Silicate minerals

About: Silicate minerals is a research topic. Over the lifetime, 1794 publications have been published within this topic receiving 67064 citations.

High-temperature metamorphism in marbles as a consequence of volatile release from crystallizing anatectic melts, Naxos, Greece

Abstract: Two generations of high-temperature silicate mineral assemblage occur in vesuvianite- and amphibolite-bearing calcitemarbles in the leucogneiss core of the Miocene M2 metamorphic complex of Naxos, Greece. The first ( 1p ) generation formed during prograde metamorphism, whereas the second retrograde ( 2r ) generation developed along contacts between prograde amphibolites and calcite marbles, in veins that crosscut pure marbles, and at the contacts between pegmatite-aplite bodies and calcite marble, and clearly grew during the infiltration of an external fluid into the marbles. Phase equilibrium calculations indicate a water-rich fluid. The oxygen isotope composition of 2r silicate minerals shows a significant decrease relative to the 1p composition in prograde marbles. The calculated δ 18 O values of water in isotopic equilibrium with the 2r silicate minerals (δ 18 O garnet = 9.0 to 9.6 ‰; δ 18 O clinopyroxene = 10.3 to 11.0 ‰; δ 18 O hornblende = 10.8 to 11.2 ‰) match those of water in equilibrium with the minerals of the pegmatite-aplite bodies, which formed during late-stage crystallization of peak-metamorphic anatectic melts. The volatiles exsolved during the pegmatite-aplite crystallization are thus indicated to be the source of the infiltrating fluid. Recrystallized calcite marble adjacent to 2r silicate minerals shows isotopic evidence for limited water-rock interaction and indicates that fluid infiltration was strongly localized in the metasomatic zones. The study confirms that high-temperature metamorphism occurs as the result of volatile release during the crystallization of anatectic partial melts and that peak metamorphism in such terrains is a continuum spanning both prograde heating and retrograde cooling.

Origin and Role of Kaolinization in Roll-Front Uranium Deposits and Its Response to Ore-Forming Fluids in the Yili Basin, China

Abstract: Kaolinite is a common mineral found in most Chinese sandstone-hosted uranium deposits. It occurs particularly in coal-bearing clastic rocks in northwest China, such as the uranium deposits in the Yili Basin, which is well known for hosting several large-scale roll-front uranium deposits. Previous studies have provided limited information on the origin of kaolinization and its role in the uranium mineralization. This study uses gas hydrocarbon, fluid inclusions, O and H isotope analysis, and scanning electron microscopy observations to investigate the formation of kaolinite in ore-hosting rocks from the Mengqiguer uranium deposit in the southern margin of the Yili Basin and to determine its role in the uranium mineralization. Results suggest that kaolinization is intense in the coal- and ore-bearing clastic rocks and that it is related to leaching of feldspar by acidic fluids. Vermicular kaolinite was formed by hydrocarbon-bearing fluid generated from coal and carbonaceous mudstone during a shallow-burial diagenetic stage at low homogenization temperatures ranging from 69 to 78°C and at relatively high salinities of 7.6−11.0 wt% . Consequently, silicate minerals (such as feldspar) were leached and created secondary pores that hosted the subsequently formed uranium minerals. In contrast, micritic kaolinite was formed by infiltration of meteoric fluid enriched in U and O2 at low homogenization temperatures of 51−63°C and low salinities of 1.2−3.7 wt% . U6+ was sorbed by the micritic kaolinite through cation exchange, forming a U-bearing kaolinite complex; it was also reduced by pyrite and carbon detrital, thereby precipitating at the acidic oxidation front. The results of this study confirm that intense kaolinization is closely related to uranium mineralization in coal-bearing clastic rocks.

Cement admixture, process for producing same, and cement composition, mortar, and concrete each containing the admixture

Abstract: Disclosed is a cement admixture comprising granules which are obtained by adding a cement as a binder material for granulation and water to a material for self-recovery from cracking and kneading the mixture and which contain the material for self-recovery from cracking as a main component. The material for self-recovery from cracking comprises at least one substance selected from a group consisting of phyllosilicate minerals (aluminosilicates, magnesium silicates), crystalline and noncrystalline silicate minerals (aluminosilicates), calcium phosphate, compounds and minerals having a carbonic acid group, compounds and minerals containing lithium, compounds and minerals containing magnesium, compounds containing fluorine, materials having pozzolanic reactivity, materials having latent hydraulicity, materials containing an expanding material or unburned expanding material, materials containing calcium oxide, and cements, the selected substances having been mixed together in any proportion.

57Fe Mössbauer study of the asbestiform silicates balangeroite and carlosturanite

Abstract: 57FeMossbauer spectra of the two silicate minerals balangeroite (BAL) and carlosturanite (CST) have been collected at 80 and 295 K under normal and magic angle geometry. For both minerals the spectra have been fitted with two ferrous and two ferric doublets; Fe2+ accounts for 80 and 62% of Fetot in Bal and CST, respectively. The number of doublets used to fit the spectra supports the hypotheses that: (i) in the serpentine-like structure of CST iron occupies only octahedra which lie between the tetrahedral silicate strips; (ii) the octahedral framework of BAL (actually monoclinic) is satisfactorily described with an orthorhombic sub-cell.

On estimating the thermodynamic properties of silicate minerals

Abstract: The refined structure (Vieillard & Tardy, 1988) and polyhedral (Chermak & Rimstidt, 1989, 1990; Holland, 1989; Robinson & Haas, 1983) models offer the most realistic approaches to predicting unknown thermodynamic properties (ΔH f , ΔG f , S 0 , C p ) of various minerals. However, routine application of the former is currently limited by the lack of model parameters for most silicate minerals. On the other hand, the polyhedral approach is potentially more powerful than alternative ΔG 0 f and ΔH 0 f estimation algorithms inasmuch as Pauling's rules for silicate minerals underlie this concept. Nonetheless, free energies of formation estimated with current polyhedral models do not yet permit accurate prediction of phase equilibria. The repeated clustering of minerals into different distortion fields (Robinson et al., 1971; Fleet, 1976; this study) as well as the separation of apparently homologous group of polyhedra into different distortion-polarization energy fields (this study) suggest assumptions that nearest neighbour ionic interactions can account for the overall features of mineral structures are too simplistic. Inclusion of polyhedral distortions and measures of covalent contributions improve estimates of thermodynamic properties based on the polyhedral approach. However, further development of the proposed estimation algorithm is limited by gaps in the thermochemical database and thus emphasizes the need for continued experimental determinations of thermodynamic properties for structurally-coherent mineral groups