Volume 101, Issue 8

The Wooley Creek batholith is a Late Jurassic, arc-related, calc-alkaline plutonic complex in the Klamath Mountain province of California. Post-emplacement tilting and erosion have exposed ~12 km of structural relief. The complex consists of an older (~159.1 Ma) lower zone (pyroxenite to tonalite) assembled by piecemeal emplacement of many magma batches, a younger (~158.2 Ma) upper zone (quartz diorite to granite), and a transitional central zone. In the lower zone, pyroxenes are too Fe rich to be in equilibrium with a melt whose composition was that of the host rock. Mass-balance calculations and simulations using rhyolite-MELTS indicate that these rocks are cumulates of pyroxenes and plagioclase ± olivine and accessory apatite and oxides. Percentages of interstitial melt varied from ~7.5–83%. The plagioclase/pyroxene ratios of cumulates vary considerably among the most mafic rocks, but are relatively uniform among quartz diorite to tonalite. This near-constant ratio results in compositional trends that mimic a liquid line of descent. In the upper zone, bulk-rock compositional trends are consistent with differentiation of andesitic parental magmas. Upward gradation from quartz dioritic to granitic compositions, modeled via mass-balance calculations and rhyolite-MELTS simulations, indicate that structurally lower parts of the upper zone are cumulates of hornblende and plagioclase ± biotite and accessory minerals, with 37–80% trapped melt. In contrast, the structurally higher part of the upper zone represents differentiated magma that escaped the subjacent cumulates, representing differentiated melt fractions remaining from 92–54%. The ratio of cumulate plagioclase/(plagioclase + mafic minerals) is ~0.48 among upper-zone cumulates, mimicking a liquid line of descent. The results suggest that compositional variation in many calc-alkaline plutons may be at least as representative of crystal accumulation as of fractional crystallization. If so, then the assumption that arc plutons geochemically resemble frozen liquids is dubious and should be tested on a case-by-case basis. Moreover, comparisons of plutonic rock compositions with those of potentially comagmatic volcanic rocks will commonly yield spurious results unless accumulation in the plutons is accounted for.

This article reviews the development of mineralogy as a science by focusing largely on the common silicate garnets of general formula {X 3 }[Y 2 ](Si 3 )O 12 . It tells of important discoveries, analyses, and proposals by various scientists relating to crystallography, crystal structures, isomorphism, and solid solution starting in Europe in the late 1700s. The critical recognition of the importance of ionic size of atoms in determining crystal-chemical properties and solid-solution behavior is emphasized. The two garnet species “pyralspite” and “(u)grandite,” which were considered to represent two independent solid-solution series, were introduced by N.H. Winchell and A.N. Winchell (1927) in their well-known book Elements of Optical Mineralogy . Critical comments on the assumptions behind the classification scheme have been pointed out for at least 50 yr, but it remains in use. There is more, though, behind this garnet classification scheme than just simple terminology. There are a long series of scientific discoveries and advances that are largely forgotten by the broader mineralogical community. They begin, here, with the work of the “father of crystallography,” René-Just Haüy, concerning the microscopic nature of crystals around 1780 and include later discoveries and proposals by Mitscherlich, Beudant, Wollaston, and Kopp relating to isomorphism and solid-solution behavior all before 1850. A second key era started with the discovery of X-ray diffraction in 1912 that allowed the atomic structures of crystals and, furthermore, atomic and ion radii to be determined. In terms of isomorphism and solid solution, the proposals and studies of Vegard, Zambonini, Wherry, A.N. Winchell, and the “father of crystal chemistry” Goldschmidt are briefly discussed. The recognition of the sizes of atoms and ions, along with an understanding of chemical bonding behavior in crystals, was critical in the establishment of what can be termed “modern mineralogy,” a quantitative science as it is largely understood today that emerged by the mid-1930s. The silicate garnet system pyrope-almandine-spessartine-grossular-andradite-uvarovite shows extensive homovalent substitutional solid solution over two structural sites and complete compositional variation between “pyralspite species” and “ugrandite species” has been documented. Thus, the prerequisites behind the terms “pyralspite” and “(u)grandite,” as originally formulated and often accepted even today, are incorrect and use of this classification is not recommended. Diffraction determinations of the volumes of garnet end-members and volumes of mixing of garnet solid solutions give physical insight into solid-solution behavior. Today, investigations of local structural and crystal-chemical properties, together with determinations of lattice strain and thermodynamic mixing properties, of silicate solid solutions are leading to an ever more quantitative understanding of mineral behavior from the microscopic to macroscopic level.

The crystal structure of svabite, ideally Ca 5 (AsO 4 ) 3 F, was studied using a specimen from the Jakobsberg mine, Värmland, Sweden, by means of single-crystal X-ray diffraction data. The structure was refined to R 1 = 0.032 on the basis of 928 unique reflections with F o > 4σ( F o ) in the P 6 3 / m space group, with unit-cell parameters a = 9.7268(5), c = 6.9820(4) Å, V = 572.07(5) Å 3 . The chemical composition of the sample, determined by electron-microprobe analysis, is (in wt%, average of 10 spot analyses): SO 3 0.49, P 2 O 5 0.21, V 2 O 5 0.04, As 2 O 5 51.21, SiO 2 0.19, CaO 39.31, MnO 0.48, SrO 0.03, PbO 5.19, Na 2 O 0.13, F 2.12, Cl 0.08, H 2 O calc 0.33, O (≡ F+Cl) –0.91, total 98.90. On the basis of 13 anions per formula unit, the empirical formula corresponds to (Ca 4.66 Pb 0.16 Mn 0.04 Na 0.03 ) Σ4.89 (As 2.96 S 0.04 Si 0.02 P 0.02 ) Σ3.04 O 12 [F 0.74 (OH) 0.24 Cl 0.01 ]. Svabite is topologically similar to the other members of the apatite supergroup: columns of face-sharing M 1 polyhedra running along c are connected through TO 4 tetrahedra with channels hosting M 2 cations and X anions. The crystal structure of synthetic Ca 5 (AsO 4 ) 3 F was previously reported as triclinic. On the contrary, the present refinement of the crystal structure of svabite shows no deviations from the hexagonal symmetry. An accurate knowledge of the atomic arrangement of this apatite-remediation mineral represents an improvement in our understanding of minerals able to sequester and stabilize heavy metals such as arsenic in polluted areas.

High P-T experiments have shown that in major rock types such as MORBs, peridotites, and pelitic sediments, increasing P (and T) leads to a gradual transfer of P from phosphates mostly represented by apatite to silicates with garnet as most important silicate P carrier. This is due to the formation of a Na 3 Al 2 (PO 4 ) 3 phase component in garnet via [8] Na [4] P [8] M-î [4] Si_ 1 that is strongly P-, and to a lesser extent T-dependent and creates garnets with significant P and Na at P as low as 2–3 GPa. Based on this experimental evidence, one would expect to routinely find P-Na-rich garnets in UH P -rocks with a wide range in composition. With very few exceptions, however, this is not the case. This discrepancy indicates that both P and Na are effectively released from garnet and re-distributed within the garnet matrix during uplift and exhumation. To explore the mechanisms of this P-Na release, P-Na-rich garnet pre-synthesized at 7 GPa and 1200 °C, containing 0.7 wt% P 2 O 5 and 0.3 wt% Na 2 O, was exposed to P-T conditions of 2 GPa and 800–1000 °C in a simplified, H 2 O-bearing, model eclogitic bulk composition. The experiments show that at subsolidus temperatures of 850–975 °C, and in the presence of a hydrous fluid, apatite quickly forms from garnet breakdown involving consumption of coexisting quartz and clinopyroxene. The apatites usually appear as rounded to lath-shaped isolated grains scattered in the garnet + clinopyroxene ± orthopyroxene + quartz + rutile matrix. More rarely, single apatite inclusions, or clusters of inclusions, may form in clinopyoxene or garnet. The observed apatite grain size is in the range ≤ ~1 × 1 to 24 × 6 μm with the largest grains occasionally containing clinopyroxene inclusions. Combined garnet breakdown and neo-formation, using pre-existing garnet as a nucleation site, may form zoned garnets with Na-P-depleted and Ti-enriched rims that represent an approach to a garnet composition typical for mid- to shallow crustal P-T conditions. Partial melting experiments indicate that eclogites containing P-rich garnet may produce P-rich and apatite-undersaturated melts for moderately SiO 2 -rich melt compositions. These melts can crystallize abundant apatite during solidification and, thus, would be effective agents for P-extraction during partial melting. Due to their very low apatite saturation concentration, the P-transport and storage capacity of granitic melts would be much more limited. An unexpected finding of this study are the substantial P and Mg contents in kyanite with 0.17–0.20 wt% P 2 O 5 and 0.20–0.56 wt% MgO, respectively. The combined P-Mg incorporation into kyanite is consistent with the coupled substitution [4] Si 4+ + [6] Al 3+ = [4] P 5+ + [6] Mg 2+ and with a strong preference of P for orthosilicate structures. The results of this study suggest that some to even all of the apatite now present in eclogites that underwent deep subduction formed by chemical adjustment of the eclogite garnet to decreasing Na 3 Al 2 (PO 4 ) 3 solubility during uplift and exhumation. This results in the appearance of apatite as a new phase in a hitherto apatite-free assemblage. Rapid transport to the surface and/or a lack of suitable reactants can suppress this re-equilibration and explain the occasionally high P- and Na-contents of eclogitic garnet-inclusions in diamond or garnets from diamondiferous eclogites sampled by kimberlites. Similarly, concurrent decreasing Ti-solubility in garnet may lead to rutile-saturation in eclogites that did not contain this phase under peak-P conditions as evidenced by the joint occurrence of (oriented) rutile and apatite inclusions in the eclogitic garnets. For the application of geothermobarometry, this delayed Ti (+P) saturation is important to be kept in mind.

Fluorapatite-monazite-xenotime-allanite mineralogy, petrology, and textures are described for a suite of Kiruna-type apatite-iron oxide ore bodies from the Grängesberg Mining District in the Bergslagen ore province, south central Sweden. Fluorapatite occurs in three main lithological assemblages. These include: (1) the apatite-iron oxide ore bodies, (2) breccias associated with the ore bodies, which contain fragmented fluorapatite crystals, and (3) the variably altered host rocks, which contain sporadic, isolated fluorapatite grains or aggregates that are occasionally associated with magnetite in the silicate mineral matrix. Fluorapatite associated with the ore bodies is often zoned, with the outer rim enriched in Y+REE compared to the inner core. It contains sparse monazite inclusions. In the breccia, fluorapatite is rich in monazite-(Ce) ± xenotime-(Y) inclusions, especially in its cores, along with reworked, larger monazite grains along fluorapatite and other mineral grain rims. In the host rocks, a small subset of the fluorapatite grains contain monazite ± xenotime inclusions, while the large majority are devoid of inclusions. Overall, these monazites are relatively poor in Th and U. Allanite-(Ce) is found as inclusions and crack fillings in the fluorapatite from all three assemblage types as well as in the form of independent grains in the surrounding silicate mineral matrix in the host rocks. The apatite-iron oxide ore bodies are proposed to have an igneous, sub-volcanic origin, potentially accompanied by explosive eruptions, which were responsible for the accompanying fluorapatite-rich breccias. Metasomatic alteration of the ore bodies probably began during the later stages of crystallization from residual, magmatically derived HCl- and H 2 SO 4 -bearing fluids present along grain boundaries. This was most likely followed by fluid exchange between the ore and its host rocks, both immediately after emplacement of the apatite-iron oxide body, and during subsequent phases of regional metamorphism and deformation.

The method of accommodation of solid solution along the OH-Cl binary in calcium phosphate apatites is not fully understood; because of steric constraints in mixtures of OH and Cl anions in the apatite [0, 0, z] anion column, the positions of OH and Cl anions in the pure hydroxylapatite and chlorapatite end-members cannot coexist in the binary anion column. We have undertaken high-precision single-crystal X-ray structure studies of eight synthetic samples along the OH-Cl apatite binary (R 1 ≈ 0.0159). We found that for all samples solid solution is attainable in space group P 6 3 / m , but the particular method of solid solution depends on composition. For samples with Cl > OH, three column anion sites (two for Cl, one for OH) provide allowable bond distances with the Ca2 atoms and allow a sequence of column anions that provides sufficient anion-anion distances and also effects reversal of the sense of ordering of the column anions relative to the mirror planes at z = ¼ and ¾. In a sample with OH > Cl, three sites exist in the anion column that also provide allowable bond distances to the triangle of Ca2 atoms or its disordered Ca2' equivalent, and afford a sequence of atoms that permits reversal of the anion column and maintenance of P 6 3 / m symmetry. One of those sites is occupied by OH and provides acceptable Ca2-OH distances, and another accommodates Cl with ideal Ca2-Cl distances. A third column anion site is unique among the calcium phosphate apatites. That site, termed the ClOH site, accommodates both OH and Cl. The site has an ideal bond distance for OH to the Ca2 atoms in the Ca2 triangle and also has an ideal bond distance for a Cl occupant to disordered Ca2' atoms; thus, because of the disordering of the Ca2-Ca2' atoms, a single site can accommodate either anion with ideal, but disparate, bond distances to Ca. Finally, in OH-Cl apatites with OH ≈ Cl, also crystallizing in space group P 6 3 / m , four anion positions are occupied in the anion column, including the ClOH site that allows occupancy by both OH and Cl. In addition to that site and distinct OH and Cl sites, OH is found to occupy the site within the mirror plane at (0,0,¼), the site occupied by F in F-bearing apatite. Occupancy of that site is essential to reversing the sense of ordering of the anion column relative to the mirror planes and preserving P 6 3 / m symmetry. Thus, the methods of effecting solid solution along the OH-Cl are composition dependent and complex.

Highly restitic garnet-kyanite-phlogopite metapelitic schists from the Goshen Dome of western Massachusetts contain: a population of prograde monocrystalline, megacrystic garnet, some with significant P in substitution for Si; precipitates of hydroxylapatite and rutile; and <1 μm zircon crystals of undetermined origin and abundance on the order of 10 5 /mm 3 . The unusual P content and the abundant internal precipitate suite are similar to features reported in garnet from ultrahigh-pressure (UHP) and mantle settings, suggesting a potential (U)HP origin for the garnet megacrysts. Zircon included in megacrysts is surrounded by radial fractures, indicating in situ volumetric expansion or new growth. Cores display rare earth element (REE) profiles and cathodoluminescence (CL) zoning consistent with magmatic growth, and yield only Paleozoic dates (447–404 Ma). The embayed core–rim boundary is marked by a several micrometers wide band of CL-dark zircon enriched in Y, P, U, and Th that is interpreted as the accumulation of redistributed xenotime component from the original zircon rim during metamorphism. Outside of this band, the rim has elevated Hf, Th/U << 1, and steep heavy REE profiles. The metamorphic rims yield concordant dates from 400 to 381 Ma. Matrix zircon grains have magmatic cores (1726–415 Ma) with similar core-rim boundaries enriched in Y, P, U, and Th. Metamorphic rims on matrix zircon yield slightly younger dates (393–365 Ma) and are compositionally heterogeneous. The difference between the youngest core and oldest rim indicates a short interval (ca. 4 Ma) between deposition of detrital zircon and the onset of metamorphism in the earliest Acadian. The oldest zircon rim dates are found within phosphatic garnet megacrysts of possible very high-pressure origin. The compositional uniformity of these rims indicates equilibrium with a single source; the anomalous composition suggests a combination of dissolution-reprecipitation and new growth of zircon that is derived from garnet. The range in both composition and dates indicates that matrix zircon rims formed in response to local changes in mineralogy and fluid/melt composition and/or availability. New growth of zircon on these grains cannot be confirmed, suggesting that dissolution-reprecipitation reactions during continued metamorphism may be the dominant mechanism that formed these rims. The data collectively suggest that dissolution-reprecipitation may be a common mechanism for producing metamorphic rims on zircon that does not require additional Zr and Hf, which are limited within most metamorphic settings.

Erupted absarokitic-shoshonitic volcanics in subduction zones make up a relatively small proportion of the total erupted volcanic material. However, they are a critical component for quantifying subduction metasomatism and mantle source heterogeneity. This study examines the geochemistry of anomalous olivine within a forearc absarokite lava and associated tephra, a compositional end-member of the Cascadia subduction zone magmatism, to infer mantle heterogeneity and magma petrogenesis. Ni concentrations from the young (42 ka) Quartzville absarokitic basalt lava flow and tephra in the Cascade forearc are up to ~6400 ppm Ni with correspondingly low Ca, averaging ~850 ppm Ca in Fo 91 olivine cores. Decreasing Ni and Ca toward olivine rims cannot be accounted for by simple fractional crystallization and instead necessitates magma mixing ± diffusive re-equilibration between the high-Ni, low-Ca olivine cores and low-Ni, high-Ca rims. δ 18 O of olivine phenocrysts (5.64‰) are elevated compared to other Cascade compositional components and outside of the range typically associated with peridotitic olivine or olivine crystallizing from peridotite-derived basaltic magmas. Trace element contents of whole rock, melt inclusions, and scoria glass have high Sr, high Dy/Yb, and low Y, characteristic of adakitic slab melts; however, major element compositions are of an alkali-rich basalt. In addition, similar trace element compositions between all analyzed glasses indicate a related petrogenesis between all components. We propose that a slab partial melt has reacted with depleted harzburgite in the mantle wedge, underlying the older Western Cascades (~40–10 Ma). Reaction of the siliceous slab-derived melt with depleted harzburgite could produce a metasomatized, zoned pyroxenite-harzburgite mantle source. High-Ni, low-Ca olivine crystallizes from melts of the subarc mantle reaction-pyroxenite. Later melting of the metasomatized harzburgite produces the absarokitic bulk composition, with olivine compositions recording diffusive re-equilibration following incorporation of pyroxenite-derived olivine in a peridotitic magma. Importantly, then, the observed olivine chemistry reflects mineralogical variations in the subarc mantle while melt trace element variations record melts/fluids derived from the subducting plate. High-Fo olivine rims (up to ~Fo 94 ) appear to be the result of late-stage oxidation in the lava flow resulting from SO 2 degassing and are unrelated to the otherwise complex magma petrogenesis.

The central Appalachian Piedmont lies in the critical juncture between the northern and southern Appalachians, portions of the orogen with distinct middle to late Paleozoic accretionary histories. Orogen-scale compilation maps link the central and southern Appalachians, but until recently, limited geochronological data prevented robust tectonic comparisons between high-grade metamorphic rocks in different parts of the orogen. We report the results of in situ U-Th-total Pb monazite geochronology that date significant deformation and metamorphism as middle Silurian (~425 Ma) through middle Devonian (~385 Ma) and demonstrate the diachronous nature of orogen development. The Rosemont Shear Zone is identified as a major tectonic boundary in southeastern Pennsylvania and northern Delaware separating the rifted Laurentian margin from younger rock units that formed in a magmatic arc setting. The Laurentian margin rocks occur in a series of nappes in which the metamorphic grade decreases from the structurally highest nappe to the lowest. The in situ monazite ages show that maximum temperature in the lowest nappe may have been attained some 15 million years after maximum temperature in the highest nappe. We interpret this to be the result of successive nappe emplacement, with the warmer overriding sheets contributing heat to lower levels. Combining geochronologic and thermobarometric results with the geometry of deformation results in a new picture of the tectonic development of the central Appalachian Piedmont that further links the evolution of the southern and northern Appalachians. For the Laurentian margin rocks, tectonism resulted from the approach and collision of peri-Gondwanan terranes during the Silurian to early Devonian in a dominantly sinistral, transpressive tectonic regime. This portion of the Pennsylvania-Delaware Piedmont inboard of the Rosemont Shear Zone is contiguous with comparable rocks in the southern Appalachians. In contrast, arc-related rock units outboard of the Rosemont Shear Zone experienced primarily thermal metamorphism in the Silurian, while crustal thickening and associated regional metamorphism is middle Devonian in age and likely the result of the accretion of Avalonia during the Acadian orogeny. These arc-related and younger rocks probably originated to the north of their present location as part of the northern Appalachians. They were ultimately emplaced in a right-lateral transcurrent regime sometime after the middle Devonian. Thus, it is in this portion of the central Appalachian Piedmont that the northern and southern Appalachians are joined.

It is well known that trace element sensitivity in electron probe microanalysis (EPMA) is limited by intrinsic random variation in the X-ray continuum background and weak signals at low concentrations. The continuum portion of the background is produced by deceleration of the electron beam by the Coulombic field of the specimen atoms. In addition to the continuum, the background also includes interferences from secondary emission lines, “holes” in the continuum from secondary Bragg diffraction, non-linear curvature of the wavelength-dispersive spectrometer (WDS) continuum and other background artifacts. Typically, the background must be characterized with sufficient precision (along with the peak intensity of the emission line of interest, to obtain the net intensity for subsequent quantification), to attain reasonable accuracy for quantification of the elements of interest. Traditionally we characterize these background intensities by measuring on either side of the emission line and interpolate the intensity underneath the peak to obtain the net intensity. Instead, by applying the mean atomic number (MAN) background calibration curve method proposed in this paper for the background intensity correction, such background measurement artifacts are avoided through identification of outliers within a set of standards. We divide the analytical uncertainty of the MAN background calibration between precision errors and accuracy errors. The precision errors of the MAN background calibration are smaller than direct background measurement, if the mean atomic number of the sample matrix is precisely known. For a simple matrix and a suitable blank standard, a high-precision blank correction can offset the accuracy component of the MAN uncertainty. Use of the blank-corrected-MAN background calibration can further improve our measurement precision for trace elements compared to traditional off-peak measurements because the background determination is not limited by continuum X-ray counting statistics. For trace element mapping of a simple matrix, the background variance due to major element heterogeneity is exceedingly small and high-precision two-dimensional background correction is possible.

Rare earth elements (REE) are technology drivers, essential for applications ranging from clean energy technologies to biomedical imaging. Thus they are “critical elements” and it is desirable to find additional RE domestic sources and explore green extraction technologies to overcome their supply risk. Phosphogypsum, a major byproduct of the phosphate fertilizer industry, incorporates a significant amount of RE from the apatite source rock and thus is a potential alternate source of RE. To know the accessibility and extractability of RE from phosphogypsum, it is important to understand the location and nature of RE binding. Here, we report the synthesis of analogs of RE-doped phosphogypsum, with europium (Eu) as a model RE. Using several characterization tools we conclude that the majority of Eu is on the surface of the calcium sulfate crystal as a separate secondary phase, namely a metastable amorphous/nanocrystalline precipitate in which Eu is associated with phosphate and sulfate as counterbalancing ions. The rapid precipitation at low temperature could be responsible for this behavior, which may not represent equilibrium, and our experiments are comparable in the timescale with the fast phosphogypsum precipitation in the industrial process. These results suggest that the Eu is not entrapped by ionic substitutions in the calcium sulfate lattice. Thus RE should be extracted relatively easily from phosphogypsum using methods that extract the RE from its surface.

Here we test the concept that a potential energy model (force field) based on an expansion of the bond-valence model can use molecular geometry to make a reasonable prediction of the thermodynamic energy. The backbone of the model is a non-standard choice of structural descriptors for the energy decomposition, which relates the energy to particular aspects of the structure. Most force fields use a many-body decomposition to describe structures (with two-, three-, and possibly four-body terms, etc.), whereas ours employs a multipole expansion of the bond valence incident to each atom. This valence multipole model separates the energy associated with each atom into terms related to total bonding (valence monopole), bonding asymmetry (valence dipole), and ellipsoidal deformation (valence quadrupole). All of these are inherently multi-body terms that are calculated by combining two-body terms (bond valences). Provided bond valence sums are satisfied to within 0.2 v.u. of the ideal for all atoms, this model can provide accuracies of ~5 kJ/mol per unique atom in the Al-Si-H-O system, at least for the equilibrium structures tested here, comparable to most quantum mechanical calculations. More development is needed to produce a fully functional force field suitable for molecular dynamics simulations, but this work shows that the development of such a force field is likely to be feasible.

The thermal behavior of optical phonons and OH vibrations of phlogopite (a trioctahedral mica) was examined at temperatures up to 1000 K using in situ infrared spectroscopy. The results showed that with increasing temperature, O–K bands in phlogopite exhibited a relatively strong variation in frequency in a manner similar to those in muscovite. The work revealed that different types of OH bands (fundamentals and combinations) have very different thermal behavior or temperature dependence, and their absorption coefficients are commonly not constant on heating. OH combination bands that are associated with summation processes of multi-phonon interactions commonly show a decrease in their intensities on heating, but in contrast combination bands due to difference processes generally exhibit an increase. This means that temperature dependencies of their absorption coefficients need to be considered when using the Beer-Lambert law to determine or estimate OH contents or hydrogen concentrations at high temperatures. The results showed a structural anomaly associated with a discontinuity in the temperature derivative of the wavenumber of Al–O and Si–O vibrations and O-H stretching near 600 K. However, framework-related phonon modes in the FIR and MIR regions do not suggest a break of the original monoclinic structural symmetry in the investigated temperature region. The complex changes are attributed to temperature-induced alteration of local configuration involving TO 4 tetrahedra and a possible change of the orientation of OH dipoles, in addition to a previously reported distortion of MO 6 octahedra. Increasing temperature to 1000 K also causes partial dehydroxylation, as evidenced by the disappearance of the OH band near 3623 cm –1 and the decrease in OH band height and area of other OH bands. The study did not record the formation of H 2 O inside phlogopite as a result of partial dehydroxylation. The work offers new data and findings that have important implications in understanding the complex structural modifications and the behavior of phonon modes and the thermal stability of hydroxyls on approaching the dehydroxylation, as well as the way hydrogen is released from micas at high temperatures. Our data also show that phologpite becomes less transparent with increasing temperature suggesting a change of radiative properties and ability to transmit heat, which could be of interest for modeling thermal-transmission in crustal rocks.

By applying the method of chemical shifts of the uranium L α 1 and L β 1 X-ray emission lines, the oxidation state of U has been determined in select samples of microlite and monazite. From the relative contents of U 4+ , U 5+ , and U 6+ species, the oxygen coefficients have been calculated as a characteristic of U-oxidation rate. It is shown that the oxidation state of U is higher in the studied microlite than in monazite. Possible mechanisms of U oxidation in these two types of minerals are discussed, and it appears that the crystal structure of monazite plays an important role in stabilizing the U redox state. Spontaneous purification of the monazite structure from alien atoms, including U, may be possible via recrystallization of the a-recoil tracks. An explanation is suggested for intensive U oxidation in microlite and in other minerals of the pyrochlore group.

Differential stresses are expected to influence the properties of minerals. The structural and Raman vibrational properties of calcite under hydrostatic and differential stresses were studied using a first-principles method based on density functional theory. Our results show that the density of calcite increases or decreases under different differential stress, relative to its value under hydrostatic pressure. The calculated effects of differential stress on bond lengths are nominal. As pressure increases, the frequencies of all Raman modes increase, with their pressure derivatives slightly depending on the differential stress. The state of stress influences the Raman modes by shifting their frequencies to either higher or lower values relative to the corresponding hydrostatic results. In particular, the largest and smallest frequency shifts were predicted for E g -156 and A 1g -1086 modes, respectively, when the additional stress was applied along the a -axis. Visualization of atomic motions associated with Raman modes suggests that the differential stress-induced shifts in Raman frequencies are controlled by out-of-plane vibrations of atoms. The stress estimated on the basis of the experimentally measured shift of the Raman frequency of calcite sample gathered from the Wuchuan earthquake fault by applying our calculated dν i /d P value of A 1g -1085 mode is 785 MPa, which appears to be comparable to the stress inferred at the Wuchuan earthquake focus. Thus, the first-principles simulations and Raman spectroscopy experiments together may help us in elucidating the state of stress in the Earth’s interior.

In contact-metamorphic environments dolomite commonly breaks down to calcite + periclase + CO 2 as a result of the infiltration of H 2 O. The transport and exchange of oxygen in dolomite rock during the breakdown reaction were examined experimentally by reacting a cylindrical core of Reed Dolomite with isotopically enriched water having the composition HD 18 O 0.5 16 O 0.5 at 700 °C and 100 MPa for 29 days. Reaction products formed along grain boundaries, fractures, and on the outside surface of the core. Some dolomite grains became enriched in Fe as a result of replacement of the host dolomite. Extensive voids are found in the grain boundaries as a result of the ~25% loss in solid volume during reaction. There are also pores, ~1 μm in diameter, in the dolomite, notably in the vicinity of the replaced dolomite. The distribution of 18 O in the dolomite and reaction products was used as a tracer of the transport and exchange of O during reaction. Electron probe microanalysis (EPMA) and secondary ion mass spectrometry (SIMS) analyses showed pervasive infiltration of fluid along grain boundaries and fractures, growth and isotopic exchange with products of reaction, and diffusion of 18 O into dolomite grains. The fluid infiltrated efficiently along grain boundaries to the dolomite grain surface. The host dolomite shows an enrichment in 18 O along grain boundaries, indicating a diffusive exchange with the fluid. An estimate of the diffusion coefficient of oxygen in dolomite, determined from ion probe spot analyses, gives D ≈ 1 × 10 –12 mm 2 /s. This value is comparable to the oxygen diffusion coefficient for calcite in an H 2 O-rich fluid. Mass balance of O in the experiment (including dolomite-fluid exchange, the amounts of neomorphic reaction products, and the fluid components) indicates that the reaction products have a 18 O concentration only about half that of the fluid. Ion probe spot analyses of calcite from both the center and the edge of the core have the fraction F = 18 O/( 18 O+ 16 O) of 0.14 ± 0.1, whereas the value calculated for the fluid is 0.31. The measured F values of calcite are intermediate between the initial F values of starting water and dolomite, indicating that the reaction products record a mix of both dolomite- and fluid-derived oxygen. The products reached about 45% of isotopic equilibrium, similar to the extent of the mineral-fluid reaction. The Fe-rich, replacement dolomite near the core edge has an elevated value of F = 0.02 ± 0.002, 10 times the value of F ≈ 0.002 for unreacted dolomite, but less than the value for the calcite. The distribution of 18 O in the minerals indicates that the breakdown and replacement reactions were faster than O diffusion in dolomite.

Thermometric properties and compositions of fluid inclusions in quartz are used to constrain the roles that fluid-soluble elements, principally Li, B, Cl, and F, have in controlling the transition from magmatic to hydrothermal mineral paragenesis in pegmatites and to ultimately understand why some pegmatites in the San Diego County pegmatite district contain abundant, gem-quality, Li-bearing minerals in pockets, whereas others do not. In this district, lithium-cesium-tantalum type pegmatites occur in the Mesozoic Peninsular Ranges Batholith. Emplacement of the dikes occurred at low pressures (200–300 MPa) that resulted in the formation of large miarolitic cavities (pockets), some of which contain gem-quality, Li-bearing minerals. Two pegmatite suites were studied: the gem-bearing Himalaya and the more barren La Posta. The inclusions measured in this study further underscore highly undercooled crystallization of pegmatites. Pressure-corrected homogenization temperatures ( T h ) of ~400 to 515 and ~70 to 425 °C were obtained for primary inclusions in the intermediate zone and the core, respectively, of a La Posta dike. Primary inclusions in the intermediate zone and the massive quartz core of the Himalaya pegmatite have T h ranges of ~350 to 420 and ~150 to 300 °C, respectively. The high portion of the latter temperature range is interpreted to represent the conditions that existed during the initial crystallization of minerals that line pegmatite pockets. The most important cations in fluid inclusions in both pegmatites are Na + , B 3+ , and Li + . Lithium concentrations are much higher in inclusions in the Himalaya pegmatite, up to 51 at% of all cations within the massive quartz in the core zone. In the La Posta pegmatite, few primary inclusions contain appreciable Li. The B content of inclusions in both pegmatites is high, up to 65 at% of cations. The dominant anions in the inclusions are Cl – , F – , and SO42−${\rm{SO}}_4^{2 - }$. The data suggest that hydrothermal fluids that collected in pockets were acidic and promoted the growth of tourmaline and other minerals that are stable in acidic solutions. In both pegmatites, Na and B dominate secondary inclusions. These inclusions reveal fluids stripped of Li and K by crystallization of lepidolite within fractures of primary minerals throughout the pegmatites, and sometimes as an alteration product in pockets. The lowering of alkali/H + ratios in the fluid-stabilized clays, including kaolinite, that line the walls of pockets. Coeval crystallization of terminated quartz crystals with clays is consistent with its precipitation from the fluids.

Auriferous sulfides, most notably pyrite (FeS 2 ) and arsenopyrite (FeAsS), are among the most important economic minerals on Earth because they can host large quantities of gold in many of the world’s major gold deposits. Here we present the first atom probe study of gold distribution in arsenopyrite to characterize the three-dimensional (3D) distribution of gold at the nanoscale and provide data to discriminate among competing models for gold incorporation in refractory ores. In contrast to models that link gold distribution to gold concentration, gold incorporation in arsenopyrite is shown to be controlled by the rate of crystal growth, with slow growth rate promoting the formation of gold clusters and rapid growth rate leading to homogeneous gold distribution. This study yields new information on the controls of gold distribution and incorporation in sulfides that has important implications for ore deposit formation. More broadly this study reveals new information about crystal-fluid interface dynamics that determine trace element incorporation into growing mineral phases.