Opaque minerals

Opaque minerals identified from active geothermal areas are pyrite, sphalerite, galena, chalcopyrite, and tetrahedrite from Okuaizu, Fushime, and Nigorikawa (Japan), Salton Sea (U.S.A.) and Broadlands (New Zealand). 

These opaque minerals are common in Pb-Zn vein-type deposits in Japan Opaque minerals identified from silica sinter containing gold and mercury are krennerite, col-oradoite and metacinnabar (Osorezan, Japan Waiotapu, New Zealand). These minerals are not found in low sulfidation-type Au-Ag deposits in Japan but are reported from Kobechizawa, and Date in south Hokkaido which are massive and disseminated types and similar to hot spring type deposits. 

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Pyrolusite is a black, opaque mineral with a metallic luster and is frequendy soft enough to soil the fingers. Most varieties contain several percent water. Pyrolusite is usually a secondary mineral formed by the oxidation of other manganese minerals. Romanechite, a newer name for what was once known as psilomelane [12322-95-1] (now a group name) (7), is an oxide of variable composition, usually containing several percent water. It is a hard, black amorphous material with a dull luster and commonly found ia the massive form. When free of other oxide minerals, romanechite can be identified readily by its superior hardness and lack of crystallinity. 

Lead(II) sulfide occurs widely as the black opaque mineral galena, which is the principal ore of lead. The bulk material has a band gap of 0.41 eV, and it is used as a Pb " ion-selective sensor and IR detector. PbS may become suitable for optoelectronic applications upon tailoring its band gap by alloying with II-VI compounds like ZnS or CdS. Importantly, PbS allows strong size-quantization effects due to a high dielectric constant and small effective mass of electrons and holes. It is considered that its band gap energy should be easily modulated from the bulk value to a few electron volts, solely by changing the material s dimensionality. 

Titanium dioxide (E171, Cl white 6) is a white, opaque mineral occurring naturally in three main forms rutile, anatase, and brookite. More than 4 million tons of titanium dioxide are produced per year and it is widely used for industrial applications (paints, inks, plastics, textiles) and in small amounts as a food colorant. ° "° Production and properties — Titanium oxide is mainly produced from ilmenite, a titaniferous ore (FeTiOj). Rutile and anatase are relatively pure titanium dioxide (Ti02) forms. Titanium oxide pigment is produced via chloride or sulfate processes via the treatment of the titanium oxide ore with chlorine gas or sulfuric acid, followed by a series of purification steps. High-purity anatase is preferred for utilization in the food industry. It may be coated with small amounts of alumina or silica to improve technological properties. 

A zonal sequence of opaque minerals is also found. Pyrite is found in zones (1) and (2). Very tiny amounts of pyrrhotite coexisting with pyrite are found in zone (1). Magnetite is common in zone (3) but this mineral is thought to be original. 

The chemical compositions of opaque minerals (sphalerite, electrum) are different in two types of deposits. The FeS content of sphalerite from vein deposits of the Te-type is generally lower than that of the Se-type (Fig. 1.118). However, FeS content of sphalerite from massive deposits of the Te-type (Kobetsuzawa, Suzaki) is high, ranging from 1 to 7 mol%. The Ag content of electrum from the Se-type is higher than that from the Te-type (Fig. 1.119). 

Deposit Se-bearing minerals Opaque minerals Gangue minerals... 

The Special Issue of Resource Geology on the Hishikari deposits (Shikazono et al., 1993) includes various aspects of the Hishikari deposits (oxygen isotopes of gangue minerals, hydrothermal alteration, precipitation sequence, fluid inclusions, vertical electric profiling and electric sounding surveys, structural geological analysis, opaque minerals. 

Main opaque minerals are chalcopyrite, cassiterite, stannite, arsenopyrite, bismuth-inite, pyrrhotite and sphalerite. The FeS content of sphalerite is high (about 18 mol% FeS). 

Opaque minerals include stibnite, jamesonite, cinnabar, gold, pyrite, pyrrhotite, arsenopyrite, marcasite, sphalerite, galena and chalcopyrite. 

From the mode of occurrence of opaque minerals it is considered that pyrrhotite and sphalerite were precipitated at an early-stage, gold, pyrite, marcasite, stibnite and cinnabar were precipitated at a late-stage, and arsenopyrite was precipitated throughout the mineralization period. 

Main opaque minerals include native gold, electrum, pyrite, pyrrhotite, chalcopy-rite, cubanite, sphalerite, arsenopyrite and tellurobismutite. The amounts of these sulfide minerals are poor, compared with those in epithermal Au-Ag vein-type deposits. It is noteworthy that silver minerals are abundant in epithermal Au-Ag vein-type deposits, whereas they are poor in gold-quartz veins. 

Main opaque minerals are chalcopyrite, pyrite, pyrrhotite, sphalerite and bornite (Table 2.22). These minerals commonly occur in massive, banded and disseminated ores and are usually metamorphosed. Hematite occurs in red chert which is composed of fine grained hematite and aluminosilicates (chlorite, stilpnomelane, amphibole, quartz) and carbonates. The massive sulfide ore bodies are overlain by a thin layer of red ferruginous rock in the Okuki (Watanabe et al., 1970). Minor opaque minerals are cobalt minerals (cobaltite, cobalt pentlandite, cobalt mackinawite, carrollite), tetrahedrite-tennantite, native gold, native silver, chalcocite, acanthite, hessite, silver-rich electrum, cubanite, valleriite , and mawsonite or stannoidite (Table 2.22). 

The reflected light from polished surfaces permits ready distinction between many opaque minerals on the basis of brightness, colour, and anisotropy. This feature is very important in the study of ore minerals and host rocks. Transparent minerals (especially in rocks) are commonly studied by transmitted light. In this case, polarized light is passed through thin sections or through fragments mounted in oil. 

Opaque minerals like iron oxides are frequently examined in the reflectance mode - and usually give diffuse reflectance spectra. Reflectance spectra provide information about the scattering and absorption coefficients of the samples and hence their optical properties. The parameters of reflectance spectra may be described in four different ways (1) by the tristimulus values of the CIE system (see 7.3.3) (2) by the Kubelka-Munk theory and (3) by using the derivative of the reflectance or remission function (Kosmas et al., 1984 Malengreau et ak, 1994 1996 Scheinost et al. 1998) and, (4) more precisely, by band fitting (Scheinost et al. 1999). 

Reflected light microscopy can be used either on thin sections or thicker polished sections. Reflected light is used primarily for identifying opaque minerals such as metals, sulfides, and some oxides. Each of these minerals has a unique appearance in reflected light. 

Larsen, E. S., and Berman, H., 1934. Microscopic Determination of the Non-opaque Minerals. Washington U.S. Geol. Survey Bulletin 848,... 

Galopin, R. Henry, N.F.M. A Microscopic Study of Opaque Minerals, Heffer and Sons Cambridge, 1972. 

Opacity of mixed-valence minerals. The opacities of many end-member Fe2+-Fe3+ oxide and silicate minerals result from electron hopping between neighbouring cations when they are located in infinite chains or bands of edge-shared octahedra in the crystal structures. Opaque minerals such as magnetite, ilvaite, deerite, cronstedtite, riebeckite and laihunite owe their relatively high electrical conductivities to thermally activated electron delocalization, contributing to intermediate valence states of iron cations which may be detected by Mossbauer spectroscopy. 

Light (or near-ir and uv radiation) that is incident on opaque minerals is partly absorbed and partly reflected by them. There are two kinds of reflection processes that occurring when light is reflected from a flat polished surface of the mineral (specular reflectance) and that occurring when the light is reflected from the mineral after it has been finely powdered (diffuse reflectance). The latter arises from radiation that has penetrated the crystals (as in an electronic absorption spectrum) and reappeared at the surface after multiple scatterings in this case there will also be a specular component to the reflectance from light that is reflected from the surfaces of the particles. The specular reflectance of a flat polished surface of an opaque mineral measured at normal incidence can be related to the n and k terms of the complex refractive index (N) in which ... 

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