Mineralogical and geochemical characteristics of scheelite-bearing skarns, and genetic relations between skarn mineralization and petrogenesis of the associated granitoid pluton at Sargipali, Sundergarh District, Eastern India
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Keywords:
Hornfels
Actinolite
Pyroxene
Grossular
Metasomatism
Titanite
Almandine
Grossular
Titanite
Almandine
Andradite
Pyrope
Cassiterite
Hornblende
Arsenopyrite
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Hornfels
Actinolite
Pyroxene
Grossular
Metasomatism
Titanite
Almandine
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Garnet skarn mineralization was recently studied at the Trohanka locality near Prakovce (Gemeric Unit, Eastern Slovakia). Ca-skarn forms lenticular bodies in green schist environment. It mainly consists of zonal garnets, pyroxenes, amphiboles and magnetite accumulations. Studied garnets are rich in andradite component (up to 89.95 mol. %) with minor grossular component (6.83 - 39.67 mol. %). Strong oscillatory zoning in andradite is caused by substitution of Fe3+ and Al3+. Most pyroxenes are rich in the hedenbergite component. In some cases, euhedral diopside crystals with marginal transition zones (composed of diopside with lower content of Mg2+ and higher content of Fe2+) were found in hedenbergite matrix. Amphiboles are dominantly represented by ferro-actinolite and ferro-hornblende in association with isolated euhedral crystals of ferro-tschermakite and ferro-pargasite. Indistinct chemical zonality of amphibole euhedral crystals is caused by presence of ferro-pargasite in the central parts and ferro-tschermakite in the peripheral parts of crystals.
Andradite
Grossular
Amphibole
Diopside
Actinolite
Almandine
Hornblende
Formula unit
Pleochroism
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Pyrope
Almandine
Grossular
Andradite
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Almandine
Grossular
Scheelite
Pyrope
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The concept that almandine-rich garnet is an anomalous phase in thermal metamorphic assemblages is based partly upon the rarity of almandine in hornfelses, and partly upon evidence of breakdown of 'regionally' formed garnets when their host-rocks have been thermally metamorphosed. Where amphibolite facies regional gneisses have been re-metamorphosed within the pyroxene-hornfels facies conditions of the late Caledonian Lochnagar aureole, garnet of composition Alm80Py11p6 (Gr+And)4 has reacted with biotite, sillimanite, and muscovite to give cordierite-rich pseudomorphs; where armoured from reaction by immersion in quartz or feldspar, no signs of dissolution are seen. In some totally reconstituted hornfelses, almandine garnet of composition AIm80P13Sp4 (Gr+And)3 appears to have coexisted stably with cordierite and orthoclase. It is concluded that the 'unstable' garnet was breaking down not because its P/T stability field had been exceeded, but because the alman-dine-bearing rock-composition field in the thermal (pyroxene-hornfels) facies was more restricted than that in the regional (amphibolite) facies. Chemical data from the hornfelses suggest that this restriction was mainly due to the stability of cordierite—the plane spinel-cordierite-quartz restricts garnet to those rocks with effective mol. (FeO+MgO-t-MnO) A12O3>1, while within that range cordierite-biotite tie-lines restrict garnet to rocks of high (FeO+MnO)/MgO ratio (Fig. 4b). Recorded instances of garnet 'instability' in thermal aureoles show similar features to the Lochnagar aureole—garnet breaks down by reaction; unequivocal instances of isochemical breakdown are rare. This, combined with the widespread occurrence of almandine in volcanic and plutonic igneous rocks, suggests that almandine is a physically stable phase not only in the hornblende-hornfels facies, but also in the pyroxene-hornfels facies and possibly in portion of the sanidinite facies as well. The rarity of almandine in thermal aureoles is the result of its very narrow rock composition field under the P/T conditions of such environments. Comparison of thermal and granulite facies garnet-cordierite assemblages suggests that P and T modify the almandine-bearing rock composition field mainly by modifying the limiting Mg/Fe ratio of garnet and the limiting Fe/Mg ratio of cordierite (Fig. 10). The wide rock-composition field of almandine in the amphibolite facies may be contingent upon the inhibition of cordierite by hydrous minerals under relatively high partial pressures of water.
Almandine
Sillimanite
Cordierite
Grossular
Hornfels
Pyrope
Pyroxene
Muscovite
Andalusite
Protolith
Pseudomorph
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Abstract The infrared (IR) spectra of almandine-grossular and almandine-pyrope garnet solid solutions have been measured using the powder method. Frequency shifts of a band related to internal vibrations associated with the 8-co-ordinate dodecahedral site are nonlinear in almandine-grossular garnets and mimic the form of its molar volume of mixing curve. Almandine-pyrope solid solutions have nearly ideal molar volumes of mixing and the frequency shift of this same 8-co-ordinate site-related band is linear. The IR data support the empirically based crystal chemical model of Equivalent Site (ES) behaviour (Newton and Wood, 1980). The IR spectra give no indication of long-range ordering between Ca and Fe 2+ in garnet, but thermodynamic calculations involving Ca-poor garnets might be affected by small volume or short-range ordering anomalies.
Almandine
Pyrope
Grossular
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Grossular
Pyrope
Almandine
Calcium aluminosilicate
Enthalpy of mixing
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The dielectric constants and dielectric loss values of a series of garnets were measured at l MHz using a two-terminal method and empirically determined edge corrections. The results are five intermediate pyrope-almandine samples, k'=11.96-12.35; spessartine, k'=11.65; two andradite samples, k'=10.53-10.59; and three grossular samples, k'=8.53-8.8l.
The deviations of measured dielectric polarizabilities as determined using the Clausius-Mosotti equation from those calculated using the sum of oxide polarizabilities according
to ɑ_D(M_2M'X_4) = 2ɑ_D(MX) + ɑ_D(M'X_2) is +5.0 to 6.5% for the pyrope-almandine samples,+ 1.9% for spessartine, -2.3% of or the andradite samples, and -5.5 to -7.0% for
grossular. These deviations from additivity are believed to result from garnet structural constraints leading to rattling Mg ions and compressed Ca ions.
Andradite
Grossular
Almandine
Pyrope
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The single‐crystal elastic properties of a natural, grossular‐rich garnet solid solution have been measured at ambient conditions using Brillouin spectroscopy. This garnet sample is unusual for a grossular in that it contains appreciable pyrope and almandine components, with a composition of Gr 48.4 Py 27.6 Alm 23.4 Sp 0.6 . It is thus suitable for investigating the variation of elastic properties with composition in Ca‐rich garnet solid solutions that are stable only at high pressure. The values obtained for the bulk modulus, K = 170.4(6), and for the shear modulus, μ = 101.6(3), of the garnet examined in this study, vary by less than 1% from those obtained from a weighted sum of the end‐member moduli. This suggests that any composition in the grossular‐pyrope‐almandine system, which comprise the majority of upper mantle garnets, is accurately estimated with this simple approach. More complex elasticity‐composition variations observed for the grossular‐andradite garnets may be specific to garnet solid solutions where the cation substitutions occur at six‐coordinated crystallographic sites.
Grossular
Pyrope
Almandine
Andradite
Elasticity
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