Band 106, Heft 6

Theoretical bond-length calculations from ideal bond valences for each ion and coordination allow for the prediction of ion site preference and partitioning in tourmaline structures at low-pressure conditions. A comparison of calculated data with published bond-length values enables the determination of the range of structurally stable bond lengths with a minimal induced distortion—the “Goldilocks zone.” The calculations provided the following conclusions: the B -site occupancy is strictly limited to B 3+ ; the T site can freely accommodate not only Si 4+ but also B 3+ and Al 3+ , although these substituents require shrinkage and expansion of the T O 4 tetrahedron, respectively; and the Be 2+ substitution results in a significant difference in charge. Satisfactory bond lengths for octahedral sites were calculated for Al 3+ ( Z -site preference), Ti 4+ , Mn 3+ , Ga 3+ , V 3+ , Fe 3+ (mixed preference), Mg 2+ , Fe 2+ , Li + , Mn 2+ , Ni 2+ , Zn 2+ , Cu 2+ , Sc 3+ , and Zr 4+ ( Y -site preference). Another group of cations, which includes U 4+ , Th 4+ , Y 3+ , and lanthanoids from Tb to Lu and Ce 4+ , have significantly longer bonds than typical Y -O and very short bonds for the X site; therefore, it is likely they would prefer an octahedron. The empirical bond length for the X site is met with Na + , Ca 2+ , Sr 2+ , Pb 2+ , and lanthanoids from La to Gd, while K, Rb, and Cs are too large in the low-pressure conditions. However, the final tourmaline composition results from the interaction of the structure with the genetic environment in terms of P-T-X and geochemical conditions. This results in structural and environmental constraints that limit the incorporation of elements into the structure. Consequently, major elements, such as Si, Al, B, Mg, Fe, Na, and Ca usually occur in abundance, whereas other elements (V, Cr, Mn, Ti, Pb) could form end-member compositions, but rarely do because of their low abundance in the environment. The elements with contents limited to trace amounts have either structural (Be, C, REE, Rb, Cs, U, Th) or geochemical (Zr 4+ , Sc 3+ , and Sr 2+ ) limits. However, environmental properties, such as high pressure or specific local structural arrangements, can overcome structural constraints and enable the incorporation of elements (K).

Magnesio-lucchesiite, ideally CaMg 3 Al 6 (Si 6 O 18 )(BO 3 ) 3 (OH) 3 O, is a new mineral species of the tourmaline supergroup. The holotype material was discovered within a lamprophyre dike that cross-cuts tourmaline-rich metapelites within the exocontact of the O’Grady Batholith, Northwest Territories (Canada). Two additional samples were found at San Piero in Campo, Elba Island, Tuscany (Italy) in hydrothermal veins embedded in meta- serpentinites within the contact aureole of the Monte Capanne intrusion. The studied crystals of magnesio-lucchesiite are black in a hand sample with vitreous luster, conchoidal fracture, an estimated hardness of 7–8, and a calculated density of 3.168 (Canada) and 3.175 g/cm 3 (Italy). In plane-polarized light, magnesio-lucchesiite is pleochroic (O = dark brown, E = colorless) and uniaxial (–); its refractive index values are n ω = 1.668(3) and n ε = 1.644(3) (Canada), and n ω = 1.665(5) and n ε = 1.645(5) (Italy). Magnesio-lucchesiite is trigonal, space group R 3 m , Z = 3, with a = 15.9910(3) Å, c = 7.2224(2) Å, V = 1599.42(7) Å 3 (Canada) and with a = 15.9270(10) Å, c = 7.1270(5) Å, V = 1565.7(2) Å 3 (Italy, sample 1). The crystal structure of magnesio-lucchesiite was refined to R 1 = 3.06% using 2953 reflections with F o > 4σ( F o ) (Canadian sample; 1.96% / 1225 for the Italian sample) The Canadian (holotype) sample has the ordered empirical formula X (Ca 0.60 Na 0.39 K 0.01 ) Σ1.00 Y M g 2.02 F e 0.62 2 + F e 0.09 3 + T i 0.25   V 0.01 C r 0.01 Σ 3.00 z A l 5.31 F e 0.69 3 + Σ 6.00 T S i 5.98 A l 0.02 Σ 6.00 O 18 B O 3 3 v ( O H ) 2.59 O 0.41 Σ 3.00 ${ }^{\mathrm{Y}}\left(\mathrm{Mg}_{2.02} \mathrm{Fe}_{0.62}^{2+} \mathrm{Fe}_{0.09}^{3+} \mathrm{Ti}_{0.25} \mathrm{~V}_{0.01} \mathrm{Cr}_{0.01}\right)_{\Sigma 3.00} \mathrm{z}\left(\mathrm{Al}_{5.31} \mathrm{Fe}_{0.69}^{3+}\right)_{\Sigma 6.00}\left[{ }^{\mathrm{T}}\left(\mathrm{S} \mathrm{i}_{5.98} \mathrm{Al}_{0.02}\right)_{\Sigma 6.00} \mathrm{O}_{18}\right]\left(\mathrm{BO}_{3}\right)_{3}{ }^{\mathrm{v}}\left[(\mathrm{OH})_{2.59} \mathrm{O}_{0.41}\right]_{\Sigma 3.00}$ W (O 0.78 F 0.22 ) Σ1.00 . The Italian (co-type) material shows a wider chemical variability, with two different samples from the same locality having ordered chemical formulas: x C a 0.88 N a 0.12 Σ 1.00 Y M g 1.45 F e 0.40 2 + A l 0.79 F e 0.36 3 + Σ 3.00 ${ }^{\mathrm{x}}\left(\mathrm{Ca}_{0.88} \mathrm{Na}_{0.12}\right)_{\Sigma 1.00} \mathrm{Y}\left(\mathrm{Mg}_{1.45} \mathrm{Fe}_{0.40}^{2+} \mathrm{Al}_{0.79} \mathrm{Fe}_{0.36}^{3+}\right)_{\Sigma 3.00}$ Z Al 6 [ T (Si 5.05 Al 0.95 ) Σ6.00 O 18 ](BO 3 ) 3 V [(OH) 2.90 O 0.10 ] Σ3.00 W (O 0.98 F 0.02 ) Σ1.00 (sample 1) and X (Ca 0.71 Na 0.21 ◻ 0.08 ) Σ1.00 Y M g 2.49 F e 0.41 2 + T i 0.10 Σ 3.00 Z A l 5.44 F e 0.46 3 + M g 0.09   V 0.01 Σ 6.00 T S i 5.87 A l 0.13 Σ 6.00 O 18 B O 3 3 V ( O H ) 3 W O 0.61 ( O H ) 0.39 Σ 1.00 ${ }^{\mathrm{Y}}\left(\mathrm{Mg}_{2.49} \mathrm{Fe}_{0.41}^{2+} \mathrm{Ti}_{0.10}\right)_{\Sigma 3.00}^{\mathrm{Z}}\left(\mathrm{Al}_{5.44} \mathrm{Fe}_{0.46}^{3+} \mathrm{Mg}_{0.09} \mathrm{~V}_{0.01}\right)_{\Sigma 6.00}\left[{ }^{\mathrm{T}}\left(\mathrm{Si}_{5.87} \mathrm{Al}_{0.13}\right)_{\Sigma 6.00} \mathrm{O}_{18}\right]\left(\mathrm{BO}_{3}\right)_{3}{ }^{\mathrm{V}}(\mathrm{OH})_{3}{ }^{\mathrm{W}}\left[\mathrm{O}_{0.61}(\mathrm{OH})_{0.39}\right]_{\Sigma 1.00}$(sample 2). Magnesio-lucchesiite is an oxy-species belonging to the calcic group of the tourmaline supergroup. It is related to lucchesiite by the homovalent substitution Y Fe ↔ Y Mg, and to feruvite by the homovalent and heterovalent substitutions Y Fe ↔ Y Mg and Z Al 3+ + W O 2– ↔ Z Mg 2+ + W (OH) 1– . The new mineral was approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification (IMA 2019-025). Occurrences of magnesio-lucchesiite show that its presence is not restricted to replacement of mafic minerals only; it may also form in metacarbonate rocks by fluctuations of F and Al during crystallization of common uvitic tourmaline. High miscibility with other tourmaline end-members indicates the large petrogenetic potential of magnesio-lucchesiite in Mg,Al-rich calc-silicate rocks, as well as contact-metamorphic and metasomatic rocks.

The Raman spectra of five [4] B-bearing tourmalines of different composition synthesized at 700 °C/4.0 GPa (including first-time synthesis of Na-Li- [4] B-tourmaline, Ca-Li- [4] B-tourmaline, and Ca-bearing ◻- [4] B-tourmaline) reveal a strong correlation between the tetrahedral boron content and the summed relative intensity of all OH-stretching bands between 3300–3430 cm –1 . The band shift to low wavenumbers is explained by strong O3-H···O5 hydrogen bridge bonding. Applying the regression equation to natural [4] B-bearing tourmaline from the Koralpe (Austria) reproduces the EMPA-derived value perfectly [EMPA: 0.67(12) [4] B pfu vs. Raman: 0.66(13) [4] B pfu]. This demonstrates that Raman spectroscopy provides a fast and easy-to-use tool for the quantification of tetrahedral boron in tourmaline. The knowledge of the amount of tetrahedral boron in tourmaline has important implications for the better understanding and modeling of B-isotope fractionation between tourmaline and fluid/melt, widely used as a tracer of mass transfer processes.

To examine the effects of chemical composition on thermal expansion in the pyroxene mineral group, X‑ray diffraction data have been collected from room temperature to ~925 °C on a chemically diverse group of 13 pyroxenes including four orthorhombic and nine monoclinic samples. Additionally, resulting unit-cell volumes computed from the XRD data have proven to be good tests for a large number of thermal expansion models; the physical model by Kroll and coworkers and the empirical one by Fei have been found to be especially useful. Modeling also has allowed the connection of present data, collected at temperatures above 25 °C, to the volume data of other workers for much lower temperatures, well below 0 °C, and also extrapolation of values for volume and thermal expansion well beyond the measurement range. We have found for orthopyroxenes that Fe 2+ -Mg 2+ substitution has little effect on thermal expansion coefficients for volume. For clinopyroxenes, however, greater thermal expansion occurs in Ca 2+ - (diopside, augite, hedenbergite, johannsenite) than in Li + - (spodumene) or Na + -bearing members (jadeite, aegirine, kosmochlor). Present data support the observation that differences in volume thermal expansion relate primarily to differences in expansion along the b crystallographic axis. This apparently is due to the greater concentration of M1 polyhedra along the b crystallographic axis, where expansion differences can be related to the shared O1-O1 polyhedral edge and inter-oxygen repulsion that is made easier by divalent, as opposed to trivalent, cation occupancy of the M1 crystallographic site.

Hydrous Fo 91 ringwoodite crystals were synthesized at 20 GPa and high-temperature conditions using a multi-anvil press. Recovered crystals were analyzed using electron microprobe analysis, Raman spectroscopy, infrared spectroscopy, synchrotron Mössbauer spectroscopy, single-crystal X‑ray diffraction, and single-crystal Laue neutron diffraction, to carefully characterize the chemistry and crystallography of the samples. Analysis of the combined data sets provides evidence for the presence of tetrahedrally coordinated ferric iron and multiple hydrogen incorporation mechanisms within these blue-colored iron-bearing ringwoodite crystals. Tetrahedral ferric iron is coupled with cation disorder of silicon onto the octahedrally coordinated site. Cation disorder in mantle ringwoodite minerals may be promoted in the presence of water, which could have implications for current models of seismic velocities within the transition zone. Additionally, the presence of tetrahedrally coordinated ferric iron may cause the blue color of many ringwoodite and other high-pressure crystals.

Element incorporation and partitioning during the evolution of clay minerals have significant implications for element cycling in geochemical processes. The main aim of this experimental study is to further our understanding of element redistribution and crystal growth during smectite evolution under different physicochemical conditions. The precursors (i.e., pure Mg- and Ni-saponite) were separately prepared by hydrothermal syntheses at the same set of temperatures (i.e., RT, 50, 150, 180, 200, and 220 °C) for one day. Then the starting materials were obtained from the mechanical mixtures of the identical molar weight of Mg- and Ni-smectite precursors prepared at the same temperature. Subsequently, Series I samples were obtained by hydrothermally treating different starting materials at 220 °C for two weeks while Series II samples were hydrothermally synthesized under various temperatures (220, 300, 400, and 500 °C) for one week using the starting materials prepared at 220 °C. Both the precursors and resultant saponites were characterized by XRD, FTIR, TEM, and STEM. The FTIR spectra of the precursors only exhibit the v Mg3OH and v Ni3OH bands, corresponding to Mg-saponite and Ni-saponite, respectively. However, the occurrence of v Mg2NiOH and v Ni2MgOH bands in the resultant saponite indicates the dissolution of the corresponding Mg- and Ni-saponite precursors and recrystallization of Mg-Ni mixed saponite. The dissolution extents of Mg- and Ni-saponite precursors, which affect the degrees of random distribution of octahedral Ni and Mg in resultant Mg,Ni-saponite, are significantly controlled by the temperature gap (Δ T ) between the precursors prepared and the resultant Mg,Ni-saponite obtained. In general, a larger Δ T leads to a higher dissolution extent of saponite precursors and a higher degree of random distribution of octahedral Ni and Mg cations in the resultant Mg,Ni-saponite structures. Thus, the distribution mode of octahedral cations in saponite, which is not only relevant to a given hydrothermal temperature but also dependent on Δ T for final products, cannot be used as a geothermometer. TEM and STEM observations provide visual evidence that the particles of saponite coarsen when Δ T is higher than zero. Both the crystal-chemical and morphological features during saponite evolution suggest that saponite particles coarsen mainly via partial/complete dissolution of precursors followed by recrystallization and growth of Mg,Ni-saponite in which crystal growth by layer attachment cannot be excluded. This study presents an experimental approach to evaluate the evolution of clay minerals in terms of crystal chemistry and crystal growth and offers a better understanding of the contributions of clay mineral evolution to element cycling.

Hydroxylpyromorphite, Pb 5 (PO 4 ) 3 (OH), has been documented in the literature as a synthetic and naturally occurring phase for some time but has not previously been formally described as a mineral. It is fully described here for the first time using crystals collected underground in the Copps mine, Gogebic County, Michigan. Hydroxylpyromorphite occurs as aggregates of randomly oriented hexagonal prisms, primarily between about 20–35 μm in length and 6–10 μm in diameter. The mineral is colorless and translucent with vitreous luster and white streak. The Mohs hardness is ~3½–4; the tenacity is brittle, the fracture is irregular, and indistinct cleavage was observed on {001}. Electron microprobe analyses provided the empirical formula Pb 4.97 (PO 4 ) 3 (OH 0.69 F 0.33 Cl 0.06 ) Σ1.08 . The calculated density using the measured composition is 7.32 g/cm 3 . Powder X‑ray diffraction data for the type material is compared to data previously reported for hydroxylpyromorphite from the talc mine at Rabenwald, Austria, and from Whytes Cleuch, Wanlockhead, Scotland. Hydroxylpyromorphite is hexagonal, P 6 3 / m , at 100 K, a = 9.7872(14), c = 7.3070(10) Å, V = 606.16(19) Å 3 , and Z = 2. The structure [ R 1 = 0.0181 for 494 F >4σ( F ) reflections] reveals that hydroxylpyromorphite adopts a column anion arrangement distinct from other members of the apatite supergroup due to the presence of fluorine and steric constraints imposed by stereoactive lone-pair electrons of Pb 2+ cations. The F – anion sites are displaced slightly from hydroxyl oxygen anions, which allows for stronger hydrogen-bonding interactions that may in turn stabilize the observed column-anion arrangement and overall structure. Our modern characterization of hydroxylpyromorphite provides deeper understanding to a mineral useful for remediation of lead-contaminated water.

Barite (BaSO 4 ) is a common mineral in sandstone that must be removed during separation of detrital zircon (ZrSiO 4 ). One widespread technique for the removal of barite exploits its lesser tenacity by milling the barite and zircon mixture in a ball mill. Here we test the extent to which such milling affects zircon and thus whether the milling could introduce bias into the detrital zircon sample. We then describe a new chemical technique for the removal of barite from detrital zircon. We find that milling a mixture of barite and zircon both breaks and causes loss of zircon grains, potentially introducing bias into a detrital zircon sample. Boiling barite in a 0.94 molar aqueous solution of sodium carbonate for 4 h converts most grains to barium carbonate. The barium carbonate grains are opaque white and thus visually distinguishable from zircon, allowing separation by hand under a stereoscopic microscope. Alternatively, the barium carbonate grains can be dissolved by boiling in 16 wt% nitric acid for 30 min. In our experiments, boiling zircon in sodium carbonate solution and/or concentrated (65 wt%) nitric acid cleaned the surfaces of and the cracks in the grains but did not visibly change the zircon surfaces in other ways. Boiling only in concentrated nitric acid did not measurably affect the U-Pb and Lu-Hf isotopic systems in zircon interiors, and boiling in a sodium carbonate solution followed by concentrated nitric acid did not detectably alter the Lu-Hf isotopic system. However, boiling in a concentrated sodium carbonate solution followed by concentrated nitric acid did disturb the U-Pb isotopic system in zircon interiors. We recognize this disturbance due to a ~0.5% reduction in 206 Pb/ 238 U dates of treated zircon. Our results highlight the importance of proper technique during zircon isolation to minimize the introduction of bias into the sample.

Minerals formed at high pressures are sensitive indicators of extreme pressure-temperature conditions that occur in nature. The discovery of the high-pressure polymorph of (Fe,Ni) 2 P, allabogdanite in the surficial pyrometamorphic rocks of the Hatrurim Formation (the Mottled Zone) surrounding the Dead Sea basin in Israel is the first terrestrial occurrence of a mineral previously only found in iron meteorites. Stepwise annealing experiments demonstrate that allabogdanite is metastable at ambient pressure and that it irreversibly transforms into its low-pressure polymorph, barringerite, upon heating to 850 ± 50 °C. High-pressure, high-temperature diamond-anvil cell (DAC) experiments confirm the results of annealing experiments. The DAC data indicate that Hatrurim allabogdanite is metastable below 7.4 GPa, and the low- to high-pressure phase transition (barringerite → allabogdanite) occurs at 25 ± 3 GPa and 1400 ± 100 °C. The observed transition pressure of Hatrurim allabogdanite is significantly higher than that of pure synthetic Fe 2 P (8 GPa) due to partial substitution of Fe for Ni (4 wt%) and Mo (2.5 wt%). Because the influence of substituting impurities on the conditions of phase transitions can be unexpectedly strong, our findings confirm that caution should be exercised when extrapolating data from experiments on synthetic compounds to natural systems. Based on the discovery of terrestrial allabogdanite (Fe,Ni) 2 P coupled with experiments probing the phase transitions in this natural composition, we contend that terrestrial allabogdanite formed via transformation from barringerite and posit potential scenarios of its formation.

Low-temperature hydrous alteration of FeTi oxide-rich ferroan peridotite, Laramie anorthosite complex, Wyoming, produced silician ferrihydrite, cronstedtite, greenalite, hisingerite, and talc. Ferrihydrite occurs as nanocrystals in ~50 nm diameter granules that form monomineralic masses up to 300 μm across. It is inferred to have formed by the replacement of an igneous sulfide such as pyrrhotite. Electron diffraction shows the ferrihydrite to be a 9-line variety. Si-rich cronstedtite formed thin rims around the ferrihydrite, and talc grew patchily around the cronstedtite. Greenalite formed in ~10 μm cracks through all the above minerals and olivine, and hisingerite microveinlets partially replaced olivine. Igneous minerals remaining include olivine Fa 46 , magnetite, ilmenite, hornblende, biotite, and trace clinopyroxene. Correlations among the constituents of ferrihydrite determined by electron microprobe, including anhydrous totals, indicate progress during the growth of two charge-balanced exchanges involving silica enrichment: an inverse cronstedtite substitution (MgFe 2+ ,Si) (Fe 3+ Mn 3+ ) –2 and an inverse hydrogarnet substitution SiH –4 . The cronstedtite exchange requires charge and size balance across nearest-neighbor T and O crystal sites, suggesting crystal-interior rather than crystal-surface control. Ferrihydrite’s composition reflects time- and space-related variations in the chemical potentials of components in the hydrous fluid at the site of alteration. An upper limit for SiO 2 of 14–15 wt%, or ≈1.0 Si per 5-cation formula unit, would seem to correspond to the limit of availability in ferrihydrite of tetrahedral sites open to the entry of Si. Our EPMA data, projected to zero SiO 2 , indicate an anhydrous total of ≈83 wt% for end-member ferrihydrite, a number that matches the formula: Fe 10 O 15. 9H 2 O. The geochemical properties of Laramie ferrihydrite are shared by some samples of altered chondritic and Martian meteorites. Ferrihydrite on Earth commonly occurs as a surface deposit; unlike the Laramie occurrence, these lack the microspatial coherence of replacements/ pseudomorphs to show systematic, structure-related element variations. The superior crystal quality of the Laramie ferrihydrite likely contributed to its unique compositional variability.

This study demonstrates the synergies and limits of multiple measurement types for the detection of smectite chemistry and oxidation state on planetary surfaces to infer past geochemical conditions. Smectite clay minerals are common products of water-rock interactions throughout the solar system, and their detection and characterization provides important clues about geochemical conditions and past environments if sufficient information about their composition can be discerned. Here, we synthesize and report on the spectroscopic properties of a suite of smectite samples that span the intermediate compositional range between Fe(II), Fe(III), Mg, and Al end-member species using bulk chemical analyses, X‑ray diffraction, Vis/IR reflectance spectroscopy, UV and green-laser Raman spectroscopy, and Mössbauer spectroscopy. Our data show that smectite composition and the oxidation state of octahedral Fe can be reliably identified in the near infrared on the basis of combination and fundamental metal-OH stretching modes between 2.1–2.9 μm, which vary systematically with chemistry. Smectites dominated by Mg or Fe(III) have spectrally distinct fundamental and combination stretches, whereas Al-rich and Fe(II)-rich smectites have similar fundamental minima near 2.76 μm, but have distinct combination M-OH features between 2.24 and 2.36 μm. We show that with expanded spectral libraries that include intermediate composition smectites and both Fe(III) and Fe(II) oxidation states, more refined characterization of smectites from MIR data is now possible, as the position of the 450 cm –1 absorption shifts systematically with octahedral Fe content, although detailed analysis is best accomplished in concert with other characterization methods. Our data also provide the first Raman spectral libraries of smectite clays as a function of chemistry, and we demonstrate that Raman spectroscopy at multiple excitation wavelengths can qualitatively distinguish smectite clays of different structures and can enhance interpretation by other types of analyses. Our sample set demonstrates how X‑ray diffraction can distinguish between dioctahedral and trioctahedral smectites using either the (02,11) or (06,33) peaks, but auxiliary information about chemistry and oxidation state aids in specific identifications. Finally, the temperature-dependent isomer shift and quadrupole splitting in Mössbauer data are insensitive to changes in Fe content but reliability differentiates Fe within the smectite mineral structure.

The understanding of crystal nucleation and growth has evolved over the past two decades from the conventional atom-by-atom model to a non-classical approach, involving particle aggregation and amorphous transformation pathways. Whereas aggregation of particles instead of individual atoms/ions/ molecules has been recognized as a common crystallization pathway at the Earth’s surface conditions, few cases are known for high-temperature (e.g., melt) mineralization, which is of great importance for understanding geological processes. Here, we present texture data for natural (e.g., igneous and metamorphic biotite and muscovite) and synthetic (e.g., fluorophlogopite) phyllosilicates suggesting that a particle attachment formation should be considered, although other crystal growth models cannot be excluded. A nonclassical crystallization model is proposed for phyllosilicates forming at elevated temperatures in magmatic and metamorphic environments whereby oriented attachment of building blocks occurs along the (001) plane or the [001] direction, or both simultaneously. In this model, the crystallization of phyllosilicates occurs in steps, with multi-ion complexes forming nanoparticles, and nanoparticles coalescing (self-assembly) to form nano-flakes that become domains in larger crystallites by oriented attachment. Adjacent domains can share a common crystallographic orientation or may be rotated at various angles relative to each other. Nanoparticles may be associated with distorted bonds or may be space separated. Thus, the phyllosilicate grows into a mosaic crystal. Mosaic crystals can also form following classical crystallization models, but the process differs in that the mosaic character involves the intergrowths of nucleation sites (classical crystal-growth process) instead of the coalescence of nanoparticles building blocks (crystallization by particle attachment). These processes may be discerned by the textural differences that result. Oriented particle attachment of building blocks in phyllosilicates is recognized by a loss of closest packing by bond distortion or by space separation at domain boundaries. Crystallization by atom attachment occurs with closest packing within layers, and particles grow independently. The two processes may occur within a single environment and are not mutually exclusive. However, defects generated, for example, by chemical inhomogeneity, mechanical deformation, or sample preparation, cannot be completely excluded, although the use of synthetic, end-member material (e.g., fluorophlogopite) generated from a melt reduces these possibilities. Nonetheless, a particle attachment model is a viable alternative to classical crystal growth processes for high-temperature phyllosilicates with the presented supporting data, although still not yet proven.

One tourmaline-bearing leucogranite dike occurs in the Yushishanxi Nb-Ta mining area in the Yushishan district of the South Qilian orogenic belt in western China. Abundant tourmalines have been identified in the leucogranite, including disseminated, crosscutting quartz-tourmaline veins and tourmaline veinlets. Detailed petrological, geochemical, and boron isotopic studies indicate that these tourmalines have distinctive core-rim zoning signatures and significant chemical variations, which can be divided into four paragenetic generations. Generation-I tourmalines in the leucogranite have a magmatic origin and were followed by hydrothermal Generation-II, Generation-III, and Generation-IV. Tourmalines from all four generations belong to the alkali group and the schorl-dravite solid-solution series. The most notable features are the variations in Mg, Fe, and Ca contents. The variable Mg/(Mg+Fe) ratios in generations I to IV may be attributed to the multiple influx and/or interaction of initially magma-derived hydrothermal fluid with surrounding rocks, magma-derived Fe-rich hydrothermal fluid, and external metamorphic fluid. In situ B-isotope analyses of tourmaline yield a total range of δ 11 B values from –11.7 to –6.0‰. The earliest Generation-I tourmalines have δ 11 B values of –11.1 to –9.6‰, whereas Generation-II and Generation-III tourmalines record a higher δ 11 B value of –9.5 to –6.0‰ and –9.3 to –6.9‰, respectively. Such an increase is mainly controlled by boron-isotope fractionation between melt-fluid and tourmaline-fluid, Rayleigh fractionation, and also, to some extent, by a hydrothermal recharge from the same magma source. Generation-IV tourmalines have the lightest B-isotope values (down to –11.7‰). This shift back to lighter B-isotopes in the Generation-IV cannot be explained by closed-system crystallization and isotope fractionation of known phases. Alternatively, a lighter source of boron via fluid circulation from the surrounding metamorphic rocks can explain this light boron isotope composition. Therefore, we propose that an isotopically light fluid may have derived from B release during mica breakdown within the surrounding metamorphic rocks.

A new mineral species, zhanghuifenite, ideally N a 3 M n 4 2 + M g 2 A l P O 4 6 , $\mathbf{N a}_{3} \mathbf{M n}_{4}^{2+} \mathbf{M} \mathbf{g}_{2} \mathbf{A l}\left(\mathbf{P O}_{4}\right)_{6},$has been found in the Santa Ana mine, San Luis province, Argentina. It occurs in irregular veinlets or patches, 5 mm thick, in a nodule of beusite interlaminated with lithiophilite. Broken pieces of zhanghuifenite are blocky or tabular. Single crystals are up to 0.8 × 0.5 × 0.5 mm. No twinning or parting is observed macroscopically. The mineral is deep green, transparent with pale green streak and vitreous luster. It is brittle and has a Mohs hardness of ~5 with good cleavage on {010}. The measured and calculated densities are 3.63(2) and 3.62 g/cm 3 , respectively. Optically, zhanghuifenite is biaxial (+), with a = 1.675(2), β = 1.680(2), γ = 1.690(2) (white light), 2 V (meas) = 74(2)°, and 2 V (calc) = 71°. The calculated compatibility index based on the empirical formula is 0.020 (excellent). An electron microprobe analysis yields an empirical formula (based on 24 O apfu) N a 2.80 C a 0.11 Σ 2.91 M n 3.09 2 + F e 0.47 2 + M g 0.36 Σ 3.92 M g 1.31 F e 0.69 2 + Σ 2.00 $\left(\mathrm{Na}_{2.80} \mathrm{Ca}_{0.11}\right)_{\Sigma 2.91}\left(\mathrm{Mn}_{3.09}^{2+} \mathrm{Fe}_{0.47}^{2+} \mathrm{Mg}_{0.36}\right)_{\Sigma 3.92}\left(\mathrm{Mg}_{1.31} \mathrm{Fe}_{0.69}^{2+}\right)_{\Sigma 2.00}$ A l 0.81 F e 0.19 3 + P O 4 6 . $\left(\mathrm{Al}_{0.81} \mathrm{Fe}_{0.19}^{3+}\right)\left(\mathrm{PO}_{4}\right)_{6}.$Zhanghuifenite is insoluble in water or hydrochloric acid. Zhanghuifenite is isostructural with bobfergusonite, a member of the alluaudite supergroup. It is monoclinic, with space group P 2 1 / n , Z = 4, and unit-cell parameters a = 12.8926(3), b = 12.4658(3), c = 10.9178(2) Å, β = 97.9200(10)°, and V = 1737.93(7) Å 3 . The crystal structure of zhanghuifenite contains six octahedral M (= Mn, Fe, Mg, Al) sites and five X (= Na, Mn, Ca) sites with coordination numbers between 6 and 8. The six M octahedra share edges to form two types of kinked chains extending along [101], with one consisting of M 1- M 4- M 5 and the other M 2- M 3- M 6. These chains are joined by PO 4 tetrahedra to form sheets parallel to (010), which are linked together through corner-sharing between PO 4 tetrahedra and M O 6 octahedra in the adjacent sheets, leaving open channels parallel to a , where the large X cations are situated. Zhanghuifenite differs from bobfergusonite in two major aspects. One is that the M 4 and M 5 sites in the former are mainly occupied by Mg, but by Fe 2+ and Fe 3+ , respectively, in the latter. The other is that the X 2- X 5 sites in zhanghuifenite are all nearly or fully filled with Na, resulting in 3 Na apfu in the ideal formula, but X 4 and X 5 are merely half-occupied in bobfergusonite, giving rise to 2 Na apfu.

Carbonates are excellent carriers for divalent cations such as Ca, Mg, and Sr, and knowledge about their stability and solid solutions is important to understanding the global strontium cycle. To shed light on the topology of the two-phase field between calcite-type and aragonite-type (Ca,Sr)CO 3 solid solutions as a function of temperature and pressure, and to learn more about the distribution of Sr and Ca between carbonates and fluid, we studied the system CaCO 3 -SrCO 3 -H 2 O at 600 °C in the pressure range 0.5–5 kbar. Conventional and rapid-quench hydrothermal syntheses were performed using a range of different starting materials. All bulk compositions were within the assumed/postulated two-phase field of calcite-type and aragonite-type (Ca,Sr)CO 3 solid solutions. The run products were analyzed by scanning electron microscopy, electron microprobe analysis, and powder X‑ray diffraction with Rietveld refinement. The results show that the heterotype solid solution is more extensive than previously assumed, with calcite incorporating up to 20 mol% SrCO 3 , which is twice as much as previously predicted. The compositional range of the aragonite-type solid solution was identical to that found in the literature. Using the data from this study, an updated version of the phase diagram P-X (Sr) at 600 °C for the CaCO 3 -SrCO 3 system was calculated. The phase diagram does not support a phase transition within the trigonal (Ca,Sr)CO 3 solid solution associated with rotational disorder of the CO 3 -groups. This order-disorder phase transition was previously postulated to explain some observed compositional trends in this system. Our new data are in line with other more recent studies. The distribution of Sr and Ca between the fluid and solid phases D = X S r solid  / X S r fluid  $D=X_{\mathrm{Sr}}^{\text {solid }} / X_{\mathrm{Sr}}^{\text {fluid }}$is near to 1.0 for calcite-type and on average around 2.0 for aragonite-type solid solutions. This contrasts with silicate-fluid systems in which Sr typically shows a strong preference for the fluid phase compared with Ca.

A recent paper devoted to unusual fine-scale tubular tunnels found in pyrope and almandine garnets suggested that the 5 to 100 μm diameter tunnels were produced by an endolithic organism that is able to chemically dissolve and penetrate the mineral, perhaps in search of nutrients. The hypothesized microbial boring of the garnets was based on the finding of endolithic remains in the tunnels, but boring alone does not adequately explain the linear, highly aligned or occasionally branched tunnels that have been imaged. We have prepared this short Letter, in the spirit of Occam’s Razor, to highlight the very probable role that dislocations play in the creation of such tunnels by preferential etching of a dislocation-rich deformation microstructure. The geometrical features of the tunnels possess all the characteristics of classical dislocation substructures that have been observed in natural and synthetic garnets.