Mineral reactions participating in intragranular fracture propagation: implications for stress corrosion cracking

Two examples of mineral reactions accompanying intragranular fracturing of silicates are described from amphibolites at the Grenville front, Coniston, Ontario, and a granodiorite within the Miéville shear zone, Switzerland. At these localities coarse grained (> 1 mm) hornblende and biotite respectively have undergone initial deformation by intense transgranular fracturing. The ambient temperature of deformation is estimated at 400–500°C for the amphibolites, and 250–300°C for the granodiorite.Fracture intensity within hornblende increases with progressive deformation until fractures coalesce and grains lose cohesion. Fractures are occupied by hornblende in close optical continuity with the parent grain, and typically contain median sutures decorated by arrays of solid inclusions. Relative to the parent grain, hornblende in fractures is depleted in Ti, Al, plus K, and enriched in Mg. Given the preferential partitioning of Mg, Al^VI and Ti into the M2 site of calcic amphiboles, the decrease of Ti and Al^VI in the host-to-crack transition is consistent with the corresponding increase of the Mg/(Mg + Fe²⁺) ratio.Fractures within biotite are bounded by an envelope of paler brown biotite which corresponds to a decrease of Ti and increase of Fe + Mg relative to regions unaffected by cracking. Fractures are occupied by secondary ilmenite, low-Ti biotite and high-Ti muscovite. Ti and Al do not vary significantly as a function of Mg/(Fe + Mg) in the host-to-crack transition, as anticipated from the approximately equal partitioning of these two cations into the M1 and M2 octahedral cation sites. The direct relationship of the mineral reactions to the fractures is taken as evidence for the participation of the reactions in crack propagation. These features may thus represent examples of natural stress corrosion cracking.     

Chlorine,titanium and barium-rich biotites: factors controlling biotite composition and the implications for garnet-biotite geothermometry

Barium-, Cl- and Ti-rich biotite occurs together with garnet, plagioclase and amphibole within narrow shear zones in 1800 Ma old noritic granulites in the Flakstadøy Basic Complex, Lofoten, north Norway. The granulite facies assemblage, plagioclase, clinopyroxene, orthopyroxene, biotite and ilmenite, was replaced by an amphibolite facies mineral assemblage including Ba-, Cl- and Ti-rich biotite during ductile deformation. Biotite shows complex compositional variations with respect to the contents of Ba, K, Cl, Ti, Al, Fe, Mg and Si. There are correlations between Si, Al^IV, K, Ba and Cl and between Al^VI and Ti. Titanium and Cl are uncorrelated. The Fe and Mg are correlated to both Cl and Ti. Multivariate analysis shows that most of the compositional variation of biotite can be described by two exchange reactions. This indicates that most of the variation in biotite composition was controlled by two chemical variables of the system. The content of the first exchange component (Ti_1.0 Fe_0.6 Al _-1.1 ^VI Mg_-0.8) in biotite can be related to the original distribution of Ti-bearing minerals in the igneous protolith. The content of the second exchange component (Al _0.4 ^IV Fe_0.8 Ba_0.5 Cl_1.0 Si_-0.4 Mg_-1.0 K_-0.5 OH_-1.0) is related to compositional variations of an externally derived Ba- and Cl-bearing fluid in equilibrium with biotite.The initially low Cl-content of the externally derived fluid was increasing during bioite forming reactions, because OH was preferentially incorporated, relative to Cl, into biotite. Continued hydration/chloridisation reactions resulted in a gradual consumption of the free fluid phase, resulting in local fluid-absent conditions. The composition of biotite reflects the composition of the last fluid in equilibrium with the mineral, i.e. the composition of the fluid immediately before the grain boundaries were fluid-undersaturated. Thus, the variations in biotite composition reflect how the fluid was gradually consumed throughout the shear zone rock. The correlations between Fe, Mg, Ba, K and Cl can be attributed to differences between the structure of the crystal lattices and the sizes of the cation sites of OH-phlogopite and Cl-annite. The dependency of the Fe/Mg ratios of biotite on the Cl-and Ti-content has a strong effect on the Fe–Mg partitioning between biotite and garnet. The relationship between lnK_D, X _Ti ^Bt and X _Cl ^Bt can be expressed by the regression equation: lnK _D =-1.82+2.60X _Ti ^Bt +5.67X _Cl ^Bt     

Mineral chemistry of banded migmatites from Hafafit and Feiran areas, Egypt

The migmatitic rocks exposed in Hafafit and Feiran areas exhibit some migmatitic structures as the banded, agmatic, boudinage and schlieren structures. The dominant type of these structures is the stromatic migmatites. Electron microprobe analyses of plagioclases, biotites and amphiboles from Hafafit and Feiran areas, in the Eastern Desert and Sinai, Egypt, are carried out and the metamorphic conditions are discussed. The present study revealed marked differences in the composition of plagioclases, biotites and amphiboles from Hafafit and Feiran localities. The obtained data indicated that plagioclases of the Feiran migmatites are of andesine and oligoclase composition, and display anorthite content from An₂₀ to An₃₈; whereas the Hafafit migmatites show a wider range of plagioclases from An₁₀ to An₆₀, and therefore plagioclases have labradorite, andesine and oligoclase composition. This may be due to the slow rate of the crystallisation processes. The analyses indicated that biotites of the studied areas are of metamorphic origin showing significant variation in Fe–Mg. It is worth mentioning that biotites from Hafafit migmatites have Mg–biotite composition while that of Feiram migmatites have Fe–biotite composition. High Mg and low Fe contents in biotite suggest higher crystallisation temperature. The composition of amphiboles in Hafafit migmatites is ferro-tschermakitic hornblende, while amphiboles from Feiram migmatites are magnesio-hornblende. High Ti content in the hornblende of Feiran migmatites suggests that they were formed at slightly higher temperatures and lower pressure than the Hafafit migmatites (i.e. Feiram migmatites and Hafafit migmatites were formed at granulite and amphibolite facies, respectively). Discrimination diagrams show that the muscovite is of secondary origin. Moreover, the present study confirmed that these migmatites are mainly formed by metamorphic differentiation via partial melting.     

Ti K-edge XANES study on the coordination number and oxidation state of Titanium in pyroxene,olivine, armalcolite,ilmenite, and silicate glass during mare basalt petrogenesis

Lunar mare basalts are a product of partial melting of the lunar mantle under more reducing conditions when compared to those expected for the Earth’s upper mantle. Alongside Fe, Ti can be a major redox sensitive element in lunar magmas, and it can be enriched by up to a factor of ten in lunar basaltic glasses when compared to their terrestrial counterparts. Therefore, to better constrain the oxidation state of Ti and its coordination chemistry during lunar magmatic processes, we report new X-ray absorption near edge structure (XANES) spectroscopy measurements for a wide range of minerals (pyroxene, olivine, Fe–Ti oxides) and basaltic melt compositions involved in partial melting of the lunar mantle. Experiments were conducted in 1 bar gas-mixing furnaces at temperatures between 1100 and 1300 °C and oxygen fugacities (fO₂) that ranged from air to two orders of magnitude below the Fe–FeO redox equilibrium. Run products were analysed via electron microprobe and XANES Ti K-edge. Typical run products had large (>?100 µm) crystals in equilibrium with quenched silicate glass. Ti K-edge XANES spectra show a clear shift in energy of the absorption edge features from oxidizing to reducing conditions and yield an average valence for Fe–Ti oxides (armalcolite and ilmenite) of 3.6, i.e., a 40% of the overall Ti is Ti³⁺ under fO₂ conditions relevant to lunar magmatism (IW ??1.5 to ??1.8). Pyroxenes and olivine have average Ti valence of 3.75 (i.e., 25% of the overall Ti is trivalent), while in silicate glasses Ti is exclusively tetravalent. Pre-edge peak intensities also indicate that the coordination number of Ti varies from an average V-fold in silicate glass to VI-fold in the Fe–Ti oxides and a mixture between IV and VI-fold coordination in the pyroxenes and olivine, with up to 82% ^[IV]Ti⁴⁺ in the pyroxene. In addition, our results can help to better constrain the Ti³⁺/∑Ti of the lunar mantle phases during magmatic processes and are applied to provide first insights into the mechanisms that may control Ti mass-dependent equilibrium isotope fractionation in lunar mare basalts.     

Crystal chemical variations in Ba-rich biotites from gabbroic rocks of lower crust (Ivrea Zone,NW Italy)

Biotites from mafic rocks occurring at different stratigraphic levels of the Ivrea-Verbano Mafic Complex are studied. The rocks are gabbros and diorites. All the biotites are intermediate between phlogopite and annite [0.282 (up to 7.14 and 9.32 wt%, respectively) with respect to those of the diorites (up to 1.26 and 6.26 wt%, respectively). Systematic compositional variations support the substitution model 2 ^IV Si+( ^IV R²⁺)2 ^IV Al+ ^VI Ti (R²⁺=Fe+Mg+Mn) in gabbros and ^IV Si+ ^VI Al ^IV Al+ ^VI Ti in diorites. A predominance of disordered stacking sequences, coexisting with 1M, 2M ₁ and 3T polytypes was observed in all biotites. It was possible to carry out structural refinements only on three biotites-2M ₁ from diorites (R-values between 2.68 and 3.77) and one biotite-1M from gabbros (R-value=3.09). It was shown that: (1) the reduced thickness of the tetrahedral sheet in Ba-rich biotites supports the coupled substitution ^IV Si+ ^XII K ^IV Al+ ^XII Ba; (2) the interlayer site geometry is affected by the whole layer chemistry and does not reflect only local chemical variations; (3) in two samples of the 2M ₁ polytype, the M(1) octahedral site is larger and more distorted than the M(2) sites because of the preferential ordering of Fe²⁺ in the M(1) site, whereas one sample shows complete cation disorder in the octahedral sites. Biotite-1M shows that Fe²⁺ can also be located in the M(2) site. Some of the differences between the biotites of gabbros and diorites (e.g. Ba concentration and exchange vectors) may be linked to the host rock composition and to its crystallization process. Biotite occurs in trace amounts in gabbros and its crystallization is related to the interstitial melt which contributed to the adcumulus growth of the main rock forming phases and became highly enriched in K, Ba and Ti. Diorites are the result of equilibrium crystallization of a residual melt rich in incompatible elements, where biotite is a major constituent.     

Amphiboles and biotites of the hornblende gneisses in an area to the North-East of the fjord Qagssit,Frederikshåb district,South-West Greenland

Summary The chemical characteristics of coexisting biotites and hornblendes and host rocks are examined and discussed. The field data indicate that biotite derives from hornblende at decreasing metamorphism. A chemical equilibrium has been reached for Fe²⁺, Mg and Mn and approached, but not reached, for tetrahedral ions, Fe³⁺, Al^vi and Ti. The disequilibrium is mainly dependent on biotite and is tentatively ascribed to the oxidizing conditions of the environment, at the time of biotite crystallization. The composition of biotite is thus partly related to that of hornblende and partly to the environment, while intracrystalline variations of hornblende are related to the chemical composition of the host rock and to varying metamorphic grade.

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Amphibole und Biotite der Hornblendegneise in einem Gebiet nordöstlich des Qagssit-Fjordes, Frederikshåb-Distrikt, Südwest-Grönland

Zusammenfassung Der Chemismus koexistierender Biotite und Hornblenden sowie von deren Muttergesteinen wird untersucht und diskutiert. Die Felddaten weisen darauf hin, daß der Biotit aus Hornblende bei abnehmender Metamorphose entsteht. Ein chemisches Gleichgewicht wurde für Fe²⁺, Mg und Mn erreicht; für die tetraedrisch koordinierten Ionen sowie Fe³⁺, Al^vi und Ti fand eine Annäherung in seine Richtung statt, es wurde aber nicht erreicht. Das Ungleichgewicht hängt im wesentlichen mit dem Biotitgehalt zusammen, und es wird versuchsweise den Oxidationsbedingungen der Umgebung bei der Biotitkristallisation zugeschrieben. Die Zusammensetzung des Biotits hängt folglich teilweise mit jener der Hornblende und teilweise mit der Umgebung zusammen, während die intrakristallinen Variationen der Hornblende von der chemischen Zusammensetzung des Muttergesteins und dem variierenden Metamorphosegrad abhängen.

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With 7 Figures     

Ti-rich hydroandradites from the Sanbagawa metamorphic rocks of the Shibukawa area,central Japan

Ti-rich hydroandradite often occurs, though usually as a minor constituent, in serpentinized ultramafic rocks and associated gabbroic rocks of the Sanbagawa metamorphic belt. The chemistry of the host rocks is commonly characterized by undersaturation with SiO₂. Two Ti-rich hydroandradites from metasomatized gabbroic rocks of the Shibukawa area have been chemically analysed by the wet method. Mössbauer experiments indicate the presence of Fe³⁺ and Fe²⁺ in the octahedral sites of these Ti-rich hydroandradites. Wet chemical analyses for total reducing capacity of the present garnets by two different methods together with Mössbauer data imply the presence of octahedral Ti³⁺ in their structure. Ti-rich hydroandratites, apart from their hydrous property, have a similar crystal chemistry to natural Ti-rich andradites. Ti-rich hydroandradites were probably formed in rocks with unusual chemical compositions within a P-T region of 300–400° C and 4–7 kb under limited conditions of relatively low oxygen fugacity and low μCO₂.     

Shock-induced thermal transformations of ries-biotites

Thermal transformations in biotites depending on increasing postshock temperatures were studied.A decrease of the unit cell volume, caused by oxidation of octahedral Fe²⁺ to Fe³⁺ at temperature rangings of about 400–500° C was observed.In the same temperature range (500° C) the biotite lattice becomes thermodynamically unstable. Intersheet and octahedral layers are decomposed, whereas individual SiO₄-tetrahedra are more resistant to thermal vibrations. The decomposition of the biotite leads to the formation of new minerals and amorphous phases. By X-ray analysesmagnetite, hercynite, pyroxene, feldspar, and quartz were identified.Thermal transformations of biotite induced by shock waves are characterized by states of distinct disequilibrium.     

Crystal chemistry of Sc-bearing synthetic diopsides

Scandium substitution in the diopside structure was studied by single-crystal X-ray diffraction on a series of synthetic diopside samples. These diopsides were doped with increasing amounts of Sc³⁺ through a coupled substitution involving ^M1(Sc³⁺)₁ ^M2[] ₁ ^T (B)₁ ^M1(Mg²⁺)_?1 ^M2(Ca²⁺)_?1 ^T(Si⁴⁺)_?1 exchange, whereby charge compensation is achieved by vacancies at the M2 sites and B at the tetrahedral sites. The substitution of scandium for magnesium at the M1 site results in an increase in volume and distortion of the M1O₆ polyhedron. The accompanying creation of vacancies at the M2 sites causes an increase in the M2 polyhedral volume. The modifications of the M1 and M2 polyhedra result in an increase of the polyhedral strip along the b lattice direction and a straightening of the tetrahedral chain. The geometrical modification of the M1 polyhedron due to scandium incorporation is comparable to those observed when similar amounts of Ti³⁺/Ti⁴⁺ substitute for Mg in the diopside structure, suggesting a structural control on the solubility of Sc and Ti in diopside that may influence the extent of the solid solutions between the Sc and Ti end-members.     

Substitution mechanisms and solubility of titanium in phlogopites from rocks of probable mantle origin

Previously proposed substitution mechanisms for Ti in phlogopites, based on experimental studies and crystal chemistry, have been examined using data for 81 phlogopites from mantle-derived rocks (primarily as nodules in kimberlites and also from alkali basalts, lamprophyres and carbonatites), 49 phlogopites from high-K rocks with basaltic affinities, and from 32 phlogopites crystallized in high pressure experiments mainly on high-K rock compositions. For the majority of phlogopites from the kimberlite group and for all those crystallized in the experimental studies, the substitution of Ti can be represented by a combination of the mechanisms represented by 2Mg^[VI]⇌ Ti^[VI]□^[VI] and Mg^[VI]2Si^[IV]⇌Ti^[VI]2Al^[IV]. Some phlogopites in ultrapotassic rocks have only the former substitution mechanism. The Ti contents of phlogopites generally increase with decreasing octahedral site occupancy and decreasing Si+Al^[VI]. For the phlogopites crystallized in the experiments on high-K rocks, the solubility of Ti increases with increasing fO₂ and temperature, and possibly with decreasing pressure at constant fO₂. The effect of the composition of the liquids used in the experimental studies from which these phlogopites have crystallized has only minor effect on either the substitution mechanism or the solubility of Ti in phlogopites. This suggests that phlogopite in high-K rocks may be a potential geothermometer and possibly a geobarometer.     

A reassignment of the optical absorption bands in biotites

The electronic absorption spectra of three biotites with largely differing Fe²⁺/Fe³⁺ ratios were studied before and after thermal dehydration and oxidation of divalent iron. Three absorption bands near 17,100, 20,500 and 24,100 cm^?1 and an absorption edge at slightly higher energies are assigned to trivalent iron present in clusters of strongly interacting ions. The presence of additional broad absorption bands due to intervalence transfer between Fe²⁺ and Fe³⁺ or Ti⁴⁺ in this region cannot be excluded for biotites with high Fe²⁺ concentrations. Three bands at lower energies show a satisfactory correlation with concentration of divalent iron and decrease in the same proportions with oxidation. We therefore assign them to split components of the spin-allowed ligand field transition of Fe²⁺ at the M ₁ and M ₂ sites. This contradicts the assignment of one of these bands to an intervalence charge transfer between Fe²⁺ and Fe³⁺ by previous authors. It is shown that there is no indisputable evidence against our assignment.     

Apparent partial loss age spectra of Neoarchean hornblende (Murmansk Terrane,Kola Peninsula,Russia): the role of biotite inclusions revealed by 40Ar/39Ar laserprobe analysis

Metamorphic hornblende frequently yields spectra with progressively increasing ⁴⁰Ar/³⁹Ar age steps, often interpreted as caused by partial resetting due to thermally activated radiogenic argon loss by solid‐state diffusion. Yet, in many cases rising Ca/K ratio spectra for such samples imply the presence of minor inclusions of K‐contaminant minerals. To avoid parts of grains with mineral inclusions or compositional zoning we drilled tiny discs from thin sections under a petrographic microscope. Laser step‐heating of drilled biotite‐free hornblende discs yielded flat age and ratio spectra. In contrast, furnace step‐heated hornblende separates from the same samples produced apparent loss age spectra. Moreover, biotite‐free samples yielded flat spectra by laser and furnace dating. Consequently, apparent loss spectra result from degassing of included substantially younger biotite before its hornblende host during laboratory step‐heating; c. 2640 Ma hornblende ages constrain the Murmansk Terrane's cooling.     

Neutron irradiation and post-irradiation annealing of rutile (TiO2−x ): effect on hydrogen incorporation and optical absorption

Neutron irradiation and post-irradiation annealing under oxidising and reducing conditions have been used to investigate H incorporation in, and the optical properties of, reduced (TiO_2−x ) rutile. Optical absorption in rutile is mainly due to a Ti³⁺ Ti⁴⁺ intervalence charge transfer effect. The main mechanism for H incorporation in rutile involves interstitial H not coupled to other defects, which has important implications for the rate of H diffusion, and possibly also on the electrical properties of rutile. Additional minor OH absorption bands in IR spectra indicate that a small amount of interstitial H is coupled to defects such as Ti³⁺ on the main octahedral site, and indicates that more than one H incorporation mechanism may operate. Concentration of oxygen vacancies has a controlling influence on the H affinity of rutile.     

Coexisting hornblendes and biotites from Precambrian gneisses of the south coast of Western Australia

The compositions of coexisting hornblendes and biotites from amphibolite and granulite facies gneisses from the south coast of Western Australia were controlled by host rock composition, paragenesis, metamophic grade, pressure, and oxygen fugacity. The Mg/(Mg + Fe²⁺) and Mn/Fe²⁺ ratios in both minerals and possibly the Al^vi contents of the hornblendes are related to host rock compositions. Metamorphic grade appears to influence, perhaps only indirectly, the Ti, Mn, and Fe³⁺ contents of both minerals and possibly the hornblende Ca content. The higher Ti and lower Mn contents of the granulite facies hornblendes and biotites are attributed to their coexistence with pyroxenes, whereas their lower Fe³⁺/(Fe²⁺ + Fe³⁺) ratios are probably due to lower oxygen fugacity in the granulite facies environment. Grade-related colour variations in both minerals were controlled by their Ti/Fe²⁺ and Fe³⁺/(Fe²⁺ + Fe³⁺ ratios. The relatively low Al^vi contents of the hornblendes suggest low- to moderate-pressure metamorphism.Variations in element distribution coefficients are related to variations in mineral compositions rather than metamorphic grade. Thus K_{D(Al^iv ?Si)} is related to the Al^iv andedenite alkali contents of the hornblendes, K_{D(Fe²⁺ ?Mg)} to the distributions of Al^iv ?Si and Al^vi + Ti + Fe³⁺, K_D(Mn) to the Mn contents of both minerals, and K_D(Al^vi) to the Al^vi contents of the biotites.     

Geochemistry of biotites from granitic rocks,Northern Portugal

The biotites from a series of rocks ranging in composition from tonalite to granite have been analysed for both major and trace elements.The relations between chemical composition and paragenesis of the biotites are studied. Most biotites co-exist with potassium feldspar and ilmenite. Variations in composition can be correlated with the occurrence of amphibole, primary muscovite and aluminosilicates in the rocks.Variation diagrams of the trace element contents and element ratios of biotite are compared to those of the host rocks. Fractionation of elements can be defined more accurately as the influence of other mineral phases is eliminated.Variations in the proportions of the octahedrally co-ordinated Al, Ti and Fe³⁺ are correlated with the conditions of crystallization and comparisons made with biotites from other suites of calc-alkali rocks.In the light of the experimental data available, the petrographic observations and the chemical data it is apparent that biotites crystallized from systems in which fO₂ was buffered, its values remaining close to that of the buffer FMQ. From the same data, a temperature of 800°C for fO₂ = 10^?14to 10^?15 bars is deduced as prevalent during the crystallization of the tonalites while for the granites, at a temperature of crystallization of 680°C, fO₂ = 10^?16to 10^?18 bars.A calc-alkali trend of fractionation is therefore apparent with decreasing fO₂ while fH₂O₂ remains relatively high.     

The distribution of fluorine among some coexisting minerals

Coexisting Ca-amphiboles, biotites, apatites, and sphenes were analysed for fluorine by a micro-probe. Fifty samples from four areas were examined. The distribution coefficient K _D(F) ^A-B for coexisting Ca-amphibole and biotite varies between 0.5 and 1.1 and is positively correlated with increasing metamorphic grade and with increasing contents of Ti in Ca-amphibole. Samples carrying hematite have normally a higher content of fluorine in the hydrous minerals due to an increasing activity of fluorine during the oxidation. It is shown that apatite mirrors a low activity of fluorine better than coexisting biotite, which in turn is a more sensitive indicator at higher F-activities. The distribution of fluorine between sphene and biotite is also discussed.     

Valence state of titanium in the Wark-Lovering rim of a Leoville CAI as a record of progressive oxidation in the early Solar Nebula

Simon et al. (2005) reported low Ti³⁺/Ti⁴⁺ values in Ti-rich pyroxenes in the Wark-Lovering rim (WL) of a Leoville CAI (144A) as compared to the interior of the inclusion. These electron microprobe analyses were interpreted as evidence that the growth of the WL rim is the manifestation of an evolution to a more oxidizing environment. Further work by Simon et al. (2007) used XANES analyses to argue for higher Ti³⁺ abundances and interpreted the data of Simon et al. (2005) as the result of X-ray contamination by neighboring phases, specifically spinel. Late-stage alteration was also included as a possible explanation.To investigate further the oxidation state of Ti in WL rims, we re-analyzed Leoville 144A to obtain a more complete data set of Ti³⁺/Ti⁴⁺ values in the Wark-Lovering rims. We conducted experiments on spinel-mixing to determine whether this was a plausible explanation for the observed paucity of Ti³⁺ in WL rims. While we found a wider range of Ti³⁺/Ti⁴⁺ in these WL rim data than in our original study, our new data show that the original conclusion that rims are lower in Ti³⁺/Ti⁴⁺ than interiors remains valid. We conclusively rule out spinel-mixing as an explanation for our data, and we see no clear inconsistency between our electron microprobe data and the XANES data. The WL rim of CAI Ef3 was also analyzed by EMPA and compared to the results of Leoville 144A.To predict compositional consequences of this hypothesis, we constructed a reaction space between Ti-rich pyroxene in the WL rim, perovskite, Mg_(g), Ca_(g), O_2(g), and SiO_(g). We find the oxidation of Ti³⁺, coupled with Ti loss via perovskite formation, explains many features of WL rim EMPA analyses. We maintain that the WL rim pyroxenes are compositionally distinct from those in the interior, and are evidence of a more oxidizing environment during WL rim formation.     

On dehydroxylation mechanisms of a biotite in vacuo and in oxygen

The dehydroxylation mechanisms of a biotite in the temperature range 300–800° C at controlled oxygen pressures were studied by infrared (IR) and Mössbauer spectroscopies. At low pressure, the lack of O₂ delays the oxidation of Fe⁺² and simultaneous loss of hydrogen but favours the loss of OH associated with octahedral vacancies in comparison with heat treatment in air. Stoichiometrical considerations based on the facts that the octahedral cations are randomly distributed and all Fe⁺² is oxidized lead to the conclusion that all the OH groups initially coordinated to Fe⁺² must be lost during the oxidation process.     

The crystal structure of disordered (Zr,Ti)O2 solid solution including srilankite: evolution towards tetragonal ZrO2 with increasing Zr

Crystal structure data are presented for seven synthetic samples of disordered zirconium-titanate solid solution (Zr,Ti)O₂, ranging in composition from x _Ti=0.43 to 0.67, thus covering compounds such as ZrTiO₄, Zr₅Ti₇O₂₄, and ZrTi₂O₆ (srilankite). The compounds, synthesized at high temperatures and various pressures in their respective stability fields, are well crystallized and of homogeneous composition. The resulting structure data are less scattered compared to previous studies that were based on compounds synthesized metastably at low temperatures and room pressure. The compounds have the structure of scrutinyite (α-PbO₂) with space group Pbcn, Z=4, unit cell parameters a=4.8495(3) Å, b=5.4635(3) Å, c=5.0462(3) Å at x _Ti=0.425 to a=4.7112(2) Å, b=5.4944(1) Å, c=4.9962(1) Å at x _Ti=0.666. The first structure refinement of pure, synthetic srilankite is presented, which is in good agreement with that of the natural counterpart. Structural trends observed in disordered zirconium-titanate solid solution along the binary join ZrO₂–TiO₂ are relatively smooth and continuous, except for rapid lengthening of an unshared octahedral edge which is anomalously short in scrutinyite-structure TiO₂. The shortness of this edge may explain the observed instability of this structure with the relatively small Ti as the dominant cation. With increasing Zr content, the average cation position moves off-centre inside the octahedron, away from two shared edges, which permits the 12 closest cation–cation distances in the structure to become more equal. The shortening of the b dimension with increasing amount of the larger cation Zr decreases the distance between octahedral Zr and two additional oxygens in an adjacent chain of edge-sharing octahedra, implying that the Zr environment is evolving towards eightfold coordination. If the two additional oxygens are considered as part of the Zr coordination polyhedron, the bonding topology of tetragonal zirconia is obtained. The compositional evolution of the cell parameters, Zr atomic coordinates and Zr coordination environment is consistent with the idea that the structure is evolving towards that of tetragonal ZrO₂. Group-theoretical relationships between scrutinyite, tetragonal zirconia, baddeleyite and fluorite structures show that the sequence of structures fluorite > tetragonal zirconia > scrutinyite > baddeleyite are all related by potentially diffusionless phase transitions driven by wavelike displacements of the oxygen substructure. The scrutinyite and tetragonal structures can act outside their stability fields as “transition states” between the structures on either side.     

Single crystal absorption spectra of synthetic Ti,Fe-substituted pyropes

Synthetic pyrope crystals up to 0.5 mm in diameter, substituted by titanium or by titanium plus iron, were grown under defined conditions of P, T, $f_{O_2 }$ in the presence of water using a piston-cylinder device. The crystals were characterized by X-ray and microprobe techniques. Their single-crystal optical absorption spectra were measured by means of a microscope-spectrometer. Two absorption bands at 16100 and 22300 cm{cm-1} in the spectra of pale-blue Fe-free Ti-bearing pyropes, grown under reduced conditions, were identified as originating from spin-allowed transitions, derived from ² T _2g → ² E _g of octahedral Ti³⁺ ions. The splitting value of the excited ²E _g state, 6200 cm^-1, and the crystal field parameter of Ti³⁺ in pyrope Δ ₀ = 19 200 cm^-1 are both in agreement with literature data. In spectra of brown Fe, Ti-bearing garnets, a broad band at 23000 cm^-1 was interpreted as a Fe^2+[8] → Ti^4+[6] charge-transfer band. The spectral position and width of this band agree with those observed for a FeTi charge transfer band in natural garnets. Fe, Ti-containing garnets synthesized at relatively high oxygen fugacity (10^-11,0 atm), which permits a fraction of Fe³⁺ to enter the garnet, show an additional Fe^2+[8] → Fe^3+[6] charge transfer band at 19800 cm^-1.