Volume 84, Issue 1-2

In a thermodynamic description of a mineral solid solution, it is customary to select a minimal group of end-members that constitutes an independent set, and yet in practical calculations it commonly occurs that a different independent set or subset is required. Given the simplification of the symmetric formalism, it is straightforward to derive the enthalpies of formation of dependent end-members as well as the interaction energies between the end-members in the new independent set, in terms of those in the original set. For example, a simplified ternary solid solution of Fe-free Ca-amphiboles might be described with independent end-members tremolite, tschermakite, and pargasite, and yet some calculations may require the use of end-members such as edenite or hornblende. Not only are their end-member properties dependent on those of the first three, but the mixing properties of any of the binary joins involving edenite or hornblende are dependent on those in the original independent set. Examples drawn from pyroxenes, amphiboles, and biotite micas show that such dependencies may prove invaluable in using experimental information or heuristics to help constrain the mixing properties of complex solid solutions. In particular, it is found that in non-ideal solid solutions that involve Fe and Mg distributed over two or more non-equivalent sites, equipartition of Fe and Mg implies extreme restrictions on the ratios of the interaction parameters and the magnitude of the Fe-Mg exchange energy. The thermodynamics can only be formulated generally and consistently when Fe- Mg ordering is explicitly included, and this is done most simply via ordered Fe-Mg endmembers.

Biotite solid-solutions are a significant H 2 O reservoir in the lithosphere, and the assemblage Bt + Opx + Kfs + Qtz is commonly used to estimate a H₂O in high-grade metamorphic and magmatic rocks. Here we report experimental constraints on subsolidus mineral equilibria involving biotite and orthopyroxene in the system KAlO 2 -MgO-FeO-SiO 2 -H 2 O-CO 2 . Our experiments address the question of stability of biotite of a given XFe in the assemblage Bt + Qtz ± Sa, or the stability of the assemblage Opx + Sa ± Qtz. Clemens (1993) and Clemens et al. (1997) concluded that CO 2 has no effect other than to lower a H₂O and thereby raise the solidus T. Our data at X Fl H₂O < 1 extend these conclusions to encompass Fe-bearing systems more similar to natural rocks. From a comparison of experimental data and calculated isopleths of biotite composition in the divariant assemblage Bt + Opx + Kfs + Qtz + fluid, it appears that phlogopite-annite solid-solutions must be significantly non-ideal (at T , 800 °C) or that enstatite-ferrosilite solid-solutions must have negative values for their Margules-type parameters. Ignoring these factors would result in any calculated a H₂O values being too low. Although various models allow us to estimate X H₂O in H 2 O-CO 2 fluids, we are still unable to use biotite equilibria to estimate a H₂O accurately during high-grade metamorphism and magma crystallization. We also consider qualitatively the effects Fe-Mg biotite solid-solution on partial melting equilibria in fluid-poor (rock-dominated) systems in which hydration-dehydration reactions control the fluid composition.

The viscosities of hydrous melts (0.65 to 2.8 wt% H 2 O) with quartzofeldspathic compositions corresponding to Ab, Ab 74 Qz 26 , and Ab 48 Qz 52 (mole proportions calculated on the basis of eight oxygen atoms; Ab = NaAlSi 3 O 8 , Qz = Si 4 O 8 ) have been determined between 980 and 1375 °C at pressures between 190 and 360 MPa using the falling sphere technique. The use of large bubble-free hydrous glass cylinders (placed in internally heated pressure vessels) previously prepared and already containing markers and platinum spheres allows falling distances up to several centimeters to be measured with a precision of ± 50 to 200 μm. This results in a precision of ± 15% relative or less for most viscosity data (± 10% relative or less if the temperature is known within ± 5 °C). For a water content of 2.8 wt% H 2 O, viscosity increases with increasing Qz content. In the investigated viscosity range, no significant deviation from Arrhenian behavior is observed and the activation energy of viscous flow increases slightly with decreasing water content of the melt (for Ab). Combining the experimental data obtained in this study with data for a haplogranitic composition investigated previously by Schulze et al. (1996) shows that the viscosities, and hence, the activation energies of viscous flow are similar for compositions with the same atom ratio (Si + Al)/(H + Na + K) (SA/HNK). Thus, melt viscosity is constant if Al, charge balanced by Na or K, is exchanged with Si + H (H incorporated as OH or H 2 O). The viscosities (in dPa·s) of all investigated hydrous haplogranite compositions with water contents ranging between 0.7 and 8.2 wt% H 2 O can be calculated to better than ±0.15 log units using the expression: logη = - 1.8 + [940 + 5598·(SA/HNK) 0.3774 ]·1/T where T is expressed in Kelvin and varies from 1073 to 1650 K.

Titanite of variable Al and F content was found in granulite- to amphibolite-facies calcsilicates in Central Dronning Maud Land, Antarctica. The highest observed Al content corresponds to an X Al [= Al/(Al + Ti)] of 0.53. Previously, such high values of X Al were reported from high-pressure rocks, but the titanite of this study is from a lowpressure terrain. The compositional variations in titanite can be described for all samples by a set of three linearly independent exchange vectors added to the CaTiSiO 5 endmember titanite. In most rocks, these vectors are Al 1 F 1 Ti -1 O -1 , Ti -0.25 M 0.25 O -1 OH 1 , and OH 1 F -1 ; in one sample, the Ti -0.25 ⃞ 0.25 O -1 OH 1 vector is replaced by a Si -0.25 ⃞ 0.25 O -1 OH 1 vector. The actual amount of exchange along these vectors and, therefore, the amount of Al in titanite, depends on P and T, on the composition of the coexisting fluid phase in terms of its H 2 O/HF fugacity ratio, and on host rock composition in terms of Al 2 O 3 /TiO 2 activity ratio. It is inferred that, in suitable chemical environments, high-Al titanite is stable over a wide P-T range. Therefore, the Al content of titanite should not be used in geothermobarometry, even qualitatively. Additionally, because of the coupled substitutions Al 1 F 1 Ti -1 O -1 and Al 1 OH 1 Ti -1 O -1 , the concentration of F in titanite is strongly dependent on the host rock chemistry. This rules out the easy use of titanite as a monitor of fluid composition.

We describe a new quench technique that allows for controlled and reproducible constant quench rates exceeding 100 °C/s in the temperature range of 1400-700 °C and ~ 10 °C/s in the range 1400-200 °C at 1 atm total pressure. Our technique uses a 1 atm infrared image furnace and a blower unit capable of discharging cooling air through the infrared furnace at speeds approaching 100 m/s. The control protocol consists of operating the furnace at a constant power setting, sufficient to reach the highest desired temperature, and modulating the blower output continuously; blower output is controlled via a siliconcontrolled rectifier (SCR) using a proportional-integral-derivative (PID) algorithm on a personal computer (PC). A special autotuning procedure was used that enables the finetuning of PID parameters necessary for precise temperature control. Temperatures are controlled to ± 1 °C over the entire temperature range under isothermal conditions and to within 10 °C of the setpoint during quench. The range of accessible quench rates using our technique opens up new temperature-time paths for quantitative study. Potential applications include detailed studies on chemical diffusion and the kinetics of bubble and crystal formation under conditions of rapid temperature changes. Such studies have direct relevance to the crystallization, degassing, and structural relaxation of silicate melts during rapid temperature changes such as those encountered during volcanic eruptions.

Pervasive metasomatism that involved the formation of tourmaline-rich rocks and influx of Li. F. and Cs into Ordovician psammo-pelitic metasediments occurred in the Valdeflores area (Cáceres. Spain). Numerous Li- and Sn-bearing. mineralized, greisen-type veins also can be observed here, in the vicinity of geochemically specialized granites. Tourmaline- rich rocks appear as: (1) massive, fine-grained, dark green to black rocks: and (2) fine- scale tourmaline-rich laminae, which alternate with quartz-rich layers parallel to the bedding. Electron microprobe analyses indicate that the tourmaline lies mostly within the space defined by the exchange vectors from dravite: FeMg -1 (schorl), ⃞AlNa -1 Mg -1 (foitite), AlOMg -1 (OH) -1 (olenite), and CaMgNa -1 Al -1 (uvite). The Fe/(Fe+Mg) ratio ranges main­ly from 0.87 to 0.54 and increases with Al in the Y-site. Analytical results and substitu­tional relations show an insignificant elbaite component. Mica in the tounnalinized rocks is very fine-grained (mostly <50 μm). White mica ranges from lithian muscovite-phengite to lepidolite/zinnwaldite, containing up to 8.40 wt% F. 6.0 wt% Li 2 O, and 10.73 wt% FeO. Dark mica shows a variable color and has compositions characterized by relatively high contents of Cs 2 O (1.14-2.78 wt%) and F (1.94-8.08 wt%), with a deficit in K 2 O (5.75- 9.04 wt%). Log (f H₂O /f HF ) of fluids in equilibrium with biotite in the tourmaline-rich rocks was 4.02-4.17 at T ≈ 400 °C. Log (f H₂O /f HF ) values of fluids in equilibrium with topaz (X F ≈ 0.8) in country rock adjacent to contacts with veins, and in equilibrium with am- blygonite-montebrasite (X amb = 0.2) in the veins were about 4.30-4.60 and 6.4-6.7, respectively. Tliese variations denote the existence of gradients in relative a HF more than differences of temperature during metasomatism. The lack of tourmaline in the veins is interpreted to reflect the alkalinity and low Fe-Mg contents in the fluids, which precluded the formation of tourmaline. Consequently, most of the boron was expelled into metase­diments where tourmaline was produced as a result.

This work presents a procedure developed for trace element analysis using the electron microprobe (EMP). The method is demonstrated by analysis of seven glasses prepared from reference rock powders, with compositions of granite, granodiorite, andesite, diabase, and basalt. The melting process was adapted to prevent the loss of volatile elements and to obtain homogeneous samples. A routine procedure for analysis of major and minor (above 1000 ppm) elements yielded results in fairly good agreement with published values. The methods presented in this study produced detection limits as low as: (1) 6-8 ppm when the Kα peaks of transition metals of the first row (Cr and Ni) were used; (2) 23 ppm with soft Lα peaks (Y, Zr, and Sr); (3) 15 ppm with high-energy Lα peaks (rare earth elements); and (4) 35 ppm with the Ma peaks of heavy elements (Pb and Th). These limits were achieved after total counting times (peak + two background measurements) of about 15 min with a beam of 35 kV and 500 nA. Precision (± 2σ) on the weight percent concentrations was below 50% for concentrations above 13 ppm for group 1 elements, 35 ppm for group 2, 25 ppm for group 3, and 55 ppm for group 4. In the present work, a commercial software package was adapted for quantitative trace element analysis. One modification addresses beam-sensitive materials for which the long counting times required for measurements need to be divided into subsets (10-20 s count), each acquired from different sites of the sample surface, to minimize damage. This analytical mode is referred to as ‘‘multi-site’’ mode.

For estimations of P - T conditions of igneous and metamorphic rocks, Fe 3+ in coexisting minerals is either assumed to be zero or is calculated from electron microprobe analyses (EMPA) based upon stoichiometry and charge balance. Geothermobarometers that involve Fe 2+ - Mg 2+ exchange can be significantly affected by either neglecting Fe 3+ or using incorrect values. Ratios of Fe 3+ /ΣFe in garnet and clinopyroxene measured by a Mössbauer milliprobe were compared to those calculated from EMPA of garnet and clinopyroxene from eclogite xenoliths from the Udachnaya kimberlite in Yakutia. The effects of Fe 3+ contents in garnet and clinopyroxene on temperature estimations were evaluated. The following Fe 3+ /ΣFe (in at%) values were obtained (EMPA/Mössbauer): Gt = 9.4/6.0; 11.5/7.0; 19.4/16.0; and 24.7/15.0; Cpx = 22.0/22.9; 34.2/22.0. The effects of Fe 3+ in clinopyroxenes on calculated temperatures are illustrated by taking eclogitic clinopyroxene compositions and changing contents of certain elements within the range of standard deviations for EMPA of those particular elements. Increasing Na 2 O contents from 5.67 to 5.74 wt% (< 2.0% relative error) would lead to increasing Fe 3+ /ΣFe from 31.6 to 47.1%, thereby decreasing the calculated temperature from 1026 to 941 °C. Various Fe 3+ /ΣFe values for garnet and clinopyroxene were also tested for their effects on calculated temperatures: for clinopyroxene, T decreases with increasing Fe 3+ /ΣFe whereas for garnet, T increases with increasing Fe 3+ /ΣFe. This compensation effect between garnet and clinopyroxene moderates the variation in temperature estimations of eclogites based on Fe 3+ corrected vs. uncorrected microprobe analyses. Little correlation exists between EMPA-calculated and Mössbauer-measured Fe 3+ /ΣFe values for these mantle-derived garnets and clinopyroxenes. Even a small relative error in Fe 3+ may significantly change calculated temperatures of equilibration, seriously affecting petrologic interpretations. In particular, uncertainty in Fe 3+ calculated from EMPA of silicate minerals leads to serious questions with regard to K D values obtained from natural assemblages.

Amphiboles were synthesized along the joins tremolite-fluorotremolite, richterite-fluororichterite and potassic-richterite-potassic-fluororichterite at 750 °C and 1 kbar P(H 2 O). Infrared spectra of the amphiboles were recorded in the principal OH-stretching region. Amphiboles of the tremolite-fluorotremolite series show one-mode behavior, a single band due to a local MgMgMg-OH- A ⃞ (⃞ = vacancy) arrangement; this behavior is consistent with no coupling between NNN (next-nearest-neighbor) O3 anions either through the O3- O3 edge or across the vacant A-site cavity. The amphiboles of the richterite-fluororichterite and potassic-richterite-potassic-fluororichterite series show two-mode behavior, two bands due to the local arrangements MgMgMg-OH- A Na-OH and MgMgMg-OH- A Na-F (and their K equivalents); this behavior is consistent with coupling between NNN O3 anions across the filled A-site cavity through Na or K that occupies the A-site. A mathematical model is developed to describe local (OH,F) ordering in amphiboles as a function of F content. The variation in infrared band intensities is consistent with complete short-range disorder of OH and F in the synthetic amphiboles of the richterite-fluororichterite and potassicrichterite- potassic-fluororichterite series.

The 29 Si MAS NMR spectrum of armenite from Wasenalp (Valais, Switzerland) indicates complete Si, Al ordering. The same chemical shifts (- 82.3, - 95.0, and - 101.8 ppm) were also measured for armenite from Rémigny (Quebec, Canada), however, the intensity ratios of the NMR bands for the latter sample differed. A full sphere of X-ray single-crystal data on an optically homogeneous domain of armenite from Rémigny was collected on a threecircle diffractometer equipped with a CCD area-detector. The crystal structure was refined in the acentric space group Pnc2 [a = 18.660(2), b = 10.697(1) c = 13.874(2) Å ] to R1 5 3.6% based on 4275 reflections. These results confirm complete Si, Al ordering. Mean Si-O distances range between 1.615 and 1.629 Å ; mean Al-O distances between 1.734 and 1.742 Å . Calcium is sevenfold coordinated by six framework O atoms and one H 2 O molecule. Barium is 12-fold coordinated by framework O atoms. Polarized IR spectra in the region of OH absorptions (between 5700 and 1300 cm -1 ) were recorded on polished slabs of Wasenalp armenite and structurally related milarite from Val Giuf. In general, milarite and armenite show similar anisotropy of H 2 O related absorptions.

Single-crystal structure-refinement and electron-microprobe analysis of synthetic amphiboles with nominal fluoro-eckermannite composition and different trivalent cations (Al, Sc, Ti 3+ , V 3+ , Cr 3+ , Ga,) show significant deviations from nominal stoichiometry. Synthetic Sc- and Cr-bearing fluoro-eckermannite are close to nominal stoichiometry, whereas the corresponding nominal Al-, V 3+ -, Ga-, and Ti-bearing species contain very few trivalent cations and approximate Na(NaMg)Mg 5 Si 8 O 22 F 2 , a composition that has not been found in natural systems. The presence of a significant cummingtonite component strongly affects unit-cell parameters, coordination geometry around the B-, C-, and T-group sites, and cation ordering at the A-group sites. The high-charge cations are completely ordered at the M2 site, and there is a well-developed linear relationship between <M2-O> and the constituent-cation radius at the M2 site. The synthetic fluoro-eckermannite structure is stabilized by large spherically symmetric trivalent cations at the M2 site; for small spherically symmetric trivalent cations, it is not stable (at least at the synthesis conditions used here). Synthetic chromium-fluoro-eckermannite is stabilized by the non-spherically symmetric 3d 2 electronic arrangement, whereas Ga and V 3+ (3d 2 ) do not stabilize the fluoroeckermannite structure.

The continuous structural transformation of tetragonal analcime (Na 15.87 Al 15.20 Si 32.64 O 96 ·16.3H 2 O) upon dehydration was studied, using Rietveld structure analysis of temperature- resolved powder diffraction data collected using synchrotron radiation. The variation of the a-c axis length difference and normalized intensity of the (200) reflection as a function of temperature suggest that tetragonal analcime evolves toward a cubic structure at high temperature. The removal of water was accompanied by a spreading of the initial Na sites into many positions bonded to the framework O atoms. The migration of H 2 O molecules through the [111] channels during dehydration caused the six-member ring apertures to open as widely as possible: this was accompanied by a twisting of the tetragonal prism, constituting the analcime framework, which led to an opposite tilting of tetrahedra connecting the prisms. These modifications induced by water diffusion are not energetically favored because they would increase the elastic energy of the system, and require a substantial thermal activation energy. The analcime framework reached a maximum distortion at about 650 K, the temperature of complete water loss, then underwent a relaxation process during which the T-O-T angles were restored to the starting value. The relative variation of cell volume associated with the opening of wide six-member ring channels during water migration, and then due to the framework relaxation process after complete dehydration, provides an explanation of the ‘‘negative thermal expansion’’ (i.e., volume contraction) effect in dehydrated analcime, which is complementary to that based on the Rigid Unit Modes theory.

Single-crystal X-ray diffraction was performed in situ at T = 20, 230, 465, and 700 °C on a partially ordered lead feldspar (PbAl 2 Si 2 O 8 , I2/c, a = 8.402, b = 13.043, c = 14.308 Å , b = 115.308, V = 1417.6 Å 3 ; Q od = 0.71). The unit-cell expansion (1.26 × 10 -5 °C -1 ) is close to that observed for other feldspars, sanidine in particular, and occurs predominantly along a*. The electron-density at the Pb site evolves with temperature toward a bean-like configuration close to that observed in disordered lead feldspar. The average Pb position approaches the c-glide plane with increasing temperature. Consequently the intensity of the b-type reflections reduces dramatically without evidence of an increase of Al-Si disorder. The evolution of atomic displacement parameters of the Pb atom with temperature supports the view that at room temperature Pb shows considerable positional disorder. Dark-field in situ TEM observations show that b antiphase domains (APD) persist unchanged in shape and size up to T = 690 °C. No diffuse component appears in b-type reflections in SAD patterns up to 935 °C, showing that the above changes in the Pb configuration do not affect the APD. The results suggest that, at T > 700 °C, Pb reaches the glide plane assuming a configuration that may favor the Al-Si disorder.

Two karrooite crystals, one with a disorder parameter (X = Ti content in the M1 site) of 0.070(5) and the other with X = 0.485(5), were mounted together in one diamond-anvil cell and studied by single-crystal X-ray diffraction at several pressures up to 7.51 GPa. The most noticeable effect of increasing cation disorder on the high-pressure behavior of the structure is to increase the compressibilities of the mean <M2-O> bond length from 0.00148(2) GPa -1 in the ordered sample to 0.00163(7) GPa -1 in the disordered one and decrease those of the mean <M1-O> bond length from 0.00243(5) to 0.00193(12) GPa -1 . These changes are responsible for the compressibility difference between the two phases observed by Hazen and Yang (1997). Both compressibilities of the mean <M-O> bond lengths and the octahedral volumes in two phases decrease linearly with increasing the Ti contents in the octahedral sites. All octahedra in two samples become less distorted as pressure increases, but those in the more disordered structure exhibit larger decreases in terms of the octahedral angle variance than the corresponding ones in the more ordered structure. The influence of pressure on the interatomic angles is small compared to the interatomic distances, suggesting that compression of the karrooite structure is controlled primarily by the bond-length shortening, rather than by bond-angle bending. The strong compressional anisotropy of the structure is a consequence of the differential compressibilities of the weaker Mg 2+ -O and stronger Ti 4+ -O bonds and the complex edge-sharing linkage involving the M1 and M2 octahedra.

Atomistic simulations using lattice and molecular dynamics were carried out on the {210}, {310}, and {410} tilt grain boundaries of MgO as a function of pressure up to 100 GPa at a single temperature of 600 K. The calculations show a significant change in the structure of the tilt grain boundaries as pressure increases. The results show that, beyond the previously identified reversible pressure induced collapse of the channel structure, an irreversible shear was identified that forms a mirror grain boundary, which does not possess well-defined dislocation cores and is consequently denser. These mirror boundaries are energetically more favorable than the symmetric boundaries at elevated pressures. As applied pressure approaches 100 GPa a reversible structural transition occurs causing the boundaries to shear perpendicular to the boundary plane forming highly dense structures that contained seven coordinate ions at the boundary and the presence of an edge dislocation-like structure. We suggest that differences between the boundary structures seen in the HREM studies with those predicted by simulations may result from stresses upon the crystal during preparation causing irreversible shearing.

In contrast to most ionic minerals studied by SFM, the periclase (001) surface dissolves not by retreat of monolayer steps parallel to (001), but by retreat of a rough surface perpendicular to (001). At pH < 2, dissolution has an additional contribution from retreating macro-steps at the edges of nearly square pits. The macro-steps have heights up to 120 nm. In general, step direction is parallel to [110] and equivalent directions. During dissolution at low pH, a soft near-surface region is formed. Other investigators have shown that the near-surface region is protonated. Protonation is supposed to stabilize the (111) surface of periclase. Due to the structural similarities between periclase (111) and brucite (001), and similar dissolution rates of periclase and brucite at pH < 5, we conclude that during dissolution the periclase (001) surface is restructured into ‘‘(111) nano-facets’’ representing brucite (001)-like layers and appearing as a rough and soft surface in SFM images. The most probable reasons that the slopes of these macro-steps (up to 508) are lower than the slopes of perfect (111) facets are the likely poorly ordered structure of these layers, microtopography on these surface facets, and tip-surface convolution in SFM. By measuring the vertical position of the surface vs. time, we calculated the dissolution rate. At pH 1 and pH 2 we found the rates to be 17.1 ± 5.8 × 10 -6 and 5.7 ± 3.7 × 10 -6 mol/m 2 ·s, respectively. These rates are in reasonable agreement with previously reported rates of periclase and brucite (001) dissolution, and are consistent with the idea that the MgO (001) surface consists of Mg(OH) 2 (001)-like layers.

High-resolution transmission electron microscopy (HRTEM) revealed that a sample of fine-grained Lewiston, Idaho, fibrolite is predominantly fibrolite with trace amounts of poorly crystalline layer silicates. The fibrolite consists of aggregates of acicular grains where the c axis of each grain is parallel to the elongation direction. Widths of 195 grains were measured: The average is 0.41 μm, the mode is 0.29 μm, and the range is 0.05-1.57 μm. No stacking faults or other extended defects were observed in any of the grains. Grain boundary energies were calculated using the symmetrical dislocation tilt wall theory (SDTW) and measurements of misorientation between the c axes of neighboring fibrolite crystals. The angles of misorientation range from 1° to 11°, yielding grain boundary energies ranging from 310 to 967 ergs/cm², respectively, with an average energy of 610 ergs/ cm 2 . Modeling the fibrolite grains as infinitely long cylinders and using the experimentally measured average grain diameter, an average molar grain boundary energy of 320 J/mol was calculated. This excess grain boundary energy could correspond to a shift of as much as + 140 °C in the andalusite-sillimanite boundary and + 30 °C in the kyanite-sillimanite boundary. Typical fibrolite grain boundaries adopt relatively high-energy configurations. We attribute this to fibrolite nucleation at pre-existing low-angle grain boundaries in layer silicates, preserving misorientations, and conferring a fine-grained texture.

Selected-area electron diffraction (SAED) combined with experimental and computed high-resolution transmission electron microscopy (HRTEM) images were used to investigate polytypism, stacking disorder, mixed layering, and vacancies in chlorites from subgreenschist facies outcrops of the Taveyanne Sandstone from the Helvetic nappes, Switzerland. SAED patterns reveal increased ordering of the stacking sequences in chlorite with increasing metamorphic grade. However, semi-randomness and rotational faults occur even if the SAED photographs imply a regular ordered stacking sequence. In diagenetic (T = 210-250 °C, P = 2.1-2.2 kbar) and anchizonal-grade outcrops (T = 270-300 °C, P < 5 kbar) the polytypes Ibb and IIbb of chlorite were found, whereas in epizonal-grade samples (T = 300-360 °C, P < 5 kbar) the exclusive polytype is IIbb. Based on SAED and HRTEM images, a polytypic evaluation in one epizonal sample indicate that the monoclinic polytype IIbb-2 (55%) occurs as frequently as the triclinic polytype IIbb-4 (45%). Our samples suggest that by far the most important influence on polytypism and stacking disorder is temperature. Vacancy clusters occur in the octahedral cation positions within the talc-like layers and brucite-like sheets. The M1 and M2 positions in the talc-like layers are affected more by the cation deficiencies than the M3 and M4 positions in the brucite-like sheets. We suggest that octahedral vacancies are a substantial feature in natural chlorites in rocks of the Taveyanne Sandstone.

The mineralogy of sedimentary iron ores from the Gunma iron mine are described to evaluate the role of microorganisms and plants in ore formation. The iron ore is composed of nanocrystalline goethite, well-crystallized jarosite and very small amounts of strengite. The ore characteristically occurs as thick-bands of alternating goethite and jarosite bands, thin-bands of different goethite grain sizes, and fossil-aggregate ore rich in moss and/or leaves. Algal fossils are clearly preserved in the goethite bands in the thick-banded ore. Lattice imaging showed characteristic crystallographic orientations of the goethite nanocrystals. The thin-banded iron ores consist of micrometer-sized chestnut-burr-like goethite aggregates, probably formed by bacterial iron biomineralization. The bands may be attributed to biological or seasonal rhythms. Various products of biomineralization are found in the present-day pH 2-3, Fe 2+ -, and SO 4 2- -rich streams. Bacterial precipitation had var- 4 iations from amorphous Fe-P-(S) precipitates near the outlet of mineral spring to Fe-P-S precipitates and to Fe-S-(P) (schwertmannite-like) precipitates in the midstream. Mosses and green algae are also collecting Fe precipitates in and around the living and dead cells. Comparison of the processes occurring in the present-day streams and the iron-ore specimens supports the interpretation of these ores as the product of biomineralization.

Microbes, their exocellular secretions, and their impact on the mineralogy and microfabric of fine-grained continental margin sediments were investigated by transmission electron microscopy. Techniques were used that retained the in situ spatial relations of both bio-organic and mineralogical constituents. Photomicrographs were taken of characteristic mineral-microbe associations in the first meter of burial at conditions ranging from aerobic to anaerobic. Single-celled prokaryotes, prokaryotic colonies, and eukaryotic organisms were observed as were motile, sessile, and predatory species. Bacterial cells dominate the assemblage. The most commonly observed mineral-biological interaction was the surrounding, or close association, of isolated heterotrophic bacterial cells by clay minerals. Almost without exception, the external surfaces of the bacteria were covered with secreted exocellular slimes composed of cross-linked polysaccharide fibrils. These fibrils act to bind sediment grains into relatively robust microaggregates, roughly ≤ 25 μm in diameter. These exocellular polymers can significantly impact the interaction between microbes and minerals, as well as the chemical and physical transport of fluids and dissolved aqueous species through the sediment. Although pore water chemical profiles from the field sites studied have dissolved Fe and Mn, no close association was found between the microbes imaged and precipitated metal oxyhydroxides or other authigenic minerals, such as is commonly reported from laboratory cultures.

A single crystal of a birefringent Cr-bearing majorite, Mg 3 Mg 0.34 Si 0.34 Al 0.18 Cr 0.14 ) 2 Si 3 O 12, was synthesized at 20 OPa and 2000 °C using “6-8” type uniaxial split-sphere apparatus. This garnet is tetragonal with the unit-cell parameters a ≈ c and deviates slightly from cubic symmetry. The structure refinements using single-crystal X-ray diffraction intensity data were carried out by assuming three space groups (one cubic and two tetragonal) to determine the most probable symmetry. The most probable space group is I4 1 /a (tetragonal). The Cr ions show a disordered distribution between the two nonequivalent octahedral sites in the I4 1 /a structure.

The crystal structure of a high-pressure Fe 3 O 4 phase was determined by in situ X-ray diffraction measurements at high pressure and temperature, using an imaging plate detector and monochromatic synchrotron X-radiation. The high-pressure phase has the Pbcm space group (CaMn 2 O 4 -type structure) with cell parameters a = 2.7992(3) Å, b = 9.4097(15) Å, and c = 9.4832(9) Å at 23.96 GPa and 823 K. Fe 3+ occupies an octahedral site and Fe 2+ is in an eightfold-coordinated site described as a bicapped trigonal prism. The high-pressure CaMn 2 P 4 -type Fe 3 O 4 phase is about 6.5% more dense than the spinel form at 24 GPa.

A new polymorph of phosphorus oxynitride (PON) a silica analog has been recovered at ambient pressure by quenching after a treatment at 850 °C under a pressure of 2.5 GPa using cristobalite- or quartz-type phases as starting materials. This PON polymorph is a thermodynamically stable phase with its own stability field in a P-T diagram. The structure of this PON phase was refined by the Rietveld method from an X-ray powder diffractogram. It is isostructural with “moganite” (SiO 2 ). The discovery of this PON polymorph should stimulate a renewed interest in the occurrence of this phase in the silica system, because “moganite” may have a small but defined P-T stability field. These results confirm the structure of “moganite” as a new structure-type in AX 2 compounds.