Magnesium-rich minerals, alteration

The clays found in sedimentary phosphate rocks include illite, kaolinite, smectites, and magnesium-rich clays such as palygorskite and sepiolite. Illite often appears to be a detrital mineral. Kaolinite, smectites, and magnesium-rich clays often appear to occur in zones within phosphate deposits. This zonation may be related to weathering and the general alteration of deposits under surface or near-surface conditions [15,16]. Clay-Iike minerals such as the zeolite clinoptilolite occasionally are found in phosphate rock. 

Clinochrysotile is a monoclinic polymorph of the magnesium-rich silicate mineral chrysotile q.v.) and belongs to the serpentine group q.v.) of minerals (Wicks and O Hanley, 1988). It has the chemical composition Mg3Si205(0H)4, and is also polymorphous with the ortho- and para-chrysotile forms. Clinochrysotile commonly occurs as white or yellowish fibres in hydrothermally altered or metamorphosed carbonate and ultramafic rocks such as those found in Cornwall (England), North Auckland (New Zealand), Transvaal (South Africa) and Cyprus. 

Talc A mineral with the approximate chemical formula Mg3Si40io(OH)2 (see chapter 2). The term has been used as a commercial name for fibrous or platy deposits formed by hydrothermal alteration of rocks rich in silica, magnesium, and iron. 

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