Volume 90, Issue 4

Electron probe microanalysis (EPMA) of accessory minerals such as monazite, xenotime, and thorite for minor- and trace-element concentrations and geochronology, requires consideration of beam irradiation effects (increasing heat and charge) as higher current densities and lengthy counting times are employed, and requires careful, detailed assessment of background intensities and interferences. A carbon coat (250 Å thickness) is generally inadequate for prevention of absorbed current fluctuation and beam damage when using the high current densities applied for high precision (e.g., 200 nA, focused beam). Beam irradiation effects include element mobility in monazite, resulting in P loss relative to REE. Coating materials of higher electrical and thermal conductivity are indicated, and use of gold (≥100 Å) is strongly suggested. Systematic compositional and, therefore, age variability can result simply from analytical effects, requiring evaluation of all aspects of data acquisition. The spectra relevant to measurement of Y, Th, Pb, and U are complex, especially in REE-bearing minerals. Acquisition of detailed spectral wavelength scans allows recognition of background and peak interferences, as well as curvature. Background intensities can be extracted directly from scan data by regression. X-ray mapping allows delineation of domains, guiding background acquisition and detailed quantitative analysis. Minor substitution or fluorescence of unexpected elements can compromise analyses, one documented effect being the fluorescence of K in monazite adjacent to, or hosted by, K-feldspar or micas. This effect, clearly evident within 10 micrometers from K-feldspar, can result in erroneous U concentrations leading to misinterpretation of rim .ages. as younger overgrowths. Absorption edges associated with Th also become relevant to the measurement of U at high Th concentrations. Because background intensity is sensitive to variation in average atomic number, backgrounds must be acquired from each identified domain, with particular attention being paid to Th variation. Misapplication of background intensities can result in large age discrepancies, for example, application of backgrounds obtained from a high-Th domain in monazite to measurement of a low-Th domain (1/3 of the amount in the high-Th domain) results in an overestimation of the UMβ background intensity of 0.008 cps/nA, and an overall “increase” in age of 70 m.y.

We examine the factors controlling accuracy and precision of monazite microprobe ages, using a JEOL 733 Superprobe equipped with 4 PET crystals, and both 1-atm gas flow Ar X-ray detectors and sealed Xe X-ray detectors. Multiple PET crystals allow for simultaneous determination of Pb concentration on up to 3 detectors, and the effects of different detector gases on spectral form can be addressed. Numerous factors in the X-ray production, detection, and counting sequence affect spectral form, including: choice of accelerating voltage, changes in d-spacing of the diffraction crystal, use of X-ray collimation slits, and type of detector gas. The energy difference between ArKα X-rays and XeLα X-rays results in, for 1-atm Ar detectors, escape peaks of second-order LREE L line X-rays that cannot be filtered using differential mode PHA. The second-order LREE energies are passed to the counter and produce, for a 140 mm Rowland circle, several problematic interferences in the Pb region of a monazite wavelength-dispersive (WD) spectrum. WD monazite spectra produced with Xe detectors are free from second-order LREE interferences in the Pb region; escape peaks of the secondorder LREE are filterable with differential mode PHA if Xe detectors are employed. Silicon, Ca, Y, Ce, P, Th, U, and Pb (2 spectrometers) are measured as part of the monazite microprobe dating protocol; ±2σ variations in elements fixed for ZAF corrections do not affect the age outside of analytical uncertainty. ThMα, UMβ, and PbMα are the analyzed lines of the age components. Corrections for interference of ThMζ 1,2 and YLγ 2,3 on PbMα are significant, but can be done precisely, and reduce the precision of theMα analysis by a trivially small amount. ThMγ, M3-N4, and M5-P3 interferences on UMβ can be corrected, as well, but ThM5 and M4 absorption edges in high-Th samples make estimation of UMβ background problematic. Background fits for UMβ peaks show that linear vs. exponential fits for UMβ do not, in general, produce statistically significant differences in microprobe ages. However, linear vs. exponential background fits for PbMα peaks do produce significantly different ages, most likely because of (1) low Pb concentrations relative to U; (2) ThMζ 1 interference on backgrounds between ThMζ 1 and PbMβ; and (3) SKα and Kβ interference in S-bearing monazite. For 6-min analyses (3 min peak, 3 min background) at 25 keV and 200 nA, 1σ Pb precisions are approximately 3.4% at 1700 ppm and 9.5% at 750 ppm; at 15 keV, precision decreases by roughly 25% of the 25 keV value. These precisions are constant for fixed current, analysis time, and concentration, but the statistical precision of distinct populations of monazite grains (domains) is a function of the total number of analyses within the domain. Instrumental errors (current measurement, dead time, pulse shift, d-spacing change) add 1.10% to random errors, but errors caused by pulse shift and d-spacing changes can be accounted for and corrected. Decreasing accelerating voltage from 25 to 15 keV decreases ZAF correction factors by as much as 50% relative, but replicate age analyses of Trebilcock monazite at 15 and 25 keV are statistically indistinguishable. Grain orientation, miscalculated background intensity, uncorrected interferences, and surface effects also introduce systematic errors. Accurate background interpolation and interference correction reduces systematic error to approximately 5.20% in addition to random (counting) error. Microprobe ages (~420 Ma) and 208 Pb/ 232 Th SIMS ages (~430 Ma) of monazite from Vermont are in agreement to within ~10 m.y. The discrepancy between U-Th-total Pb microprobe ages and 208 Pb/ 232 Th ages is removed when the high background measurement for PbMα is shifted to the short-wavelength side of PbMβ, removing a possible ThMζ 1 interference.

High resolution X-ray maps of Th, U, Pb, and Y in monazite can be used to construct age maps, which reveal the continuous spatial distribution of ages in a single grain of monazite. The age mapping algorithm and three examples are presented to illustrate the capabilities and applications of this mapping technique, the insights it can provide into monazite geochronology in general, and its limitations compared to electron microprobe (EMP) quantitative dating and other in-situ geochronologic techniques. Age maps offer critical information for unraveling metamorphic and tectonic histories and for interpreting results from other geochronologic techniques, and they are a valuable aid for rigorously locating in-situ analytical points. Age mapping also can be used to better understand the behavior of the U-Th-Pb system in monazite during metamorphism, deformation, and fluid-circulation events. Age maps presented in this paper reveal unsuspected age heterogeneities on the micrometer scale, like a now-healed fracture not visible in back-scattered electron (BSE) images or young domains less than 5 μm in width located inside an older core. In both cases, using age maps as a template for locating in-situ analysis points will minimize the peril of age mixing and erroneous geological interpretations. In addition to providing critical information for illustrating and interpreting the history of complex polygenetic monazite, age mapping may ultimately lead to a better understanding of the processes involved with monazite growth and recrystallization, and thus, even more powerful applications of the monazite geochronometer. The AgeMap program is available in a Windows version and can be downloaded from the internet at the following address: http://www.geo.umass.edu/probe/agemap.

Using a three-dimensional U-Th-Pb concordia diagram, electron microprobe (EMP) U-Th-Pb analyses are shown to yield isotopic age constraints on isotopically concordant and discordant data that have not been recognized previously. The three-dimensional U-Th-Pb concordia diagram is discussed. It is demonstrated that a date obtained from an EMP analysis is as old as or younger than the 207 Pb/ 235 U and 207 Pb/ 206 Pb ages, and as old as or older than the 208 Pb/ 232 Th age. EMP analyses always yield a minimum age for the oldest Pb component.

Four distinct generations of monazite growth have been identified in samples from the Chesham Pond Nappe, and three (monazite compositional domains 2, 3, and 4) have been correlated with both temperature and mineral assemblage. Domain 1 cores were interpreted previously to be detrital relics or vestiges of an earlier Acadian metamorphism. The four monazite domains have been dated by in situ isotope and chemical methods; the following are chemical ages of each domain (weighted average ±2 standard errors of the mean): 400 ± 10 Ma (domain 1); 381 ± 8 Ma (domain 2); 372 ± 6 Ma (domain 3); 352 ± 14 Ma (domain 4). Heating and cooling rates derived from combining monazite ages, monazite thermometry, and 40 Ar/ 39 Ar closure temperatures are approximately 10.15 °C/m.y. for heating from 470 to 740 °C, approximately 8 °C/m.y. for cooling from 740 to 375 °C, and approximately 1.2 °C/m.y for cooling from 375 to 150 °C. Temperature-time paths calculated with monazite ages and monazite thermometry indicate that (1) plutonism at ca. 400 Ma was the likely heat source for the formation of monazite domain 1 and (2) monazite domains 2.4 were produced during a regional low-pressure, high-temperature metamorphism active between 380.350 Ma. The regional metamorphism is ascribed to lithospheric mantle delamination, followed by asthenospheric mantle upwelling, which heated a wide area of the Merrimack basin (southwestern New Hampshire, central Massachusetts, central Connecticut) to temperatures in excess of 725 °C. Monazite ages in the Chesham Pond Nappe and adjacent structural units to the west constrain the commencement of nappe overthrusting to roughly 355 Ma.

High spatial resolution dating of monazite by the electron-probe microanalyzer (EPMA) enables systematic and detailed studies of small minerals. Like zircon, monazite records the complex history undergone by the host rocks. Recent improvements in the statistical treatment of many in situ data now make it possible to decipher the related thermal events and so obtain reliable and precise ages. Our work shows that a significant number of individual spot analyses is required to reach such precise information (i.e., more than 30.40 data). Using the examples of monazites from three migmatites and one granite, we show how to select the most efficient method of age calculation according to the U and Th geochemistry of the grains, or grain domains, that we are trying to date. Three situations may be met: (1) monazites exhibiting significant Th/U ratio variation, (2) monazites exhibiting a fairly constant Th/U ratio, but significant U + Th heterogeneity, and (3) monazites of constant U and Th concentrations. For the first case, a precise mean age can be calculated using a method of data reduction in the Th/Pb = f(U/Pb) diagram, whereby a precision of ±5−10 Ma (2σ) is commonly achieved. For the second case, an isochron age can be calculated according to the Pb = f(Th*) method, with a common precision of around 20 Ma (2σ), whereas for the third case, a simple weighted average age can be calculated. Using these approaches, coupled with a back-scattered electron image study, we demonstrate that inheritance is probably as common for monazite as for zircon. In addition, the combination of high spatial resolution and precise age determination show the limited extent of Pb diffusion in monazite. Finally, an example from a migmatite from southern French Guiana demonstrates the especially robust behavior of the Th-U-Pb system in monazite. This system remains closed during late migmatization and during the subsequent zircon crystallization and zircon overgrowth of protolith zircons. The monazite yielded exactly the same age as the protolith zircons.

Polygenetic monazite grains in diverse Precambrian crystalline rocks from the Black Hills, South Dakota, have been analyzed in situ by ion and electron microprobe methods (SHRIMP and EMP), to evaluate the accuracy and precision of EMP ages determined using a new analytical protocol that incorporates improved background acquisition and interference corrections. Parallel evaluations were conducted by comparing EMP chemical and SHRIMP isotopic ages at regional-, rock-, and grainscales. The monazite data set includes 354 EMP chemical analyses from 26 grains in six metamorphic rocks, which resolve into 54 age-composition domains, and 31 SHRIMP isotopic ages from 13 grains in one of the rocks, with six grains microanalyzed in common by the two methods. The data set also includes monazite-bearing garnets in two of the rocks, whose isotopic compositions were analyzed using Pb stepwise-leaching (PbSL) methods. Both the EMP and SHRIMP data sets reveal a continuum of apparent monazite ages spanning a ~1790.1680 Ma timeframe, with a relatively high probability of ages at ~1755 and ~1715 Ma that correspond spatially to core and rim domains. PbSL ages of ~1742 and ~1734 Ma obtained from monazite-bearing garnet in two rocks are intermediate compared to the corresponding EMP ages, and are thereby interpreted as mixed ages. EMP data for two grains in the structurally deepest of the six rocks record ~1785 and ~1755 Ma ages in the cores and (higher-Y and lower-Th) rims, respectively, and these results are duplicated by SHRIMP ages in these and/or other grains from the same rock. Overall, the EMP, SHRIMP, and PbSL ages are internally consistent at the various scales of observation, which serves to validate EMP chemical dating as an accurate and precise method of discerning monazite age populations in polymetamorphic terrains. The EMP data set is interpreted geologically as reflecting multiple episodes of monazite growth that are provisionally related to known metamorphic events in the Black Hills. Taking the most precise EMP data at face value, it is possible to resolve the timing of the two older events at ≤1784 ± 4 Ma (or ≤1786 ± 6 Ma) and 1756 ± 3 Ma (or 1753 ± 4 Ma), with 95% confidence. These events are considered to be related to sequential episodes of N-directed thrusting and ~E-W compression associated with Paleoproterozoic crustal assembly in the mid-continent. A younger metamorphism, related to granite intrusion known to have occurred at 1715 ± 3 Ma, is dated independently at 1717 ± 2 Ma from the EMP monazite ages.

Time-of-flight neutron powder data and synchrotron infrared absorption spectra were collected for katoite hydrogarnet [Ca 3 Al 2 (O 4 D 4 )3] at pressures to 9.4 and 9.8 GPa, respectively. The phase transition from space group Ia3d to I4̄3d was observed in the neutron spectrum at ~7.5 GPa, as indicated by the presence of two weak reflections (730 and 530) that violate the hkl conditions (hk0, h ≠ 2n) imposed by the a-glide operation. However, attempts to refine the high-pressure structure in space group I4̄3d did not significantly improve the fit and produced a chemically unreasonable O-D bond length at the second D position. Structure refinements in Ia3d indicate that (1) the O-D bond length, corrected for the effects of thermal motion, remains essentially constant (~0.95 Å) with increasing pressure; (2) hydrogen bond lengths shorten with increasing pressure; however, the variation in O-D···O angles indicates a preferential strengthening of H bonds; and (3) the compression mechanism is characterized by bond shortening rather than bond bending. The new results are in excellent agreement with both high-pressure X-ray diffraction experiments and ab initio calculations, and illustrate the need to eliminate peak broadening in high-pressure neutron powder experiments. IR spectra collected for the same sample showed discontinuities in both O-H and O-D vibrational frequencies at ~5 GPa, suggesting that deuteration does not significantly affect the pressure of the transition. The higher pressure observed for the transition in the neutron data is probably due to lower signal-to-noise levels, which mask the weaker, symmetry-forbidden reflections at lower pressure.

Elastic behavior and high-pressure (HP) structural evolution of a natural zeolite levyne, (Ca 0.5 ,Na,K) 6 (Al 6 Si 12 O 36 )⋅18H 2 O, has been investigated up to 5 GPa by means of in situ single-crystal X-ray diffraction with a diamond-anvil cell using a non-penetrating pressure-transmitting medium. Peculiar elastic behavior has been observed in the range 0.1 GPa: the c parameter decreases between 0 and 0.2 GPa, then increases up to 0.5.0.6 GPa. Above this P-value, the parameter decreases as expected. Anomalous behavior is also shown by the a-parameter, which first increases up to 0.2 GPa, then decreases as expected. However, these anomalous lattice variations are only slightly reflected in the cell volume behavior. The low-P trend (P < 1 GPa) is also followed by the lattice parameters in decompression. The isothermal Equation-of-State (EoS) at P > 1 GPa, refined with a second-order Birch-Murnaghan EoS, yields the following parameters: V 0 = 3539(3) Å 3 and KT 0 = 48(1) GPa. Comparison of structural refinements performed at 0.0001, 0.79(5), and 3.00(5) GPa highlights two distinct deformation mechanisms of the Si/Al-framework, one predominant at low pressures (0.1 GPa) and the other at high pressures (1.5 GPa). The extra-framework content does not show any evident modification within the pressure range investigated.

Several compounds of the Na 2 (Mn 1-x Fe x 2+ ) 2 Fe 3+ (PO 4 ) 3 solid solution have been hydrothermally synthesized at 400 °C and 1 kbar; pure alluaudite-like compounds have been obtained for x = 0.00, 0.25, 0.50, 0.75, and 1.00. Rietveld refinements of the powder X-ray diffraction patterns indicate the presence of Na + at the A1 and A2' sites, Mn 2+ and Fe 2+ at the M1 site, and Mn 2+ , Fe 2+ , and Fe 3+ at the M2 site. The presence of small amounts of Na + at the M1 site and Mn 2+ at the A1 site indicates a partially disordered distribution of these cations. An excellent linear correlation has been established between the M1-M2 distance and the energy of the infrared band attributed to the M 2+ -O vibrations. The Mössbauer spectra, measured between 85 and 295 K, were analyzed in terms of a model which includes the next-nearest neighbor interactions at the M2 and M1 crystallographic sites. Fe 2+ and Fe 3+ isomer shifts are typical of the alluaudite structure and exhibit the expected second-order Doppler shift. The derived iron vibrating masses and Mössbauer lattice temperatures are within the range of values expected for iron cations in an octahedral environment. The Fe 2+ and Fe3+ quadrupole splittings are typical of the alluaudite structure, and the temperature dependence of the Fe 2+ quadrupole splitting was fit with the Ingalls model, which yielded a ground state orbital splitting of ca. 460 to 735 cm -1 for the Fe 2+ sites. The isomer shifts and quadrupole splittings of Fe 2+ at the M1 site are larger than those of Fe 2+ at M2, indicating that the M1 site is both larger and more distorted than the M2 site.

The low-temperature heat capacity of uvarovite (Ca 3 Cr 2 Si 3 O 12 ) was measured between 2 and 400 K, and thermochemical functions were derived from the results. The measured heat-capacity curve shows a significant lambda-shaped anomaly peaking at around 9 K. The nature of this transition is unknown. From our data, we suggest a standard entropy for uvarovite at 298.15 K of 320.9 ± 0.6 J/(mol·K).

Phase transformations of calcium carbonates (CaCO 3 ) were investigated using a laser-heated diamond anvil cell combined with a synchrotron X-ray diffraction method. Calcite, which is the stable phase at ambient conditions, transforms to aragonite at high P-T conditions that correspond to the uppermost part of the upper mantle. The phase transformation from aragonite to a new form of calcium carbonate was observed at pressures higher than about 40 GPa, corresponding to the lower mantle. The new carbonate has orthorhombic symmetry (P2 1 2 1 2) and was confirmed to remain stable at least up to 86 GPa (2000 kilometer depth). This indicates that carbon might be stored in the new calcium carbonate phase in the deep mantle.

The dehydration mechanism of natural single crystals of gypsum was investigated in the temperature range 300-430 K by in situ infrared (FTIR) spectroscopy. The thermal evolution of the second-order modes of H 2 O and SO 4 groups in gypsum, in the wavenumber range 4850.5450 cm -1 and 2050.2300 cm -1 respectively, were used to probe the dehydration and rehydration sequence. A total disappearance of the combination modes of H 2 O and the replacement of four SO -2 4 bands (2245, 2200, 2133, and 2117 cm -1 ) observed at room temperature by three bands (2236, 2163, and 2131 cm -1 ) observed at 390 K indicates the direct formation of γ-CaSO 4 upon heating. Upon cooling water re-enters into the γ-CaSO 4 structure at around 363 K to form bassanite. This observation, that the dehydration of gypsum directly yields γ-CaSO 4 (anhydrite) without the intermediate formation of hemi-hydrate (bassanite), is further corroborated by the dehydration behavior of bassanite. The second-order SO 4 modes of bassanite observed around 2218, 2136, and 2096 cm -1 were replaced with the bands of γ-CaSO 4 at about 378 K upon heating.

The iron sulfate FeOHSO 4 studied was obtained as a dehydration product of metahohmannite Fe 2 (H 2 O) 4 [O(SO 4 ) 2 ] during a synchrotron real-time powder diffraction experiment. As quoted in the literature, FeOHSO 4 has iron atoms octahedrally coordinated with two hydroxyl groups and four sulfate O atoms, while each hydroxyl group is bonded to two iron atoms. This compound is commonly described in the orthorhombic system with space group Pnma, lattice parameters a J = 7.33, b J = 6.42, and c J = 7.14 Å (a J , b J , and c J are the Johansson lattice parameters), and Z = 4. However a preliminary Rietveld refinement of the pattern at about 220 °C using the structural model from the literature yielded a poor fit of the observed data and a final R p value of about 23%. A careful analysis of the calculated powder diffraction pattern showed unexpected peaks, not observed in the experimental trace, for h = 2n + 1, while sharp reflections for h = 2n seemed to point to different lattice constants and space group. The recognition of the order-disorder character of the FeOHSO 4 compound was the key to successfully interpreting the unexpected features of the experimental powder pattern and the misfit with respect to the calculated pattern. In fact, FeOHSO 4 belongs to a family of OD structures formed by equivalent layers of symmetry Pbmm. Only two MDO (Maximum Degree of Order) polytypes are possible. MDO1 results from a regular alternation of stacking operators 2 1/2 and 2 -1/2 , and yields an orthorhombic structure with space group Pnma and lattice parameters a J = 7.33, b J = 6.42, and c J = 7.14 Å. MDO2 results from the 2 1/2 |2 1/2 |2 1/2 ... sequence of symmetry operators and yields a monoclinic structure with space group P2 1 /c, a M = 7.33, b M = 7.14, c M = 7.39 Å, and β = 119.7°. The analysis of one-dimensional stacking disorder was performed by fitting the observed XRPD pattern with a calculated intensity curve generated by DIFFaX. The disorder model was investigated by taking into account a probability matrix for the occurrence of OD layer sequences. The best fit (R p = 0.009) to the observed powder pattern was obtained with a 61:39 ratio of monoclinic and orthorhombic polytypes for a fully disordered OD layers sequence.

The solid solubility of CaTiOGeO 4 with CaZrOGeO 4 and the structural phase transitions occurring in this solid solution as a function of temperature and composition were examined using in situ high temperature X-ray powder diffraction. Structural phase transitions were identified based on the determination of spontaneous strain. The transition P2 1 /a-A2/a, which is typical for titanite and CaTiOGeO 4 , was observed in samples with Zr concentrations up to 10%. The addition of Zr destabilizes the ordered P2 1 /a phase and T c decreases accordingly. The aristotype structure of titanite in space group symmetry A2/a was observed for intermediate compounds. Compounds with high Zr contents exhibit a triclinically distorted titanite structure. Their structure was modeled in space group A1̄. The triclinic structure appears at Zr contents of about 70% with Tc close to ambient temperature. Further increase of the Zr content stabilizes the triclinic structure and leads to an increase of T c up to T c = 488 K in CaZrOGeO 4 .

A Na B (NaMg) C Mg 5 Si 8 O 22 (OH,D) 2 amphibole was hydrothermally synthesized at 850 °C and 0.3 GPa. SEM, EPMA, and X-ray powder-diffraction data showed the experimental product to consist of a high amphibole yield (90.95%), plus minor quartz and rare enstatite. Neutron powder-diffraction data were collected using a time-of-flight diffractometer at room T and at 8 K, respectively, and structure refinement was carried out using the Rietveld method. The space group of the amphibole is P2 1 /m at both temperatures, as confirmed by the presence of b-type reflections (h + k = 2n + 1). FTIR OH- and OD-stretching spectra at both room and low T (30 K) show two main absorptions, which are assigned to two non-equivalent OH groups in the structure, and a third lower-frequency band, assigned to A-site vacant environments (local cummingtonite environments). At room- and low-T, the cell parameters are (in Å): a 9.7188(1) and 9.7016(2), b 17.9385(3) and 17.8953(4), c 5.2692(1) and 5.2574(1); β (°) is 102.526(1) and 102.597(2). Cell volumes (Å 3 ) are 896.78(2) at room T and 890.80(2) at 8 K, with a relative reduction of less than 1%. Accurate structural positions for the hydrogen atoms were obtained from diffraction data. The O5A-O6A-O5A and O5B-O6B-O5B angles, diagnostic of the A- and B-chains kinking along c, are 190.0° and 159.2° at 293 K and 193.8° and 156.8° at 8 K, respectively. The orientation of the thermoelastic strain ellipsoid was calculated and the principal unit-strain tensor components are reported. A comparison between the low-temperature data reported here and the high-temperature data for a similar amphibole composition, reported by Cámara et al. (2003) up to 643 K, is discussed.

Gaseous secondary electron (GSE) imaging of eclogite garnets under the environmental scanning electron microscope (ESEM) at low chamber gas pressures reveals detailed image contrast patterns (charge-contrast images, CCI) that are not present in back-scattered or secondary electron images. Image intensity is a function of the amount of surface charge accumulation. Successful acquisition of CCI depends on frame size and beam scan rate at a given chamber gas pressure and beam current. Images are obtained in a few seconds, and are stable and reproducible. CCI patterns do not correlate with cracks or grain boundaries, but do correspond closely to variations in major-element composition, both in the form of concentric (growth) zoning, and branching, linear features interpreted as cracks that have been healed by new garnet growth. Causes of CCI are not yet well understood, but may be related to variations in lattice defect density and their influence on charge-trapping and dissipation. These in turn influence the rate of charge build-up at or very close to the specimen surface. One interesting possibility is that CCI images detect vacancies related to non-homovalent coupled substitutions involving, for example, REE and hydroxyl, so the method offers a way of imaging the distribution of these trace species in garnets. The CCI images are rich in microstructural detail and offer the potential for rapid, high-resolution, low-noise reconnaissance mapping of intragranular microstructure and compositional variation in both natural and synthetic garnets.

In this study, we present a modified double-capsule technique to perform experiments on H 2 O undersaturated, Fe-bearing systems at elevated pressures and temperatures and oxygen fugacities (f O₂ ) relevant for natural calc-alkaline magmas. Welded shut, Fe-preconditioned Au 90 Pd 10 capsules were placed in an outer Pt capsule that contains the same starting material. Experiments were performed at 1.0 GPa and 1200 °C using a synthetic, hydrous basalt and run with either boron nitride (BN) or MgO surrounding the welded capsules. Optimum results were obtained by using Fe-preconditioned Au 90 Pd 10 inner capsules in combination with MgO assemblies. The application of the modified doublecapsule technique with Fe-preconditioned inner AuPd capsules reduced Fe loss to less than 3% relative, conserved H 2 O within the error of ion-microprobe analyses, and kept the f O₂ (QFM+1.1) within 1 log unit of the initial value constrained by the Fe 2 O 3 /FeO ratio of the starting material (QFM+0.43). These conditions are similar to estimates of f O₂ during the crystallization of natural calc-alkaline magmas.

Ferrihydrite and a vernadite-like mineral, in samples collected from the riverbeds and floodplains of the river draining the largest mining-contaminated site in the United States (the Clark Fork River Superfund Complex), have been studied with transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) analysis. These poorly crystalline minerals are environmentally important in this system because contaminant heavy metals (As, Cu, Pb, and/or Zn) are always associated with them. Both two- and six-line ferrihydrite have been identified with selected-area electron diffraction. For the vernadite-like mineral, the two d values observed are approximately between 0.1 and 0.2 Å larger than those reported for vernadite, the Mn hydrous oxide that is thought to have a birnessitelike structure, but which is disordered in the layer stacking direction. In several field specimens, the ferrihydrite and vernadite-like minerals are intimately mixed on the nanoscale, but they also occur separately. It is suggested that the vernadite-like mineral, found separately, is produced biogenically by Mn-oxidizing bacteria, whereas the same mineral associated with ferrihydrite is produced abiotically via the heterogeneous oxidation of Mn 2+ aq initially on ferrihydrite surfaces. Evidence from this study demonstrates that the vernadite-like mineral sorbs considerably more toxic metals than does ferrihydrite, demonstrating that it may be a good candidate for application to heavy-metal sorption in permeable reactive barriers.

Wonesite, an interlayer-deficient trioctahedral sodium mica, has been investigated mainly by highresolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) using a Gandolfi camera. The true structure of wonesite is triclinic with a large layer offset, i.e., displacement between the two tetrahedral sheets across the interlayer region, which is partially occupied by sodium ions. The direction and amount of the layer offset is approximately [11̄0] and 1.25 Å respectively, with unit-cell dimensions of a = 5.31, b = 9.18, and c = 9.75 Å, α = 96.2, β = 96.5, γ = 89.9°. This interlayer structure, which is similar to that in talc, explains well the exsolution lamellae of talc in wonesite with continuous 2:1 layers. The volume ratio of wonesite and exsolved talc is estimated to be 4:1 to 5:1 from the XRD patterns. Disorder in the directions of the layer offset is occasionally observed in TEM. The interlayer in sodium micas (preiswerkite, paragonite, aspidolite, wonesite, etc.) possesses various amounts of layer offset, depending on the cavity space in the tetrahedral sheet that is primarily determined by the ditrigonal rotation angle.

Re-examination of holotype “clinoholmquistite”, ideally A □ B Li 2 C (Mg 3 Al 2 ) T Si 8 O 22 X (OH) 2 (Ginzburg 1965) from the Tastyg spodumene deposit, Tuva, Siberia, Russia by EMP and SIMS analysis and structure refinement shows that the sample consists of a mixture of two distinct amphibole compositions, tremolite and a new amphibole end-member, fluoro-sodic-pedrizite, ideally A Na B Li 2 C (Mg 2 Al 2 Li) TSi 8 O 22 X F 2 (IMA-CNMMN 2004-002). Fluoro-sodic-pedrizite from Tastyg has the following crystal-chemical formula and unit-cell parameters: A (Na 0.64 K 0.01 ) B (Li 1.93 Ca 0.04 Na 0.03 ) M1 (Mg 1.69 Fe 2+ 0.31 ) M2 (Al 1.98 Cr 0.01 Zn 0.01 ) M3 (Li 0.64 Fe 2+ 0.21 Mg 0.13 Mn 0.02 ) T1 (Si 3.96 Al 0.04 ) T 2 Si 4 O 22 X (F 1.10 OH 0.90 ), a = 9.368(8), b = 17.616(10), and c = 5.271(4) Å, β = 102.38(4)°, V = 849.6 Å 3 , Z = 2. The structure has been refined to R obs = 2.3% (I > 3σ I ) and R all = 3.8%. Refined site-scattering values and site-geometries were used, together with EMP and SIMS results, to obtain site populations. Fluoro-sodic-pedrizite is the first amphibole end-member with dominant C Li found in Fe-poor geologic environments. The coexisting tremolite contains only 0.002 wt% Li 2 O and 0.06 wt% B 2 O 3 , probably ordered at the T1 site. Crystal-chemical arguments, as well as preliminary experimental work, suggest clinoholmquistite is unstable.

Software for processing quantitative X-ray maps collected on an electron probe microanalyzer is described. The techniques used in collection of maps and capabilities of the software to produce data useful for petrologic analysis are illustrated using three examples from pelitic schists from Mica Creek, British Columbia. The program utilizes a Bence-Albee algorithm to convert raw X-ray counts at each map pixel to wt% oxide, and allows for spectrometer deadtime corrections and background subtraction. Additional features include phase identification, modal analysis, calculation of cations, calculation of integrated rock composition over specified regions of the map, display of maps and compositional graphs, output of compositional statistics, and the ability to remove pixels and screen for bad analyses. The first example utilizes a sample lacking phases with significant chemical zonation, and outlines the basic functionality of the program. The second example compares an integrated map composition to an X-ray fluorescence analysis on the same rock chip. The results indicate it is possible to obtain integrated map compositions approaching that obtained from an X-ray fluorescence analysis. The third example compares mean X-ray map compositions for individual phases to means obtained using routine point analysis procedures, and the results indicate mean phase compositions obtained from the X-ray maps are of generally high quality. The sample in this example contains compositionally zoned garnet, and is used to show the ability to remove selective portions of garnet to model the approximate change in effective bulk composition during garnet growth.

The Raman mapping technique provides a non-destructive means of studying internal growth textures and other micro-structural heterogeneity inside diamond single-crystals. Raman maps showing distribution patterns of the bandwidth (FWHM) of the main first-order lattice vibration of diamond (LO=TO phonon at ~1332 cm -1 ) along two-dimensional planes inside diamond crystals may reveal the internal growth zoning of these crystals. The observed zoning is affected, and in some cases even obscured in micro-areas adjacent to inclusions, by patterns of heterogeneous strain in the diamond. We present Raman maps obtained from diamond crystals containing large, single-crystal graphite inclusions, from the Panda kimberlite, Ekati Diamond Mine, Canada. The diamond growth texture was always found to start from the graphite inclusion. This result implies that graphite must have been the primary phase and was overgrown by diamond, whereas syngenetic growth of diamond and graphite was unlikely.

Major-, minor-, and trace-element compositions, as determined by X-ray fluorescence (XRF) analysis, were obtained on 34 samples of vermiculite to ascertain whether chemical differences exist to the extent of determining the source of commercial products. The sample set included ores from four deposits, seven commercially available garden products, and insulation from four attics. The trace-element distributions of Ba, Cr, and V can be used to distinguish the Libby vermiculite samples from the garden products. In general, the overall composition of the Libby and South Carolina deposits appeared similar, but differed from the South Africa and China deposits based on simple statistical methods. Cluster analysis provided a good distinction of the four ore types, grouped the four attic samples with the Libby ore, and, with less certainty, grouped the garden samples with the South Africa ore.

Single crystals of CaGeO 3 garnet were synthesized at 3 GPa and 1000 °C using a cubic anvil type of high pressure apparatus and the crystal structure was refined from single crystal X-ray diffraction data. This garnet is tetragonal with lattice parameters of a = 12.535(2) Å, c = 12.370(2) Å, V = 1943.5(5) Å 3 and belongs to space group I4 1 /a. Two dodecahedral sites are occupied only by Ca with mean Ca- O bond lengths of 2.480(4) and 2.467(4) Å. The Ca and Ge cations are completely ordered at two octahedral sites with mean Ca-O = 2.301(3) Å and mean Ge-O = 1.910(3) Å. Three tetrahedral sites are occupied only by Ge, and their mean Ge-O bond lengths are 1.753(3), 1.787(4), and 1.764(4) Å. Furthermore, the present tetragonal garnet has an unusual feature in that the mean value [2.704(5) Å] of the shared edge lengths of GeO 6 octahedron is larger than that [2.699(5) Å] of the unshared ones, as has also been observed for other tetragonal garnets with I4 1 /a.

The extent and rate of Fe-isotope exchange between aqueous Fe(III) atoms and those at the nanoparticulate ferrihydrite surface have been constrained using a 57 Fe-isotope tracer approach. Isotopic exchange was determined between hexaquo Fe(III) and 3 nm ferrihydrite under conditions analogous to natural environments at the equilibrium solubility of nanoparticulate ferrihydrite. Within approximately 11 days, the percent Fe-isotope (atom) exchange increased to a maximum of 26 ± 5%, and remained constant within error over the remaining 12-week time of the experiment, suggesting that Fe-isotope exchange is limited to Fe atoms in available surface sites. These results confirm earlier work that used NTA- and EDTA-bearing solutions, which concluded isotopic exchange is largely limited to Fe at surface sites. Our results, however, which were obtained in dilute aqueous solutions at the natural solubility of 0.12 ppm aqueous Fe(III) (pH = 4.7), demonstrate that isotopic exchange rates are one to two orders of magnitude slower in the absence of Fe(III) aq -complexing ligands.

The cation distribution in magnesioferrite MgFe 2 O 4 was investigated using a “piston-cylinder” apparatus at three pressures (ambient, 1, and 2 GPa) over a temperature range of 600 to 1200 °C. Quenched samples of magnesioferrite were investigated by X-ray powder diffraction. The inversion parameter was derived from the unit-cell parameter at ambient temperature. The inversion parameter increases with pressure, being a smooth function of P-T conditions.The changes in the cation distribution in magnesioferrite caused by the changes in P-T conditions produce the contributions to the bulk modulus (<0.01%) and thermal expansion coefficient (12.60%). On average, the pressure increment of 0.01 GPa is compensated for by 1 °C heating. The effect of pressure on the cation ordering obeys the fundamental relationship (dV/dx)T,P (dx/dP)T < 0. The terms are (dV/dx) < 0 and (dx/dP) > 0 for MgFe 2 O 4 and MgAl 2 O 4 .