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Semi-precious stones
Published in Francis P. Gudyanga, Minerals in Africa, 2020
Andradite is a calcium-iron garnet Ca3Fe2(SiO4)3 of variable composition in the following colours: black, brown, green, red or yellow. It has the following varieties: topazolite (yellow to green), demantoid (green) and melanite (black). It occurs both in deep-seated igneous rocks like syenite as well as serpentine, schists and crystalline limestone.
Literature Review
Published in Habeeb Lateef Muttashar, Sustainable Construction Materials, 2019
Varieties of garnet minerals exist, each having its characteristic chemical composition. The predominant minerals in the garnet group are almandine, pyrope, spessartine, andradite, grossular, and uvarovite. All these minerals possess vitreous luster, varying diaphaneity from transparent to translucent, brittle tenacity, and a lack of cleavage. They occur in the form of individual crystals, stream-worn pebbles, granular aggregates, and massive clusters (Evans, 2006). The chemical formula, hardness, specific gravity, and colors of these garnets are summarized in Table 2.4.
Origin and evolution of nephrites, diopsidites and giant diopside crystals from the contact zones of the Pounamu Ultramafics, Westland, New Zealand
Published in New Zealand Journal of Geology and Geophysics, 2023
Giant diopsides grew in a geologically-young extensional fracture, nucleating on diopside-rich segregations formed by rodingitisation along a meta-serpentinite-schist contact within the Pounamu Ultramafics. With crystallisation temperatures likely to be considerably lower than those of the earlier Alpine metamorphism, the dominance of diopside relative to tremolite suggests crystallisation from H2O-rich fluids (Spear 1993). The diopsidite mineral assemblages at Mt Inframeta also contain the aluminous phases grossular-andradite-rich garnet (with variable uvarovite component) and an epidote group mineral with a variable Fe component. Gordon and Greenwood (1971) showed that at a nominal pressure of 2 kb, the zoisite-grossular assemblage is restricted to fluid compositions with XCO2 of < 0.06. Taylor and Liou (1978) have subsequently shown that the addition of Fe extends the stability field of andradite and epidote relative to grossularite and zoisite, but at pressures of 2 kb, the fluid composition at temperatures less than 500° C is still H2O-dominated, with XCO2 < 0.12. All stability fields are sensitive to the effect of fO2.
Strain localisation and transcurrent reactivation in the granulite facies Kalinjala Shear Zone at Port Neill, South Australia
Published in Australian Journal of Earth Sciences, 2022
C. J. L. Wilson, J. R. Stewart, P. G. Betts
An isolated elongate, green coloured calc-silicate body (1 m wide and 180 m long) occurs on Headland 1 within quartz–feldspar orthogneisses and adjacent to a Jussieu dyke (Figure 10). It is composed of relatively coarse (1.5 mm) epidote (10%) set in a matrix of plagioclase (0.3–1 mm, 60%), clinopyroxene (0.3–0.5 mm, 10%), andradite garnet (<0.1 mm, 15%), vesuvianite (5%) and magnetite (<5%). This body has the appearance of a dyke; however, we interpret it as a sedimentary enclave within the gneiss, and the 600 mm wide Jussieu dyke, on its eastern boundary, appears to have intruded along the granite–sediment contact. This is also a region where thinner mafic units are juxtaposed (Figure 12a), and the shear strain (Figure 12b) is heterogeneously distributed through the partitioning into discrete shear zones along mafic–felsic boundaries, which are commonly accompanied by K-feldspar-rich pegmatites (Figure 12c).
Regional isotopic fingerprinting of hydrothermal barite mineralisation and its possible association to epigenetic Fe and Mn ore formation in the Paleoproterozoic Transvaal Supergroup of South Africa
Published in Applied Earth Science, 2019
V. Papadopoulos, H. Tsikos, A. Boyce, D. Mark
The giant sedimentary manganese deposits of the Kalahari manganese field (KMF) in South Africa have been locally upgraded by hydrothermal alteration involving introduction of Na and K, elemental redistribution, and formation of a voluminous gangue mineralogy which includes silicates, borates and sulphates such as aegirine, andradite, gaudefroyite and barite (Beukes et al. 1995). Barite and high Ba-Na-K concentrations have been reported from the orebodies of the Postmasburg Fe–Mn Field (PMF) stretching as far as ∼150 km to the south of the KMF. However, these ores are thought to have been formed through lateritic processes and accumulation in surficial karst environments (Gutzmer and Beukes 1996; Beukes et al. 2003), and hence, barite has been regarded as evidence for localised hydrothermal fluid flow postdating ore formation.