Opaque minerals in unmineralized to lightly mineralized granites

Although the mineralogy of granites is variable, they are essentially quartz and feldspar rocks that carry micas, amphiboles and pyroxenes, together with a wide variety of accessory minerals. Accurate characterization of the minor and accessory phases within granites should complement geochemical studies, especially if mass-balance calculations are to be performed. Reflected light investigation of the opaque and accessory phases has an important role in helping to discriminate fertile from infertile granites, in terms of their potential for economic metalliferous deposits or for geothermal energy (the so-called high heat producing granites).

Accessory minerals are found as 10-100µm diameter grains between quartz and feldspars, or commonly within mafic and especially altered mafic minerals. Pleochroic halos in mafic minerals usually suggest the presence of one or more accessory minerals and repay careful study.

Amongst the more common primary opaque and accessory minerals are zircon, allanite, sphene, tourmaline, epidote; ilmenite-haematite, magnetite-ulvöspinel, cassiterite, columbite-tantalite, uraninite, thorite, TiO2 minerals, pyrochlore group minerals, wolframite; apatite, monazite, xenotime; pyrite, pyrrhotite, chalcopyrite, sphalerite, arsenopyrite and molybdenite. In addition, numerous rare earth element-bearing carbonates and fluorocarbonates and U-Nb-Ti-bearing oxide minerals have been recorded. The accessory minerals occur as loose aggregates, often with symplectite-like intergrowths, at crystal boundaries of the silicates, or lie along the cleavage planes and grain boundaries of mafic minerals.

--------------------------------------------------
Note added at 1 hr (2011-02-22 14:43:41 GMT)
--------------------------------------------------

I'm not very fond of geology either...

A rock is a rock, if you ask me :)

PS: pyrite is classified as an opaque mineral. Go figure.