Volume 90, Issue 11-12

This study demonstrates how high-resolution X-ray computed tomography can be used to determine the geometry of curved inclusion trails in garnet porphyroblasts. For the first time, the three-dimensional geometry of these features is shown as an animation, a stereo pair, a VRML model, and two-dimensional sections. The spatial resolution of the data in this study, approximately 10 μm, is high enough to resolve inclusions, and finer resolutions are possible. Comparisons of scan imagery with thin section and microprobe data demonstrate that many typical inclusion phases in garnet porphyroblasts can be clearly distinguished, including quartz, ilmenite, and monazite. A sodium metatungstate solution with a density of 1.19 g/cm 3 was used for a wedge calibration during scanning, reducing artifacts to a minimum and allowing maximum image contrast. The overall conclusion of this study is that highresolution X-ray computed tomography is a quick, non-destructive technique that is invaluable in the study of porphyroblast microstructure. The three-dimensional data generated by this technique can be compared with geometries predicted by the varying theoretical models for the formation of curved inclusion trails in garnet porphyroblasts. They can also be analyzed to determine the three-dimensional orientation of curvature axes in individual porphyroblasts, which has not been possible until now.

The detailed crystallography of natural pyrite framboids has been determined for the first time using electron backscatter diffraction techniques. The crystallographic ordering of microcrystals correlates positively with morphological ordering; the crystallographic orientations are random in morphologically disordered framboids and are almost ordered in morphologically ordered framboids. Morphologically ordered framboids involve two types of systematic misorientations across the microcrystal boundaries: low-angle (ca. <20°) and high-angle (ca. 70.90°) misorientations. The low-angle misorientation probably reflects slight physical misalignment of microcrystals in the packing structure, whereas the high-angle misorientation is considered to result from the dichotomy of the pyrite microcrystals having fourfold morphological symmetry but only twofold crystallographic symmetry about <100>. Thus, the crystallographic orientation of microcrystals is not uniform, even in highly ordered framboids. This suggests that the self-organization of microcrystals in pyrite framboids is not crystallographically controlled, for example by sequential replication of existing microcrystals, since this would not result in high lattice misorientation angles between adjacent microcrystals. Presumably, the self-organization process is a consequence of the aggregation of multiple equidimensional and equimorphic microcrystals that have nucleated in a fixed volume. We suggest that the regular arrangement of microcrystals occurs by the physical rotation (reorientation) of individual microcrystals, driven by the reduction in surface free energy between neighbors.

The iridescence colors of abalone shell arise from Bragg diffraction of light from the layers of the nacre. The thickness of the aragonite nacre tiles is locally regular but varies during the growth cycles of the shell and this can give rise to complex color play. In Paua shell (Haliotis iris) and in H. fulgens (particularly the muscle scar shell) the thickness of the nacre tiles varies from 0.25(3) to 0.39(3) μm, but locally the thickness is constant within domains of hundreds of tiles. Other species such as H. laevigata and H. rufescens are similar, but their tile thicknesses range from 0.35(4) to 0.52(4) μm. In all species, the color displayed changes with observation angle and is due to layer diffraction. In H. iris and H. fulgens, the colors displayed encompass the complete visible spectrum; color hues are pure and are well-defined first-order diffraction colors. Shells of the other species display red and green, but not blue colors. The colors are rendered most vividly where dark organic growth layers are formed. These absorb or scatter light and enhance the iridescence colors. The origin and nature of the diffraction colors are compared with those observed in labradorite and opal. The degree of regularity in tile thickness needed to allow diffraction colors to be generated is modeled using pearl (sheet nacre) and abalone shell (columnar nacre) as examples. The wavelength dispersion is proportional to the product of the squares of the refractive indices of the material, the normalized standard deviation of the thickness, and the order of the diffraction color. For this reason, only first-order diffraction color is seen from shells.

A metapelite from the easternmost Wyoming craton (Black Hills, South Dakota) has been analyzed by microstructural methods to unravel polyphase deformational history associated with 1800.1700 Ma assembly of southern Laurentia. Three deformational fabrics are recognized in oriented thin sections: an ENE-trending S 1 fabric, preserved as oblique inclusion trails in garnet porphyroblasts; a NNWtrending S 2 fabric, preserved as microlithons in the rock matrix; and a flattening fabric, S 3 , which transposed S 1 -S 2 and dominates the matrix. A complex monazite porphyroblast has been analyzed in situ with the electron microprobe (Ultrachron) to constrain the timing of S 1 -S 3 fabric formation associated with monazite growth. The core of this grain uniquely preserves the S 1 -S 2 fabrics as sigmoidal inclusion trails. The mean total-Pb age of this domain is 1750 ± 10 Ma (all dates reported at 95% confidence; n = 39 spots), which is equivalent to the published 207 Pb/ 206 Pb age for the same domain. These results validate the total-Pb dating method in general and the Ultrachron in particular, for reliable age determination in low-Th monazite, and are interpreted as 1750 Ma minimum ages for the S 1 -S 2 fabrics and sequential, D 1 -D 2 collisional events that imposed them (~N-directed arc accretion and ~E-W continental collision, respectively). A higher-Th,Y rim of this same .Rosetta. grain truncates the S 1 -S 2 sigmoid, and is associated with resorption textures in garnet porphyroblasts, coupled release of Y, and an S 3 fabric that pervasively overprinted S 1 -S 2 in the rock matrix. The mean Ultrachron date of this domain is 1692 ± 5 Ma (n = 17 spots), which is slightly younger that the published isotopic age for all monazite rims combined. These results support a ~1715.1690 Ma timeframe for localized doming (D 3 ) related to granite magmatism, the onset of which has been dated independently at 1715 ± 3 Ma. The timing of post-D 3 cooling through 350 and 300 °C is constrained by 40 Ar/ 39 Ar dates of ~1610 and ~1480 Ma obtained for separates of D 3 matrix muscovite and biotite, respectively, which are interpreted as closure ages. This study shows that fabrics in poly-deformed rocks can be dated by linking monazite spot ages to key microtextures. Further, the results of this micrometer-scale study enhance previous knowledge of local thermotectonism (Black Hills) and regional terrane assembly (Laurentia).

Five unique mineral assemblages that include the sulfates millosevichite, alunogen, anhydrite, tschermigite, coquimbite, voltaite, and godovikovite, as well as the halide salammoniac and an unidentified phase, according to X-ray diffraction and EDS data, were found as encrustations on quartzofeldspathic sand and sandstone adjacent to coal-fire gas vents associated with underground coal fires in the Wuda coalfield of Inner Mongolia. The mineral assemblage of alunogen, coquimbite, voltaite, and the unidentified phase collected from the same gas vent, is documented for the first time. Coquimbite also occurs as rosettes secondarily nucleated on a cryptocrystalline mass of alunogen, coquimbite, voltaite, and the unidentified phase during storage in a sealed container at room temperature. Field observations, analyses of vent gases, SEM images, and mineral compositions suggest that the sulfates millosevichite, alunogen, coquimbite, voltaite, godovikovite, and the unidentified phase, crystallized in response to a complex sequence of processes that include condensation, hydrothermal alteration, crystallization from solution, fluctuating vent temperatures, boiling, and dehydration reactions, whereas the halide salammoniac crystallized during the sublimation of coal-fire gas. Tschermigite and anhydrite formed by the reaction of coal-fire gas with quartzofeldspathic rock or by hydrothermal alteration of this rock and crystallization from an acid-rich aqueous solution. Variations in the mineral assemblages found at five gas vents are possibly due to differences in coal-bed chemistry, exchange reactions involving coal-fire gas, and the composition of sediment, rock, and aqueous solutions prior to the exhalation of gas at the surface, as well as the temperature and cooling rate at a vent. Few studies have addressed the interaction of coal-fire gas with sediment, rock, and aqueous solutions and the subsequent mineralization processes. Coal fires present opportunities for discovering rare and new mineral occurrences. These minerals have potentially important environmental significance and may be vectors for the transmission of toxins. Coal fires also provide insight for the recognition in the geologic record of preserved mineral assemblages that are diagnostic of ancient fires.

Diamond inclusions are of particular research interest in mantle petrology and diamond exploration as they provide direct information about the chemical composition of upper and lower mantle and about the petrogenetic sources of diamonds in a given deposit. The objective of the present work is to develop semi-quantitative analytical tools for non-destructive in situ identification and characterization of mineral inclusions in diamonds using synchrotron micro-X-ray Fluorescence (μSXRF) spectroscopy and micro-X-ray Absorption Near Edge Structure (μXANES) spectroscopy at a focused spot size of 4 to 5 micrometers. The data were collected at the Pacific Northwest Consortium (PNC-CAT) 20-ID microprobe beamline at the Advanced Photon Source, located at the Argonne National Laboratory, and yielded the first high-resolution maps of Ti, Cr, Fe, Ni, Cu, and Zn for natural diamond grains, along with quantitative μSXRF analysis of select chemical elements in exposed kimberlite indicator mineral grains. The distribution of diamond inclusions inside the natural diamond host, both visible and invisible using optical transmitted-light microscopy, can be mapped using synchrotron μXRF analysis. Overall, the relative abundances of chemical elements determined by μSXRF elemental analyses are broadly similar to their expected ratios in the mineral and therefore can be used to identify inclusions in diamonds in situ. Synchrotron μXRF quantitative analysis provides accurate estimates of Cr contents of exposed polished minerals when calibrated using the concentration of Fe as a standard. Corresponding Cr K-edge μXANES analyses on selected inclusions yield unique information regarding the formal oxidation state and local coordination of Cr.

The microstructure and composition, including chemical speciation of S, of the calcitic and aragonitic shell layers of two mollusc shells were investigated using a combination of thermogravimetric analyses, scanning electron microscopy, electron probe microanalyses, and X-ray absorption near-edge structure spectroscopy. Microprobe analyses show the different chemical contents of the shell layers, whereas in situ XANES maps and spectra show the dominance of organic sulfate over S aminoacids (cysteine, cystine, and methionine). Also, the distribution of S species is different for different structures within the shell. Growth lines are clearly seen in the chemical species maps. These results suggest that studies focussing exclusively on the protein contents of biominerals are not sufficient to understand the role of the organic matrices in the biomineralization and diagenetic processes.

To evaluate the dissolution processes of diamond crystals in kimberlitic and lamproitic magmas, a series of diamond dissolution experiments were carried out in the graphite stability field at 1300-1420 °C and 1 GPa under the WI, MW, and HM buffers. Dissolution agents used include an aphanitic kimberlite from Wesselton Mine, South Africa, and a lamproite from Mount North, West Kimberley, Australia. With increasing run duration, diamond morphology changed from a sharp octahedron through combinations of octahedron and tetrahexahedroidal forms to spherical tetrahexahedroid having rounded faces. Negatively oriented trigons were formed on the octahedral {111} face. As the degree of diamond dissolution increased, the trigons changed from smaller shallow types to larger deep types. The dissolution rate in the kimberlitic solvent at 1300 °C was 0.12 mm/h under the HM buffer, 0.0034 mm/h under the MW buffer, and 0.0017 mm/h under the WI buffer, whereas that at 1420 °C was 0.014 mm/h under the WI buffer. In the lamproitic solvent, the dissolution rate was 0.0024 mm/h at 1420 °C under the WI buffer. The data indicate that diamond dissolves in silicate melts as carbonate ions through an oxidizing reaction. The degree of dissolution strongly depends on temperature, oxidation state, and the compositional dependence of CO 2 solubility in the melts.

Laser ablation.inductively coupled plasma.mass spectrometry (LA-ICP-MS) has become recognized as a sensitive, efficient, and cost-effective approach to measuring the major-, minor-, and trace-solute compositions of individual fluid inclusions in minerals. As a prerequisite for the routine analysis of natural inclusions in our laboratory, the precision and accuracy of the technique was assessed using sets of multi-element synthetic fluid inclusions. Five multi-element standard solutions were prepared, and incorporated as fluid inclusions in quartz crystals at 750 °C and 7 kbar. Fluid inclusions were ablated with a 193 nm ArF excimer laser and analyzed with a quadrupole ICP-MS, equipped with an octopole reaction cell for the removal of Ar-based interferences. The internal standard used in all cases was Na. Analytical precision for K, Rb, and Cs is typically better than 15% RSD, whereas Li, Mg, Ca, Sr, Ba, Mn, Fe, Cu, Zn, and Cl analyses are typically reproducible within 30% RSD. Measured concentrations approximate a Gaussian distribution, suggesting that analytical errors are random. Analyses for most elements are accurate within 15%. Limits of detection vary widely according to inclusion volume, but are 1 to 100 µg/g for most elements. These figures of merit are in excellent agreement with previous studies. We also demonstrate that, over the range investigated, precision and accuracy are insensitive to inclusion size and depth. Finally, the combination of our LAICP- MS analyses with microthermometric data shows that charge-balancing to NaCl-H 2 O equivalent chloride molality is the most valid approach to LA-ICP-MS data reduction, where chloride-dominated fluid inclusions are concerned.

A simple computer model for the dissolution kinetics of crystalline matter governed by etch-pit formation predicts different development, paths, and states for geometric, total (BET), and reactive surface area during the dissolution process. The model also explores the dynamics of the dissolution rate of a given model crystal surface as a function of the development of surface area. Because the surface area term is used in the normalization of bulk dissolution rates, results of this normalization reflect the large differences explored. Based on this evaluation, we discuss the application of the diversely defined surface area terms. In the light of this discussion, the likelihood of an unambiguous definition or application of reactive surface area is problematic. The model focuses on the relationship between the variation in total surface area and the global dissolution rate, and thus is independent of specific surface reaction mechanisms. The actual model calculations presented as an example in this paper utilize experimentally determined dissolution data of three dolomite [CaMg(CO 3 ) 2 ] cleavage surfaces obtained by vertical scanning interferometry (VSI). Similar data from minerals such as calcite, feldspars, and barite can be used and make this model applicable to a range of different crystalline phases.

The crystal-chemistry of 13 elbaite-schorl tourmaline crystals from the Cruzeiro pegmatite (Minas Gerais, Brazil) was studied with a multi-analytical approach (SREF, EMPA, SIMS, MS). Effective cation radii at the Y and Z sites and site populations were refined by a minimization procedure. The results indicate that the crystals belong to the alkali group. Elbaite crystals are O2.-free at the W and V sites and show OH content at the O2 site (up to 0.2 apfu). Conversely, schorl crystals always show O2. at the W site. The main substitutional mechanism is the dehydroxylation type: Y Fe 2+ + Y Fe 3+ + WO → Y Li + Y Al + W (OH+F). The T site is characterized by T Si → T Al substitution. <X-O> is linearly correlated with vacancy content in crystals with (OH + F) ≤ 4, whereas it is almost constant in crystals with OH at the O 2 position. Along the series, <Y-O> is inversely correlated with Y Al. The Z site is almost fully occupied by R 3+ (with Z Al largely dominant) and the Z Fe tot ↔ Z Al substitution explains the inverse correlation of <Z-O> with Z Al. In the elbaite compositional range, lattice parameters are functions of <Y-O>, whereas in the schorl range they are essentially functions of <Z-O>. Along the whole elbaite-schorl series, both chemical substitutions and size increase of Y are far larger than those of Z. In spite of this, lattice parameters increase with <Y-O> as much as with <Z-O>. This is due to the role of the [ZO 6 ] polyhedra, which extend along a and c to form the skeleton of the tourmaline structure. Therefore, any change in the size of Z leads to a change in the whole structure.

A series of controlled abiotic syntheses of magnetite nanocrystals were carried out to explore the possibility of using morphological criteria, crystal size distributions, and shape ratio as tools for identifying nanocrystals that would be specifically produced by magnetotactic bacteria. High-quality magnetite crystals synthesized with various controlled total iron concentrations were shown to have cubo-octahedral shapes and sizes varying from 4 to 24 nm. The mean particle size of the population was found to be 10.5 ± 0.7 nm and no significant effect of the total iron concentration on the particle size was observed. Systematical analyses of size and morphology also allowed for the determination of crystal size and shape ratio distributions. Crystal sizes were observed to follow log-normal distributions. Shape factors are bounded by one, with maxima between 0.80 to 1.00. Their distributions are asymmetric, with a cut off toward the high values. Crystal morphologies and shape factors appear not to be a powerful diagnostic tool for the differentiation of abiotic vs. biotic particles. However, crystal size distributions of abiotic crystals are significantly different from those of biotic populations. Indeed, opposite asymmetry of the size distributions from biogenic and non biogenic crystals was observed, with cut off toward larger sizes for biogenic nanocrystals and with cut off toward smaller sizes for abiogenic nanocrystals. This therefore constitutes a potential diagnostic tool for deciphering magnetite origin.

Fragmentation of crystals is an important mechanism, and a component of particle dynamics in igneous and metamorphic rocks that has received surprisingly little attention. Recent advances in textural analysis, extraction techniques, digital imaging, and computer-assisted measurements enable rapid accumulation of 3D data on particle shapes and size distributions. This paper reviews fragment size distributions (FSD) that result from fragmentation: lognormal, fractal, loggamma, and Weibull; discusses their genesis mechanisms; and presents relevant examples of fragmentation from experimental physics. Next, the paper considers FSDs of feldspars on digitized images of thin sections and on published images, and quartz extracted from vesicular pumice by acid from eight well-known large-volume eruptive units. The acid solution of pumice enables examination of volume abundance, 3D shapes, proportions of fragmented crystals, and measurements of their CSDs and FSDs. FSDs were also measured in samples of welded tuff and a granite disaggregated by electric pulse. Products of syneruptive shock wave fragmentation, and fragmentation by an electric pulse are found to be fractal with large breakage probabilities, branching ratios, and fractal dimensions of 2 to 3. In contrast, most quartz fragments in pumice obey a lognormal distribution and fragmentation is driven by a melt inclusion decrepitation mechanism, which results in low breakage probability and small number (2−3) of fragments per breakage cycle. These results are consistent with one atmosphere heating experiments of quartz phenocrysts that led to melt inclusion decrepitation and caused quartz to break up into several smaller pieces collectively having lognormal FSD. Measured melt inclusion size distributions suggest decrepitation of outermost melt inclusions, and low survival rate for large inclusions, and inclusions with large radius/crystal size ratio. The modeling of periodic fragmentation of crystals with melt inclusions due to overheating and/or decompression, which may occur many times during the lifetime of a long-lived magma body, may explain concave-down, lognormal CSDs abundant in igneous rocks. The genesis of lognormality can be explained by the fragmentation algorithm of Kolmogorov (1941). Other algorithms may generate lognormal-like loggamma distributions. Fragmentation serves as an important size limiting factor, a nucleation aid, and it facilitates isotopic and trace elemental exchange.

The kinetics of the Fe-Mg intracrystalline exchange reaction in P2 1 /c pigeonite (Wo 10 En 47 Fs 43 ) free of exsolved augite, from the Paraná rhyodacite sample BTS308, was studied by single-crystal X-ray diffraction (XRD). Isothermal disordering annealing experiments, with oxygen fugacity controlled at the IW buffer, were performed on two crystals at 650, 700, 750, and 800 °C until the Fe-Mg exchange equilibrium was reached. The XRD data were collected from the two untreated crystals and after each annealing experiment. Structure refinements were carried out taking into account the recently discovered stronger preference of Mn for the M2 site compared to Fe 2+ . The linear regression of ln k D * vs. 1/T yielded the following equation: ln k D * = -2925(± 110)/T(K) + 0.574(± 0.111); (R 2 = 0.997) The T c values calculated using this equation were 566 (±6) and 571 (±6) °C for the two crystals. Analysis of the kinetic data was performed according to Mueller's model, which allowed retrieval of the disordering rate constants C 0 K dis + for all four temperatures. The Arrhenius relation: ln K dis + = ln K 0 - Q/(RT) = 20.45(± 1.91) . 25191 (± 1900)/T(K); (R 2 = 0.989) yielded an activation energy of 50.03 (±3) kcal/mol for the Fe-Mg exchange process. Cooling time constants, calculated at the QMF buffer conditions of the host rock were, for the two crystals, η = 0.94 × 10 -1 K -1 year -1 and η = 1.10 × 10 -1 K -1 year -1 , and gave cooling rates on the order of 10 °C/h consistent with very fast lava cooling.

The mechanical behavior of minerals under high pressure has attracted considerable interest in recent years. Clay minerals, including kaolinite, are common minerals found in sedimentary environments and their behavior under pressure is an important factor in understanding seismogenic zones. Kaolinite is a ubiquitous member of the kaolin group of 1:1 phyllosilicates that have recently been found to undergo a structural phase transition between 2.0 and 2.5 GPa. In this study, the bulk modulus and elastic constants of kaolinite have been calculated from first principles within density functional theory (DFT) for the first time. The bulk modulus is predicted to be 23 GPa for kaolinite. The calculated elastic constant tensors indicate that the a direction is slightly more flexible than the b direction. The calculated elastic constant tensor along c is much lower than the constants calculated along a and b consistent with the crystal structure of kaolinite. Elastic wave velocities for P- and S-waves were calculated to be 7.34 and 3.50 km/s, respectively. Since an agreement between the theoretical and experimental values is satisfactory, we conclude that the theoretical calculations presented here are useful in seismic research for predicting the mechanical properties of minerals that are difficult to obtain experimentally because of their small particle size (typically <2 micrometers).

Trans-vacant and cis-vacant polymorphs of smectites and illites were distinguished by studying different cation substitutions in octahedral and tetrahedral sheets and in the interlayer. The standard Kohn-Sham self-consistent density functional method was used in the generalized gradient approximation (GGA) with numerical atomic orbitals as the basis set. The calculations reproduce the differences in the lattice parameters between the cis-vacant and trans-vacant configurations as observed from experimental studies of phyllosilicates. This theoretical approach is a useful tool for predicting crystallographic properties that must be calculated for smectites and illites because they cannot be determined experimentally in these clay minerals, especially the cis-vacant and trans-vacant configurations. The effect of cation substitutions in the octahedral and tetrahedral sheets on various structural features is also presented. The calculated effects are consistent with experimental results. The energy differences between the cis-vacant and trans-vacant polymorphs for a given composition are lower than the energy changes produced by the relative cation distributions in the octahedral sheet. Nevertheless, in the most illitic samples the trans-vacant arrangement is more stable than the cis-vacant form, in accord with experimental studies.

Raman analyses of single crystals of ikaite, CaCO 3 ·6H 2 O, synthesized in a diamond-anvil cell at ambient temperature yield spectra from 0.14 to 4.08 GPa; the most intense peaks are at 228 and 1081 cm -1 corresponding to E g (external) and A 1g (internal) modes of vibrations in CO 3 2. ions, respectively. These are in good agreement with Raman spectra previously published for ikaite in powder form at ambient temperature and pressure. Visual observations of a sample consisting initially of a mixture of calcite + water in a hydrothermal diamond-anvil cell yielded a P-T phase diagram up to 2 GPa and 120 °C; the boundary for the reaction ikaite ↔ aragonite + water has a positive slope and is curved convexly toward the aragonite + water field similar to typical melt curves. This curvature can be explained in terms of the Clapeyron equation for a boundary between a solid phase and a more compressible liquid phase or largely liquid phase assemblage.

Variations in unit-cell volumes of mantle minerals as functions of P and T are important parameters in the description of the interior of the Earth and the behavior of materials. Recently, Thompson and Downs (2004) presented a model for the crystal structures of pyroxenes parameterized in terms of the O3-O3-O3 angle, θ, and the oxygen radius, r. This model has proven useful in the analysis of compression and expansion mechanisms in pyroxenes, providing an understanding of θ and r as functions of P and T. However, it did not provide a basis for analyzing changes in some properties that are strongly dependent on composition. In this paper, we show that ambient unit-cell volumes of the C2/c pyroxenes are correlated with M1 cation radius. This relationship can be used to calculate model ambient unit-cell volumes as a function of chemistry. From this starting point, pyroxene unit-cell volume variation with P and T can be modeled as a function of θ(P,T) and r(P,T). These relationships are investigated for diopside, hedenbergite, acmite, jadeite, and kosmochlor. The model reproduces observed unit-cell volumes of these phases recorded at P to within 0.09% and at T to within 0.10%, at simultaneous P and T for jadeite to within 0.57%, and at simultaneous P and T for diopside to within 1.20%. K o and K’ from third-order Birch-Murnaghan fits to the observed volume vs. pressure relationships and those calculated from the Thompson-Downs model are statistically the same. The fit of the Thompson-Downs EOS to the observed data is compared to the fit of the third order Birch-Murnaghan. The model is used to create an algorithm that estimates volumes for C2/c pyroxenes as a function of P, T, and x.

The transformations of 2-line ferrihydrite to hematite (pH 10.7) or goethite (pH 13.7), and of phosphate-doped 2-line ferrihydrite to goethite (pH 13.7), were studied at 60.137 °C using synchrotron- based, in-situ energy dispersive powder diffraction (EDPD). The time-resolved data for the growth of the diffraction peaks were fitted with a pseudo first-order kinetic model. As shown in previous studies, the conditional rate constant of goethite formation increases with increasing pH and is significantly lower than that for hematite crystallization. The activation energies of nucleation for hematite (pH 10.7), pure goethite (pH 13.7), and phosphate-doped goethite (pH 13.7) are 24, 7, and 21 kJ/mol, respectively, whereas the activation energies of crystallization are 69, 39, and 26 kJ/mol. The crystallization of phosphate-doped ferrihydrite produced large rectangular goethite crystals with dense ferrihydrite cores on which the goethite grew epitaxially. The rate of goethite formation is greatly reduced in the presence of phosphate due to an increase in the entropic component of the free energy of activation. This increase in entropy arises from adsorption of phosphate on to the (210) crystal faces, with an associated increase in relative growth rate on the (101) faces.

The high sensitivity of the satellite-transition (ST) MAS NMR technique was exploited to obtain high-resolution 17 O MAS NMR spectra of the three polymorphs of Mg 2 SiO 4 : forsterite (α-Mg 2 SiO 4 ), wadsleyite (β-Mg 2 SiO 4 ), and ringwoodite (γ-Mg 2 SiO 4 ). High NMR sensitivity was important in this application because 17 O-enriched, Fe-free materials are required for 17 O NMR and high-pressure syntheses of the dense β and γ polymorphs result in a only a few milligrams of these solids. In all, eight distinct O species were identified and assigned: three in forsterite, four in wadsleyite, and one in ringwoodite, in agreement with the number of O sites in their crystal structures. The isotropic chemical shifts extracted are in excellent agreement with a previously published correlation with Si-O bond length. However, unexpectedly large quadrupolar coupling constants were found for the non-bridging O species in the dense polymorphs wadsleyite and ringwoodite.

Thermal histories of non-cumulate eucrites, Pasamonte and Haraiya, were determined from Ca/(Ca + Fe + Mg) profiles of host/lamellae pyroxene pairs, from compositional profiles of pyroxene rims affected by metasomatism, and from lamellae thicknesses using a rate law for lamellar coarsening. Pasamonte, a Type 2 eucrite, experienced “partial” equilibration during a relatively short period of thermal annealing. This equilibration is expressed texturally by metasomatized rims in Mg-rich and Fe-rich pyroxene grains and by coarsening of nanometer-scale exsolution lamellae. A cooling rate of ~10.2 °C/day corresponds to post-impact mixing and reburial of the polymict Pasamonte assemblage to a depth similar to that of the initial lava flow. Haraiya, a Type 7 eucrite, has experienced a complex thermal history involving multiple brecciation events and mixing of a single flow to produce a monomict assemblage containing clasts of variable grain size and texture. A period of thermal annealing, ~50 000 years in duration at temperatures between ~1000 and 700 ºC, corresponding to a cooling rate of ~10 -4 to 10 -5 ºC /day, produced augite exsolution up to 3 μm thick in pigeonite. Rapid burial by successive flows during a period of high magma productivity and high heat flow due to 26 Al decay in the crust and mantle was a possible source of heat for metamorphism in the lower crust. However, cooling rates during relaxation of the lower crustal thermal gradient are much less than the mineralogically determined cooling rates. Heat derived from impact or from mid- to upper-crustal level intrusions is a plausible heat source for metamorphism and is consistent with observed cooling rates.

The Li-F-Be-B-P bearing Pinilla de Fermoselle (PF) pegmatite occurs in the apical part of a leucogranite body. It shows a clear non-symmetrical vertical zoning from the contact with the leucogranite to a contact with the metamorphic country rocks. The pegmatitic facies evolve upward from (1) the undifferentiated Lower Border Zone (LBZ), with quartz, feldspars, muscovite, biotite, and black tourmaline, through (2) the Intermediate Zone (IZ), with quartz, muscovite, zinnwaldite, black tourmaline, and Fe-Mn phosphates, to (3) the highly evolved Upper Border Zone (UBZ), with quartz, albite, lepidolite, zinnwaldite, elbaite, and beryl. The composition of the pegmatite-forming minerals suggests that a residual melt become progressively enriched in F and Li until the crystallization of the apical UBZ, whereas P partitions in the melt only until the intermediate levels of differentiation attained in the IZ. Chemical variations in the mica and tourmaline as well as in the feldspar and Fe- Mn phosphate minerals are consistent with an internal evolution by crystal fractionation processes. A plausible model for the crystallization of the PF pegmatite involves a rapid, in situ, bottom-up crystallization from significantly undercooled liquids. The lack of metasomatic effects in the metamorphic host-rock and the estimated P content of the initial leucogranite melt suggest that the PF pegmatite mainly crystallized under closed system conditions.

Fourteen spinels from two types of mantle xenoliths from the San Carlos Volcanic Field in Arizona were characterized using single-crystal X-ray diffraction and electron microprobe analysis. The dominant feature seen in the chemistry of spinels from the Group I xenoliths is the extensive substitution of Cr for Al (Cr 0.20 Al 1.76 to Cr 0.83 Al 1.10 ) correlated with Mg for Fe 2+ (Mg 0.69 Fe 2+ 0.31 to Mg 0.80 Fe 2+ 0.19 ). Although Group II spinels display consistently low Cr values, they also show a well-correlated substitution of Mg for Fe 2+ (Mg 0.63 Fe 2+ 0.37 to Mg 0.69 Fe 2+ 0.31 ). Unit-cell parameters for spinels from the Group I xenoliths range from 8.1259 to 8.2167 Å, while those from the Group II xenoliths range from 8.1247 to 8.1569 Å. The cell parameters are linearly correlated with Fe 2+ and Cr contents. Cation distributions were determined from experimental bond lengths and refined site occupancies using the algorithm of Lavina et al. (2002). The San Carlos spinels display variable degrees of order, with inversion parameters ranging from 0.10 to 0.16 for Group I and from 0.17 to 0.22 for Group II. Closure temperatures were computed with the Princivalle equation, giving averages of 808(37) °C for spinels from Group I xenoliths and 822(62) °C for samples from Group II xenoliths. We show that these results are reasonable, and thus extend the use of the Princivalle equation, or at least its functional form, to samples with significant Cr and Fe 2+ contents. This study demonstrates that, in spite of the extensive chemical variability of the San Carlos spinels, and given that the origins of the two groups of xenoliths are different, the oxygen coordinates remain fixed, suggesting that the oxygen coordinate is a function of thermal history.

Solid-fluid interactions often involve the replacement of one phase by another while retaining the morphology and structural details of the parent phase, i.e, pseudomorphism. We present in situ observations of the evolution of both the solid and fluid compositions at the interface during such a replacement reaction in the model system KBr-KCl-H 2 O, in which a single crystal of KBr is replaced by a single crystal of KCl. The pseudomorphism is initiated by epitaxial growth at the fluid-mineral interface, when the dissolution of the parent phase results in an interfacial fluid layer that is supersaturated with respect to a different solid composition. The subsequent evolution of the coupled dissolution and growth can be related to local equilibrium defined by a Lippmann diagram. The reaction features, including the development of porosity in the new solid phase, share many characteristics of replacement reactions in nature as well as in technical applications

Polarized IR-absorption spectroscopy in the range 7000-1000 cm -1 has been used to examine natural cordierites of different geological origin and chemical composition. In the literature there is much information on H 2 O and CO 2 , as channel constituents in cordierite, though no isotopic species of these molecules have been distinguished. We newly identify 12 C 16 O 2 , 13 C 16 O 2 , 12 C 16 O 18 O, as well as 12 C 16 O from the single-crystal polarized IR spectra. The vibrational parameters, the temperature behavior, and polarization dependence of the identified carbon oxide molecules were studied in the temperature range 80-700 K. The molecular axes of the CO 2 and CO species are parallel to the a-axis of cordierite crystals. Repeated spectral measurements after heating confirm the low mobility along the c-axis channels of CO 2 and CO molecules at temperatures below 700 K. On the other hand, vibrational frequencies of CO 2 and CO species obtained in this investigation are close to those of the vapor phases, suggesting that the molecules are loosely bonded in the large cavities of the silicate framework.

Two distinct occurrences of wollastonite- and melilite-bearing rocks from Ricetto and Colle Fabbri, which are located in the central Italy Apennine Range, are referred to as “paralavas” generated by melting and recrystallization of marly sediments “likely due to coal fires” by Melluso et al. (2003). We submit data demonstrating that these conclusions are incorrect. Ricetto is clearly pyrometamorphic, as described by Capitanio et al. (2001, 2004). Colle Fabbri is clearly an igneous outcrop as described in Stoppa (1988). It is quite different in scale, field relationships, and chemistry, and thus Ricetto is irrelevant to its petrogenesis. The sediment mixing model of Melluso et al. (2003) does not relate to the Colle Fabbri field data. Furthermore, the Ricetto Miocene flysch country-rock and the Colle Fabbri enclosing Pliocene clays were not analyzed by Melluso et al. (2003). A limestone-shale mixing calculation using Sr and Nd isotopic data in Melluso et al. (2003, Tables 3.4) and a 143 Nd/ 144 Nd ratio of 0.51213 for Apennine shales (op.cit. p. 1297) fails to account for the Colle Fabbri samples, which remain well clear of the hypothetical mixing line. Rocks of mantle origin, including micaceous kimberlites and Western Australia lamproites, plot in the variation field of Colle Fabbri and regionally associated igneous rocks. Colle Fabbri is but one of a series of similar igneous melilite-bearing and carbonatite occurrences that constitute the Intramontane Ultra-alkaline Province of central Italy (IUP), although Colle Fabbri does have unique features, as do other of these occurrences (Stoppa et al. 2003).

We briefly review the petrological and geochemical features, and physical characteristics, of the Colle Fabbri and Ricetto rocks to discuss their possible paramagmatic origin, i.e., derived from complete melting, after devolatilization, of marly protoliths, without any orthomagmatic, ultimately mantle-derived, chemical or thermal input. Contrary to the various comments that have been made, the overall mineralogical, petrological, and geochemical data provided in the literature well agree with this finding.

The bimodal rock of Ricetto consists of a felsic glassy component, which prevails volumetrically at the top of the rock, and mafic spherules of acicular diopside + anorthite + wollastonite pseudomorph after melilite, which are concentrated at the bottom of the rock. This texture clearly indicates liquid immiscibility, which can be modeled in the compositional space SiO 2 - Al 2 O 3 -MgO-CaO and Al 2 O 3 -CaO-(FeO T + MgO). The spinifex and dendritic forms are due to quenching from a melt at temperature of 1000-1100 °C: melilite was the first phase to crystallize and the subsequent rapid cooling caused its breakdown to a more stable assemblage. The strict chemical similarity between the rock under study and the surrounding siliciclastic sandstones, together with the results of 14 C radiometric analysis on coal fragments found at bottom of the convex central part of the body, demonstrate that the rock of Ricetto is a man-made pyrometamorphic product. The genetic attribution of the Colle Fabbri occurrence to pyrometamorphic phenomena somewhat similar to Ricetto is difficult to be asserted, as the crystallization history at Colle Fabbri has been much more complicated than at Ricetto. The common presence of melilite and wollastonite cannot account for a direct comparison between the Ricetto and Colle Fabbri rocks, as conspicuous mineralogic, petrologic, and geologic differences exist between these two occurrences.

Two small outcrops of wollastonite- and melilite-bearing paralavas, with a pyrometamorphic genesis, have been found along the Apennine chain, in Central Italy, at the localities of Colle Fabbri and Ricetto. Recent papers added new data strengthening the hypothesis that the Ricetto wollastonite- and melilite-bearing rocks were produced by pyrometamorphism of carbonate-rich siliciclastic sedimentary rocks. In addition, these new papers depicted the Ricetto paralavas as having wide mineralogical and compositional variations due to liquid immiscibility and/or carbonate devolatilization, paralleling the trend observed for other paralavas (e.g., Colle Fabbri, Italy; British Columbia, Canada; Wyoming, U.S.A.; etc.). The source of heat for inducing pyrometamorphism was not investigated in the original paper, which was the subject of the comment. Capitanio et al. (2004), on the basis of 14 C age determination and of a mineralogical study, argued convincingly that heat was supplied to the Ricetto rocks by wood-combustion for charcoal production.