment on the M2 site is rotated further away from the b-axis than on M1.

Crystal structures of four aragonite-group carbonates-aragonite (Ca 0.997 Sr 0.003 CO 3 ), calcian strontianite (Ca 0.147 Sr 0.853 CO 3 ), cerussite (Ca 0.001 Pb 0.999 CO 3 ), and witherite (Sr 0.019 Ba 0.981 CO 3 )-have been refined at ambient conditions, and thermal expansion has been measured over a range of temperatures from 143 to 586 K by single-crystal X-ray diffraction. Average linear thermal expansion coefficients α 0 (V) are 58(2), 58.3(7), 64(2), and 57(2) (× 10 −6 K −1 ) for aragonite, strontianite, cerussite, and witherite, respectively, throughout the experimental temperature range. Aragonite, strontianite, and witherite have very similar α 0 (V) values, whereas that of cerussite is significant larger, primarily due to the caxis thermal expansion for cerussite being much larger than those of the other carbonates. There are no significant differences for α 0 (a) values among the four carbonates, whereas α 0 (b) values decrease in the order of aragonite > strontianite > cerussite ≈ witherite, and α 0 (c) values increase in the order of aragonite < strontianite < witherite < cerussite. Crystal structures were refined for aragonite (184 to 527 K). <Ca-O> vs. T (K) is fitted linearly quite well, with a slope of 5.8(8) × 10 −6 (Å/K). Corrected for assumed rigid body motion, the CO 3 groups showed no significant change in C-O distances over the temperature range.

Although the transition of the blueschist to greenschist facies has been modeled by the reaction glaucophane (Gln) + paragonite (Pg) + water (V) = clinochlore (Chl) + albite (Ab) + quartz (Qtz), we have found in the system Na 2 O-MgO-Al 2 O 3 -SiO 2 -H 2 O that smectite (Smec) forms repeatedly and persistently through the approximate reaction Gln + Chl = Smec ± Ab. The reaction was verified using both synthetic and natural Gln and Chl, where in all cases complete reaction of Chl occurs in the presence of Gln to yield Smec. We have, therefore, investigated the reaction Gln + Pg + Qtz ± V = Smec + Ab, as a possible alternative to the first reaction. The reaction Gln + Pg + Qtz ± V = Smec + Ab was studied over the P-T range of 600-800 °C and 1.4-2.0 GPa keeping the water content in the range of 0-4 wt% to avoid the direct reaction of Gln to Smec + Ab at water contents above 4 wt%. The was experimentally reversed at 600 °C and 1.60(5) GPa, 700 °C and 1.70(5) GPa, and at 800 °C and 1.90(5) GPa (at 0 wt% water content), but with partial melting at 800 °C and pressure ≤2.0 GPa with water ≥3 wt%. The compositions of the phases and the state of Al-Si disorder in the albite were determined. The amphibole showed minor increase in Al and decrease in Mg and Si relative to the glaucophanic amphibole starting material, whereas paragonite showed minor enrichment in Mg and Si at the highest temperatures, complementary to the change in amphibole composition. Albite showed only limited ordering with decrease in temperature. Smectite at 600-700 °C has the average composition Na 0.25 Mg 2.21 Al 1.19 Si 3.44 O 10 (OH) 2 ·nH 2 O and shows slight Mg depletion and Al enrichment at 800 °C. Evidence for at least some Mg enrichment in the interlayer cation sites was supported by powder X-ray diffraction with in situ heating stage analyses using changes in the d 001 reflection at 200 °C as an approximate indication of Mg for Na substitution. The composition of smectite formed in this study is quite similar to that of other natural trioctahedral phyllosilicates reported in the literature. This, together with the experimental reversibility of the reaction observed here, indicates that smectite can form as a progressive metamorphic mineral at greenschist- and amphibole-facies conditions, particularly at the expense of chlorite and glaucophane.

Coralloite is a new mineral found at the Monte Nero Mine (Rocchetta Vara, La Spezia, Liguria, Italy) having the simplified formula Mn 2+ Mn 2 3+ (AsO 4 ) 2 (OH) 2 ·4H 2 O. It occurs as sub-millimetric lamellar cinnabar-red crystals elongated on [100] and flattened on (001), isolated or forming wisps up to 0.5-1 mm long. Associated phases are calcite, inesite, quartz, brandtite, sarkinite, and tilasite in a chert matrix. Crystals are pleochroic, yellow along [100] and orange-red in directions normal to it. Extinction is parallel to the cleavage traces and elongation is negative. The small crystal size does not allow accurate determination of refraction indices. Crossed polar observations of crystals placed in diiodomethane (n = 1.74) suggest that the mean refractive index is close to that value. Coralloite is triclinic, space group P1, a = 5.5828(7), b = 9.7660(13), c = 5.5455(7) Å, α = 94.467(3), β = 111.348(2), γ = 93.850(2)°, V = 279.26(6) Å 3 , Z = 1. The five strongest lines in the simulated powder diffraction pattern (d obs , I, hkl) are: 9.710 Å, 100.0, (01̄0), 5.166 Å, 77.1, (100); 5.136 Å, 79.7, (001); 3.342 Å, 64.8, (1̄2̄1); 3.324 Å, 33.6, (12̄1̄). The structure of coralloite (final R all 0.044 for 3092 observed reflections) shows similarities with bermanite: Mn 2+ Mn 2 3+ (PO 4 ) 2 (OH) 2 ·4H 2 O and ercitite: Na 2 Mn 2 3+ (PO 4 ) 2 (OH) 2 ·4H 2 O. In particular, these three minerals exhibit the same structural slab formed by [ [6] Mn 2 3+ ( [4] XO 4 ) 2 (OH) 2 ]; X = As or P. However, these structural slabs are connected by interposed layers of different polyhedra for each mineral species.

Metanatroautunite, Na[(UO 2 )(PO 4 )](H 2 O) 3 , from the Lake Boga granite, Victoria, Australia, has tetragonal symmetry, space group P4/ncc, with the unit-cell parameters: a = 6.9935(7), c = 17.5101(12) Å, V = 856.40(13) Å 3 , and Z = 4. The crystal structure has been solved and refined to R 1 = 0.0398 for 368 unique reflections [F > 4σ(F)] and 0.0456 for all 496 unique reflections. Metanatroautunite has an almost identical corrugated polyhedral sheet to meta-autunite-group minerals, consisting of corner-sharing uranyl square pyramids and phosphate tetrahedra. Hydrogen bonds (and cation-oxygen bonds) link the water molecules in the interlayer into square-planar sets, which are connected together creating 8-membered arrays. Metanatroautunite is identical to synthetic Na[(UO 2 )(PO 4 )](H 2 O) 3 .

The phase relations of petedunnite [CaZnSi 2 O 6 (pd)] were determined experimentally at P-T conditions up to 2.5 GPa and 1100 °C. Single-phase petedunnite is formed at high pressures (P > 0.8 GPa). Reversed experiments show that at lower pressures and temperatures >650 °C petedunnite decomposes to willemite [Zn 2 SiO 4 (wil)], hardystonite [Ca 2 ZnSi 2 O 7 (har)], and quartz [SiO 2 (qtz)] according to the reaction 4 pd = wil + 2 har + 3 qtz. The boundary curve for this equilibrium reaction is given by P (GPa) = -0.093 (0.029) + 0.0014 (0.0003) T (°C), by disregarding the phase transition of quartz. The stability field of wil + har + qtz is restricted toward lower temperatures by zinc-feldspar [CaZn- Si 3 O 8 (zfsp)] according to the known reaction: wil + 2 har + 7 qtz = 4 zfsp. These reactions intersect at 650(1) °C/0.78(0.01) GPa, generating an invariant point I pd . Additionally, petedunnite-breakdown reaction is intersected by the low/high-quartz phase transition curve, generating an invariant point I pd qtz at ~840 °C, 1.04 GPa. At temperatures <650 °C, further reactions occur in the system CaO-ZnOSiO 2 including the doubly degenerate reaction zfsp = pd + qtz and 3 pd = har + wil + zfsp, which also intersect the invariant point I pd . All reactions involving petedunnite display shallow positive slopes within the P/T-field, indicating that the crystallization of petedunnite is highly pressure sensitive over a wide temperature range. This means that an increasing petedunnite component in pyroxene shifts its stability field to higher pressures, similar to the effect of a jadeitic component. The study of natural clinopyroxene and the correlation of its zinc content with published P-T conditions of these mineral assemblages confirmed a significant relationship between extraordinary high-zinc concentrations in pyroxene and high-metamorphic pressure conditions. In addition, the petedunnite component is obviously sensitive to the prevailing fluid conditions in terms of the fugacity ratio f S2 /f O₂ . Furthermore, a distinct temperature dependency of the zinc component was observed in the range of trace element concentration. In consequence, Zn turns out to be a key element with regard to its implementation as a sophisticated petrogenetic indicator of metamorphic conditions. Therefore, routine measurement of zinc in element analyses of clinopyroxenes is strongly recommended.

The previously published structure determination of weeksite from the Anderson mine, Arizona, U.S.A., suggested that it is orthorhombic, Cmmb, with a = 14.209(2), b = 14.248(2), c = 35.869(4) Å, and V = 7262(2) Å 3 , and an ideal chemical formula (K,Ba) 1-2 (UO 2 ) 2 (Si 5 O 13 )·H 2 O. Using single-crystal X-ray diffraction, electron microprobe analysis, and thermal analysis, we reexamined weeksite from the same locality. Our results demonstrate that weeksite is monoclinic, with the space group C2/m and unit-cell parameters a = 14.1957(4), b = 14.2291(5), c = 9.6305(3) Å, b = 111.578(3)°, V = 1808.96(10) Å 3 , and an ideal formula K 2 (UO 2 ) 2 (Si 5 O 13 )·4H 2 O. The previously reported orthorhombic unit cell is shown to result from twinning of the monoclinic cell. The structure refinement yielded R 1 = 2.84% for 1632 observed reflections [I obs > 3σ(I)] and 5.42% for all 2379 reflections. The total H 2 O content derived from the structure refinement agrees well with that from the thermal analysis. Although the general topology of our structure resembles that reported previously, all Si sites in our structure are fully occupied, in contrast to the previous structure determination, which includes four partially occupied SiO 4 tetrahedra. From our structure data on weeksite, it appears evident that the orthorhombic cell of the newly discovered weeksite-type mineral coutinhoite, Th x B 1-2x (UO 2 ) 2 Si 5 O 13 ·3H 2 O, needs to be reevaluated.

Applications of electron backscatter diffraction (EBSD) to obtain crystallographic information of minerals in petrographic thin sections in mineralogy and petrology are increasing. However, platy phyllosilicates that mostly appear with their silicate layers terminated by the surface in the thin sections generally do not show sharp EBSD patterns in spite of gentle mechanical polishing. TEM examination indicated that this is due to crystal bending of phyllosilicates from the surface to a few micrometers in depth, caused by the polishing process. Ion beam etching commonly used to prepare TEM specimens was found to be applicable to remove the surface region with crystal bending. As a result, clear and sharp EBSD patterns were acquired from the phyllosilicates (micas, chlorite, etc.) in petrographic thin sections, by which their crystal orientations and polytypes were unambiguously determined.