【Throwback: Fossils in Hong Kong Footprints of the Geological History 】
by Prof. LEE Cho-min
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Hong Kong Mining History
Gemstone of The Month: Quartz Pseudomorph
Quartz crystallizes from silica rich solution which are usually associated with volcanic or hydrothermal activities. As a very common mineral, quartz always exists in a definite crystal form like hexagonal prism plus rhombohedra, or appears massive. However, the sample shown in the above photo, displays a fibrous vein which does not look like quartz. Indeed, it is quartz pseudomorph after crocidolite (an asbestos mineral) with preserved fibrous habit of crocidolite, alternatively known as “tiger-eye”.
Crocidolite is a type of asbestos mineral and essentially a silicate mineral of the amphibole group. When silica fluid flows along structures and interacts with crocidolite, SiO2 replaces the chemical composition of the original crocidolite. However, the original blue colour and the fibrous structure of crocidolite are preserved. The golden colour of the tiger-eye is subsequently tainted by iron oxides. Because of its attractive appearance, it is often used as gemstone.
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【The 40th Anniversary of The Geological Society of Hong Kong 】