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All available volume and elasticity data for the garnet end-members grossular, pyrope, almandine and spessartine have been re-evaluated for both internal consistency and for consistency with experimentally measured heat capacities. The consistent data were then used to determine the parameters of third-order Birch–Murnaghan EoS to describe the isothermal compression at 298 K and a Mie–Grüneisen–Debye thermal-pressure EoS to describe the PVT behaviour. In a full Mie–Grüneisen–Debye EoS, the variation of the thermal Grüneisen parameter with volume is defined as γ = γ 0 V V 0 q . For grossular and pyrope garnets, there is sufficient data to refine q which has a value of q  = 0.8(2) for both garnets. For other garnets, the data do not constrain the value of q and we therefore refined a q- compromise version of the Mie–Grüneisen–Debye EoS in which both γ / V and the Debye temperature θ D are held constant at all P and T , leading to ∂ C V ∂ P T = 0 , parallel isochors and constant isothermal bulk modulus along an isochor. Final refined parameters for the q- compromise Mie–Grüneisen–Debye EoS are: Pyrope Almandine Spessartine Grossular V 0 (cm 3 /mol) a 113.13 115.25 117.92 125.35 K 0T (GPa) 169.3 (3) 174.6 (4) 177.57 (6) 167.0 (2) K 0 T ′ 4.55 (5) 5.41 (13) 4.6 (3) 5.07 (8) θ D0 771 (28) 862 (22) 860 (35) 750 (13) γ 0 1.185 (12) 1.16 (fixed) 1.18 (3) 1.156 (6) for pyrope and grossular, the two versions of the Mie–Grüneisen–Debye EoS predict indistinguishable properties over the metamorphic pressure and temperature range, and the same properties as the EoS based on experimental heat capacities. The biggest change from previously published EoS is for almandine for which the new EoS predicts geologically reasonable entrapment conditions for zircon inclusions in almandine-rich garnets.

The enrichment of manganese in peraluminous (S-type) granitic melts beginning with the anatexis of metapelitic rock and ending with the crystallization of highly evolved pegmatites is explained using experimentally derived mineral-melt partition coefficients and solubility data for Mn-rich garnet. Mineral-melt partition coefficients for Fe, Mg, and Mn between garnet, cordierite, tourmaline, and peraluminous, B-bearing hydrous granitic melt were measured between 650 and 850°C at 200 MPaH2O. The compositions of garnet and tourmaline synthesized in these experiments are similar to those found in nature. Garnets evolve from Sps51Alm23Prp25 to Sps81Alm15Prp4 with decreasing temperature. The Mn content of cordierite increases with decreasing temperature. The composition of tourmaline does not vary with temperature. Partition coefficients, DMα/L, and exchange coefficients, KDα/L=DMα/L/DNα/L where α is a mineral, L is liquid (melt), and M and N are different elements, are presented for mineral-glass pairs. Partition coefficients for Mg, Fe, and Mn increase with decreasing temperature for garnet, tourmaline, and cordierite. The precipitation of garnet alone results in a progressive increase of MgO/FeO and a decrease of MnO/FeO in the melt. Crystallization of cordierite and tourmaline results in a decrease of MgO/FeO and an increase of MnO/FeO in melt. Tourmaline is most efficient at concentrating Mn in residual liquids. The trend toward increasing Mn/Fe in natural garnets in granites and pegmatites is not controlled by garnet itself, but instead by the crystallization of other mafic minerals in which Mg and Fe are more compatible than is Mn. A Rayleigh fractionation model constitutes a test of the partition coefficients reported in this manuscript. The starting composition for the model is that of a liquid (melt inclusions) from an anatectic S-type source. Normative modes of cordierite and biotite are calculated from that composition and are similar to modes of these minerals in natural occurrences. The model consists of crystallization of a cordierite-biotite granite from 850 to 650°C. The model predicts that ∼95% crystallization of the starting composition is required to reach saturation in spessartine-rich garnet at near-solidus temperatures. The model, therefore, is consistent with the occurrence of spessartine as restricted to highly fractionated granite-pegmatite systems at the end stages of magmatism.

A new member of the epidote supergroup, ferriandrosite-(Ce), ideally MnCeFe3+AlMn2+(Si2O7)(SiO4)O(OH), was found at the Július manganese ore occurrence near Betliar, Rožňava Co., Kosice Region, Slovakia. It occurs as subhedral grains and polycrystalline aggregates, up to 0.3 mm in size, enclosed in pyroxmangite. Other associated minerals are spessartine, rhodochrosite, quartz, baryte and pyrosmalite-(Mn). Ferriandrosite-(Ce) is dark brown, with a light brown streak and vitreous lustre. The Mohs hardness is ∼6 1/2 to 7 and tenacity is brittle with no observable cleavage or fracture. The calculated density is 4.321 g·cm-3. Ferriandrosite-(Ce) is optically biaxial (+), with weak pleochroism, high surface relief and the mean calculated refractive index is 1.832. The empirical structural formula of ferriandrosite-(Ce), based on 13 anions per formula unit, is A1(Mn2+0.63Ca0.35Ce0.02)Σ1.00A2(Ce0.53La0.27Nd0.14Pr0.05REE *0.01)Σ1.00M1(Fe3+0.41Al0.12V3+0.01Mg0.40Ti0.05)Σ0.99M2Al 1.00M3(Mn2+0.75Fe2+0.22Mg0.03)Σ1.00T1-3Si3.00O11O4(O0.67F0.33) (OH), where REE* are minor rare earth elements. Ferriandrosite-(Ce) is monoclinic, space group P21/m, a = 8.8483(4) Å, b = 5.7307(3) Å, c = 10.0314(5) Å, β = 113.3659(15)°, V = 466.95(4) Å3 and Z = 2. The crystal structure of ferriandrosite-(Ce) was refined to a final R1 = 0.0210 for 1910 reflections with Fo > 4σ(Fo) and 127 refined parameters. Structural features of ferriandrosite-(Ce) are discussed and compared with other members of the androsite-series.

The incorporation mechanisms and diffusional loss of hydrogen in garnet have been experimentally investigated. A suite of gem-quality hydrous spessartine- and grossular-rich garnets were analysed by Fourier transform infrared spectroscopy (FTIR) and by ion microprobe (SHRIMP-SI) to determine the calibration coefficients for quantification of FTIR data. The excellent agreement between measured absorption and OH/O indicates that the same molar extinction coefficient can be used for spessartine and grossular. The coefficient of 14400 l mol− 1 cm− 2 proposed by Maldener et al. (Phys Chem Miner 30:337–344, 2003) seems the most appropriate for both minerals. A grossular with 6.4% andradite and 1.6% almandine containing 834 ppm H2O, and an almost pure spessartine with 282 ppm H2O, were selected for diffusion experiments. 1.5-mm cubes of garnets were heated between 12 h and 10 days at 1 atm under various temperature (750–1050 °C) and oxygen fugacity (\\[{f_{{{\\text{O}}_2}}}\\]) conditions, (ΔQFM + 15.2 to − 3.0). Diffusion profiles were acquired from sections through the cubes using FTIR, with a deconvolution algorithm developed to assess peak-specific behaviour. Different families of peaks have been identified based on their diffusive behaviour, representing hydrogen incorporated in different H-bearing defects. A dominant, fast, strongly \\[{f_{{{\\text{O}}_2}}}\\]-dependent oxidation-related diffusion mechanism is proposed \\[\\left( {\\{ {{\\text{M}}^{2+}}+{{\\text{H}}^+}\\} +\\frac{1}{4}{{\\text{O}}_2}={{\\text{M}}^{3+}}+\\frac{1}{2}{{\\text{H}}_2}{\\text{O}}} \\right)\\] (M=Fe, Mn) with a relatively low activation energy (158 ± 19 kJ mol− 1). This diffusion mechanism is likely restricted by availability of ferrous iron in grossular. At low oxygen fugacity, this diffusion mechanism is shut off and the diffusivity decreased by more than three orders of magnitude. A second, slower hydrogen diffusion mechanism has been observed in minor bands, where charge balance might be maintained by diffusion of cation vacancies, with much higher activation energy (≈ 200–270 kJ mol− 1). Spessartine shows clear differences in peak retentivity suggesting that up to four different H sites might exist. This opens exciting opportunities to use hydrogen diffusion in garnet as speedometer. However, it is essential to constrain the main diffusion mechanisms and the oxygen fugacity in the rocks investigated to obtain timescales for metamorphic or igneous processes.

A rounded fragment of a multicoloured tourmaline crystal (2.5 cm diameter), collected from the secondary gem deposit of Mavuco, Alto Ligona pegmatite district, Mozambique, has been investigated using a multi-analytical approach, with the objective of reconstructing its growth history. The sample represents a core-to-rim section, perpendicular to the c axis, of a crystal characterised by a variety of colours. These change from a black core to an intermediate zone with a series of colours, yellow, blue-green and purple, to a final dark-green prismatic overgrowth. These changes are the result of a wide variation in Fe, Mn, Ti and Cu concentrations and their redox state. The black core is characterised by enrichment in Fe and Mn, with iron present in its divalent state. The yellow zone shows a progressive depletion in Fe and its colouration is caused by Mn2+ and Mn2+-Ti4+ IVCT interactions. The progressive decrease in Mn coupled with the absence of Ti, and the lack of Fe, implies that Cu2+ acts as the only chromophore in the pale blue-green zone. The dominant colour-causing agent of the purplish zone is Mn3+, denoting a change in redox environment; however, even though the amount of Cu remains significant, its chromophore effect is obscured by Mn3+. The dark-green prismatic overgrowth, characterised by a sharp increase in Fe, Mn and also Ca, is interpreted as a late-stage partial re-opening of the geochemical system. This occurrence could potentially be related to mechanical instability of the cavity in which the crystal grew.

In this study about spessartines,13 spessartine samples with proper conditions are used for FTIR spectrum and UV-Vis spectrum tests to find the gemological characteristics and coloration mechanism. In the UV-Vis spectrum, spessartines’ special color has a relationship with the absorption bands at about 460 nm and 480 nm. Based on the CIE 1976 L*a*b* colour system, we come to the conclusion that both color coordinates a* and b* control the value of chroma C*, and color coordinate b* mostly controls the hue angle h°. We also explore how the different chromogenic ions FeOtot and MnO and their ratio FeOtot/MnO influence spessartines’ color, finding that the color of spessartine samples is influenced by both Mn and Fe. By analyzing the FTIR spectrum, we discovered that as the content of Mn decreases, and the A, C and D peaks move to the position of a longer wave, the color of spessartine samples also changes significantly. By using the standard light source D65, we find that N9.5 Neutral Grey Background is the best background to grade the color of spessartines.

Manganese mineralization was discovered at the Zimná Voda occurrence near Prakovce located in the Spišsko-gemerské rudohorie Mountains, eastern Slovakia. The mineralization is hosted in the Early Paleozoic metamorphic rocks of the Gemeric Unit in the Western Carpathians and it represents the only occurrence of metamorphosed manganese mineralization discovered within the Drnava Formation (Devonian). Manganese minerals are directly associated with magnetite bodies forming isolated lenses of a strata-bound oxidic Fe-ores. The manganese mineralization underwent metamorphism during the Variscan and Alpine tectonometamorphic events, resulting in two types of distinct mineral assemblages. The older assemblage includes rhodonite-ferrorhodonite series, rhodochrosite, kutnohorite, spessartine, fluorapatite and quartz with magnetite impregnations. These assemblages are intersected by younger mineral assemblages present in veins and vugs. This later assemblage consists of rhodochrosite, kutnohorite, baryte, clino-suenoite to clino-ferro-suenoite, minerals of the epidote supergroup, quartz and relatively rare sulfidic mineralization including pyrite, galena, sphalerite and cobaltite along with hübnerite. This stage also features the formation of garnets predominantly composed of the andradite molecule, with locally preserved spessartine remnants in the garnet centres. The subsequent younger post-Variscan metamorphic event is characterized by a high influx of rare earth elements (REE), leading to the formation of arare ferriallanite-(Ce), ferriallanite-(La), ferriakasakaite-(La) and ferriakasakaite-(Ce). Allanite-subgroup minerals form a strongly zonal poly crystalline dark brown aggregates with size up to 50 pm, generated from hydrothermal fluids affecting the mineral composition of the late-stage mineral assemblage.

The Penouta Sn-Ta deposit, in the northwest of Spain, is a greisenized granitic cupola where Ta minerals occur mainly as disseminations in a leucogranite body intruded in Precambrian-Lower Cambrian gneisses and mica-schists. This leucogranite is a medium- to fine-grained inequigranular rock consisting mainly of quartz, albite, K-feldspar and muscovite. Accessory minerals are mainly of spessartine, zircon, cassiterite, Nb-Ta oxides, monazite, xenotime, native bismuth and pyrite. The alteration processes were mainly albitization, muscovitization and kaolinitization. This leucogranite is peraluminous and P-poor, with 0.03-0.07 wt.% P2O5, 900-1500 ppm Rb, 30-65 ppm Cs, 120-533 ppm Li, 80-140 ppm Ta, 51-81 ppm Nb and up to 569 ppm of Sn. Mineralogical characterization of Nb-Ta oxide minerals was determined by X-ray diffraction, scanning electron microscopy, electron microprobe analysis and mineral liberation analysis. Mn-rich members of the columbite-group minerals (CGM) are the most common Ta-bearing phases, but microlite, wodginite, tapiolite and Ta-rich cassiterite occur also. CGM crystals are commonly zoned concentrically, with a Nb-rich core surrounded by a Ta-rich rim, with a sharp boundary between them. Convoluted zoning occurs also. Dissolution textures resulting from the corrosion of columbite and tantalite rims, in particular, are common. The Mn/(Mn + Fe) ratio varies between 0.33 and 0.97 and the Ta/(Ta + Nb) ratio between 0.07 and 0.93. Wodginite and microlite formed as late replacements of CGM and occur associated with tantalite and cassiterite. Subhedral to anhedral cassiterite crystals, usually up to 200 µm across, occur in two generations: the earlier one is Nb,Ta-poor whereas in the later generation, the Ta content can reach >9 wt.% of Ta2O5 and 1.7 wt.% of Nb. The presence of a fluid phase in the apical zone of the granite, probably related to the separation of a fluid/vapour of the melt, could explain the sponge-like textures, the Ta enrichment associated with these textures, the occurrence of Ta-enriched mineral phases (microlite and wodginite) and their common interstitial character.

The pressure–volume–temperature ( P – V – T ) equation of state (EoS) of two natural garnet samples along spessartine–almandine (Spe–Alm) join has been measured at high temperature up to 800 K and high pressures up to 15.46 and 16.17 GPa for Spe 64 Alm 36 and Spe 38 Alm 62 , respectively, using in situ angle-dispersive X-ray diffraction and diamond anvil cell. Analysis of room-temperature P – V data to a third-order Birch–Murnaghan EoS yields: V 0  = 1,544.4 ± 0.4 Å 3 , K 0  = 180 ± 3 GPa and K 0 ′  = 4.0 ± 0.4 for Spe 38 Alm 62 , and V 0  = 1,557.5 ± 0.3 Å 3 , K 0  = 176 ± 2 GPa and K 0 ′  = 4.0 ± 0.3 for Spe 64 Alm 36 . Fitting of our P – V – T data by means of the high-temperature third-order Birch–Murnaghan EoS gives the thermoelastic parameters: V 0  = 1,544.6 ± 0.6 Å 3 , K 0  = 180 ± 4 GPa, K 0 ′  = 4.0 ± 0.4, (∂ K /∂ T ) P  = −0.028 ± 0.005 GPa K −1 and α 0  = (3.16 ± 0.14) × 10 −5  K −1 for Spe 38 Alm 62 , and V 0  = 1,557.7 ± 0.9 Å 3 , K 0  = 176 ± 4 GPa, K 0 ′  = 4.0 ± 0.5, (∂ K /∂ T ) P  = −0.029 ± 0.005 GPa K −1 and α 0  = (3.04 ± 0.16) × 10 −5  K −1 for Spe 64 Alm 36 . The results confirm that almandine content increases the bulk modulus of the spessartine–almandine join following a nearly ideal mixing model. The relation between bulk modulus and almandine mole fraction ( X Alm ) in this garnet join is derived to be K 0 (GPa) = 171.6(±2.6) + 10.9(±1.8) X Alm . Present results are also compared with previously studies determined the thermoelastic properties of other garnets.

The low-temperature magnetic properties and Néel temperature, TN, behavior of four silicate substitutional solid solutions containing paramagnetic ions are analyzed. The four systems are: fayaliteforsterite olivine [Fe22+SiO4-Mg2SiO4], and the garnet series, grossular-andradite [Ca3(Alx,Fe1-x3+)2Si3O12], grossular-spessartine [(Cax,Mn1-x2+)3Al2Si3O12], and almandine-spessartine [(Fex2+,Mn1-x2+)3Al2Si3O12]. Local magnetic behavior of the transition-metal-bearing end-members is taken from published neutron diffraction results and computational studies. TN values are from calorimetric heat capacity, CP, and magnetic susceptibility measurements. These end-members, along with more transition-metal-rich solid solutions, show a paramagnetic to antiferromagnetic phase transition. It is marked by a CP λ-anomaly that decreases in temperature and magnitude with increasing substitution of the diamagnetic component. For olivines, TN varies between 65 and 18 K and TN for the various garnets is less than 12 K. Local magnetic behavior can involve one or more superexchange interactions mediated through oxygen atoms. TN behavior shows a quasi-plateau-like effect for the systems fayalite-forsterite, grossular-andradite, and grossular-spessartine. More transition-metal-rich crystals show a stronger TN dependence compared to transition-metal-poor ones. The latter may possibly show superparamagnetic behavior. (Fex2+,Mn1-x2+)3Al2Si3O12 garnets show fundamentally different magnetic behavior. End-member almandine and spessartine have different and complex interacting local superexchange mechanisms and intermediate compositions show a double-exchange magnetic mechanism. For the latter, TN values show negative deviations from linear interpolated TN values between the end-members. Double exchange seldom occurs in oxides, and this may be the first documentation of this magnetic mechanism in a silicate. TN behavior may possibly be used to better understand the nature of macroscopic thermodynamic functions, CP and S°, of both end-member and substitutional solid-solution phases.