The Study of Opaque and Ore Minerals

Reflected light microscopy is the standard method for the characterization of ore minerals and hence its role in economic mineral studies is well established. However, the technique has been neglected in routine petrographical studies of unmineralized rocks. This is partly because the recognition and identification of opaque minerals using their optical properties in reflected light is a skill that takes time to master and partly because electron microscopy and microprobe techniques are used as a substitute for, rather than alongside, the polarizing microscope.

Unlike translucent minerals in transmitted light, many of the optical properties of opaque minerals in reflected light are perceived to change as the viewing conditions are altered. In well polished, untarnished sections the perceived surface colour and reflectance of opaque minerals will depend upon the presence of coloured filters in the light path, the strength of the illumination, the magnification of the lenses and whether air or oil immersion techniques are used. Even when these are standardized the apparent optical properties of a mineral will be partially dependent on those of adjacent minerals. A mineral, therefore, can appear quite 'different' in different associations and so the most effective method of teaching opaque mineral identification is to use mineral associations rather than mono-mineralic material.

There are a number of excellent textbooks, although some are now dated or out of print. Galopin and Henry (1972) and, more recently, Craig and Vaughan (1981) are comprehensive introductory texts giving both theoretical and practical aspects of ore microscopy together with mineralogical tables for the common ore minerals. Extensive identification tables include Schouten (1962), Uytenbogaardt and Burke (1971) and, most recently, Criddle and Stanley (1986). Texts with large numbers of black and white photomicrographs include Edwards (1947), Oelsner (1966), Kostov and Minceva-Stefanova (1981) and most notably Ramdohr (1969). Texts with colour photomicrographs are fewer but include Picot and Johan (1977) and Fander (1985).

This virtual atlas contains nearly four hundred colour photomicrographs, each accompanied by a description and used to illustrate common mineral assemblages. The photomicrographs are grouped together into blocks of associations under a broad genetic classification scheme, and each block is cross-referenced and linked to a text that is intended as a brief background and introduction to the figured examples and an expansion of their mineralogy and petrography. The text, therefore, is biased towards descriptive petrography and away from economic geology, detailed descriptions of deposits or ore genesis. The majority of the references, nearly all of which are to be found in easily accessible textbooks, conference volumes, or journals, reflect this bias. However, for the more important classes of mineral deposits, some recent generalized references are included.

Inevitably, a certain selectivity has been imposed upon the choice of material. Uncommon mineral assemblages, notably those with platinum group minerals, tellurides and selenides, are covered only briefly, as are assemblages that comprise minerals with very similar optical properties, including manganese minerals, iron-nickel-cobalt arsenides and sulpharsenides and most sulphosalts. For these latter assemblages an accurate characterization of their mineralogy usually requires microanalytical and X-ray diffraction techniques in addition to microscopy.

Standard polished sections and polished thin sections are illustrated, and include some showing polishing faults and tarnishing. The majority of the photomicrographs show the minerals in plane polarized light using low to medium power air lenses since these are the usual viewing conditions. Photomicrographs taken in partially crossed polarized light or using high power oil immersion lenses have been included to illustrate further optical properties (anisotropy effects, internal reflections and reflection pleochroism) and to extend the range of viewing conditions. The minerals are identified in terms of their reflectance, surface colour and spatial position.

The identities of the mineral species have been confirmed wherever necessary by X-ray diffraction or microanalytical techniques but where this has not been possible a number of conventions have been adopted. These include the use of 'chalcocite' for uncharacterized blue-white copper sulphides of the chalcocite-anilite-digenite-djurleite group; 'TiO₂ mineral' for uncharacterized anatase-rutile-pseudobrookite minerals; 'limonite' for goethite-lepidocrocite-akaganeite-feroxyhyte; 'psilomelane' for manganese oxide or hydroxide minerals; 'tetrahedrite group' for minerals of the tetrahedrite-tennantite series and 'stannite group' for uncharacterized anisotropic stannite group minerals. The figure caption heading accompanying each photomicrograph lists the main opaque minerals in decreasing order of abundance.

This site has been made possible by the generosity of many people, including colleagues, teachers, mining company and museum staff, but mostly that of research students, who over the last two decades have donated their time, experience and material. Unfortunately, a list of them would be too long, and probably incomplete. Nevertheless, we are grateful. Drs J. P. N. Badham, C. Pointon and C. J. Stanley are thanked for their help and permission to use their material, as are NASA, The British Museum (Natural History), The National Museum of Wales, La Societe Jersiaise, BP Minerals International, Billiton and Amerada Hess. Professor D. J. Vaughan is thanked for his encouragement and for critically reading the original manuscript and photomicrographs. The assistance of Professor D. D. Hawkes and staff of the Department of Geology, Aston University, especially Bob Brueton who prepared many of the sections, and Chris Gee and Phoebe Kent who typed and retyped everything is acknowledged. Finally, we thank Rosie, Rob's wife, for her love and tolerance and the late S. Wu-Wei for companionship during the writing of the atlas.

In memory of Constantine XI Palaeologos 29th May, 1453