Talc - rocks and minerals - The University of Auckland

Talc is the softest mineral, demonstrated by its position at the bottom of Mohs' Scale of Hardness with a relative hardness value of 1. It has a soapy, ... Home ThegeologyofAuckland Minerals Rocks Glossary Apatite Augite Biotite Calcite Chlorite Corundum Diamond Fluorite Garnet Gypsum Hornblende Ilmenite Magnetite Muscovite Olivine Orthoclase Plagioclase Pyrite Quartz Talc Topaz Introduction Properties Commonrock-formingminerals Economicminerals TalcTalcisthesoftestmineral,demonstratedbyitspositionatthebottomof Mohs'ScaleofHardnesswitharelativehardnessvalueof1.Ithasasoapy,greasyfeel. Talcisformedbythehydrothermalalterationofultrabasicrocks,orlowgradethermalmetamorphismofsiliceous dolomites. Mostpeopleknowtalcastheprimaryingredientintalcumpowder.However,talcisanimportantindustrialmineral. Itsresistancetoheat,electricityandacidsmakeitusefulforlabcountertopsandelectricalswitchboards.It isanimportantfillerinpaints,rubberandinsecticides. Talcoftenreplacesothermineralsatombyatomtoformpseudomorphs,takingtheformofthereplacedmineral.Thus, aspecimenofwhatappearstobemilky quartzwouldactuallybetalc,havingasoapyfeelandbeingabletobescratchedwithafingernail. Chemicalcomposition-Mg 3Si 4O 10(OH) 2 Hardness-1 Specificgravity-2.7-2.9 Transparency-Crystalstranslucent,massesopaque Colour-Green,greyandwhitetoalmostsilver Streak-White Lustre-Pearly,greasy Cleavage/fracture-Perfectinonedirection/uneven Crystalhabit/modeofoccurrence-Tabular(thinflakes,neverlargecrystals)/ granular, cryptocrystalline © TheUniversityofAuckland2005