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(Extracted from Collins Photo Guide Rocks, Minerals and Gemstones. 1992)

CLASS V111 – SILICATES - Serpentinite

Hardness 2.5 - 4.

More or less green metamorphic rock consisting predominantly of serpentine minerals. The name (Latin for snake) refers to the flecked appearance or it may be derived from its alleged antidotal effect against snake poison. Major constituents chrysotile or antigorite. Large number of medium constituents possible, among others being olivine, pyroxene, garnets, amphibole, chromite and magnetite. Calcite is almost always present. Texture compact, sometimes fibrous or lamellar, almost always massive, occasionally thickly-bedded. A slight schistosity is evident very occasionally. As a result of the varying proportions and different distribution of the minerals the coloration is usually irregular (flecked, striped, wavy, veined). Colour varies from light grey green through to green black. Bluish, brown and reddish tones also sometimes occur. Originates during regional metamorphism from ultra-basic igneous rocks (peridotite, pyroxenite, picrite) through the alteration (serpentinization) of olivine or more rarely of pyroxenes, amphibole or chlorite. The iron compounds released in this process can give rise to ore deposits. Very occasionally serpentinites can also form from dolomite rocks. LOCALITIES: Fichtelgebirge, Schwarzwald/Germany, Erzgebirge/ Germany, Tyrol, Karriten, Waldviertel/Austria, Graubfinden/Switzerland, Pyrenees, Vosges/France, Liguria/ Italy, Lizard/Cornwall/England, Shetland Isles/Scotland, Montana, Oregon, California, Maine/USA, Quebec/Canada. Used since ancient times for jewellery and objets d'art because of its low hardness, good polishability and decorative appearance. In the trade and the stone industry serpentinite is usually just called serpentine like the mineral.

Many serpentinites contain ores which are worth mining or are very closely associated with deposits - among others with iron, - chromium, - magnesite - talc, and asbestos - occurrences.

The varieties are designated according to their most significant medium constituent (bronzite - or garnet-serpentinite) or to the rock from which they are derived (harzburgite serpentinite). The green colour of serpentinite is frequently mentioned in the names of the types used in the trade. Often also classified as a MARBLE.

Moh’s Mineral Hardness Scale
1. Talc - can be scratched with a fingernail
2. Gypsum - can be scratched with a fingernail
3. Calcite - can be scratched with a copper coin
4. Flourite - easily scratched with a knife

Scale goes up to 9. Corundum and finally 10. Diamond.

Viennese mineralogist Friederich Mohs 1773 - 1839.