TETRAHEDRITE – TENNANTITE
TETRAHEDRITE – TENNANTITE Tetrahedrite-tennantite – Cu12(Sb,As)S13 – are very frequent sulfides, important ores mainly of Cu and Sb, but also of Ag and Hg. Today the terms “tetrahedrite” and “tennantite” no longer refer to individual minerals, but to two Subgroups of the Tetrahedrite Group. The classification of these sulfides is constantly evolving. Of tetrahedrite, for […]
TOPAZ
TOPAZ Topaz – Al2[(F,OH)2SiO4)] – is a rare nesosilicate that constitutes an important gemstone of high value. It has three varieties: “imperial topaz” (orange), “killiecrankie Diamond” (a Tasmanian variety) and “pyknite” (fine-grained topaz). It rarely fluoresces under ultraviolet light, exhibiting yellow, white, orange, or greenish-yellow fluorescence. When colorless, even well formed and especially when granular, […]
TREMOLITE
TREMOLITE Tremolite – Ca2Mg5Si8O22(OH)2 – is an inosilicate of the Amphibole Group. It is an important rock-forming mineral, typical of metamorphic rocks. Due to its fibrous habit, it was occasionally mined as “asbestos”, but its use has been decreasing due to health problems generated by the inhalation of dust from the fibers, which are considered […]
TITANITE
TITANITE Titanite – CaTiSiO5 – is a very common nesosilicate, an accessory mineral to many types of igneous rocks. A traditional name for titanite, still widely used, is “sphene”. It is a TiO2 ore, an important white pigment. Furthermore, it can be used as a gemstone and for dating by the U-Pb method. It may […]
TALC
TALC Talc – Mg3Si4O10(OH)2 – is an uncommon phyllosilicate, important as a rock former and very important as an industrial mineral. It has 5 varieties and can contain Fe, F, Ni, Al, Ca, Na and H2O. Sometimes talc is contaminated with fibrous amphiboles (“asbestos”) and, in these cases, the inhalation of dust poses a health […]
TOURMALINE
TOURMALINE Tourmaline – NaFe3Al6(Si6O18)(BO3)3(OH)3(OH) (Schorl) – is a relatively common cyclosilicate that can occur in many types of rock. It is a mineral with very high prices in the market of collectible minerals and as gemstone. “Tourmaline” is actually not a mineral, but just a generic term applied to members of the Tourmaline Group, which […]
TURQUOISE
TURQUOISE Turquoise – CuAl6(PO4)4(OH)8 · 4H2O – is a very rare phosphate, typical of recent orogenic regions, where it is mined as a valuable gemstone and as an ornamental stone. Imitations are very common in the market for gemstones and collector minerals. It is classified in the Turquoise Group, being isostructural with chalcosiderite and planetite. […]
TRIDYMITE
TRIDYMITE Tridymite – SiO2 – is a rare and difficult to identify tectosilicate, typical of acid volcanic rocks. It has no economic importance. Tridymite is one of the polymorphs of SiO2, with quartz, β-quartz, cristobalite, coesite and stishovite. It is a high temperature, low pressure polymorph. It has 7 crystalline forms: 2 hexagonal, 3 orthorhombic […]
TRONA
TRONA Trona – Na3H(CO3)2.2H2O – is a relatively rare carbonate that occurs in arid regions. It is an important ore of sodium carbonate (“soda ash”) mainly in the USA, where the Green River Formation deposit is the largest known deposit of its type in the world. In the rest of the world, Na carbonate is […]
TROILITE
TROILITE Troilite is a rare sulfide, found here on Earth in only a few very specific geological occurrences. It has no economic use. Troilite is classified in the Pyrrhotite Group. It is the S analogue of achávalite (FeSe) and chemically the Fe analogue of covellite (CuS). It can transform into hexagonal pyrrhotite at T<122ºC, in […]