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Semi-precious stones
Published in Francis P. Gudyanga, Minerals in Africa, 2020
Tanzanite Ca2Al3(SiO4)3(OH) belongs to the epidote group and is a blue/violet variety of zoisite, a calcium aluminium hydroxyl sorosilicate mineral. It has been mined since 1967 in the Mererani Hills [1000] near the city of Arusha and Mount Kilimanjaro, Tanzania. Tanzanite appears to have different colours when viewed under alternate lighting conditions. Several colours have been observed in various specimens: blue, brown, cyan, green, shades of purple, red, violet and yellow. It is usually reddish brown in its rough state. Heat treatment enhances its dichroic colours ranging from bluish-violet to violetish-blue [1001,1002].
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.
Provenance of the Pakawau Group and Farewell Formation (Late Cretaceous – Paleocene), Taranaki Basin, northwest Nelson, New Zealand
Published in New Zealand Journal of Geology and Geophysics, 2020
Sarah L. Smithies, Kari N. Bassett, Greg H. Browne, Alexander R. L. Nichols
The low-grade metamorphism of the clasts is similar to that seen in the Takaka Terrane volcanics. The Takaka Terrane Devil River Volcanics show extensive very low to low-grade metamorphism, with a mineral assemblage of albite + chlorite ± carbonate ± quartz ± epidote ± prehnite ± pumpellyite ± actinolite ± sericite ± (clino)zoisite ± stilpnomelane (Münker and Cooper 1999). Primary volcanic textures are usually preserved (Maclean 1994; Münker and Cooper 1999), with the volcanics generally andesitic to basaltic in composition and including tuffs and silicified volcanics creating cherts (Maclean 1994). The low-grade metamorphism of the volcanic clasts supports a Takaka Terrane source, in combination with the island arc geochemistry.
Reconnaissance composition of river sand from northern South Island, New Zealand: a modern analogue for southern Taranaki Basin
Published in New Zealand Journal of Geology and Geophysics, 2020
Linda M. Doran, Kathleen M. Marsaglia, Greg H. Browne
Dense minerals include unaltered clinopyroxene (mostly augite) and orthopyroxene, and represent approximately 2–5% of total points counted (Figure 8). Other dense minerals include epidote, garnet, opaques, and clinozoisite/zoisite. Accessory minerals include chlorite and serpentine. Serpentine often retains the texture of the olivine and pyroxene it has replaced. In the very fine-sand fraction, serpentine makes up more than a quarter of all points counted; it is least abundant (approximately 12%) in the fine sand fraction.