Clues for the Identification of Those Pesky Rock Forming Minerals (Follow along on p 62) Colby Geology A FELDSPARS (two varieties to identify) All have two directions of cleavage PLAGIOCLASE (2 types to identify) (Ca,Na)(Si,Al) O Have striations on cleavage planes (end at edge of cleavage plane) a OLIGOCLASE (white) Na-rich feldspar b LABRADORITE (gray to black) Ca-rich feldspar STRIATIONS OLIGOCLASE LABRADORITE Labradorite has labradorescence or chatoyance LABRADORITE A FELDSPARS (two varieties to identify) All have two directions of cleavage ORTHOCLASE (pink, tan or white) KAlSi Has stringers that go through the mineral - looks superficially like striations Look at display so that you know what striations are ORTHOCLASE FERROMAGNESIANS (iron and magnesium silicate All have two directions of cleavage AMPHIBOLES (2 types to identify) Cleavage planes intersect forming 124 o and 56 o cleavage angles Crystals usually long and thin a HORNBLENDE (black) Cleavage Angles b ACTINOLITE (greenish gray or grayish green) Crystals usually in radiating masses (check with me if not visible!) ACTINOLITE PYROXENES (1 to identify) Cleavage planes intersect forming 88 o and 92 o cleavage angles Crystals usually short and stubby a AUGITE (greenish black) AUGITE C MICAS (3 to identify) All have one direction of cleavage, producing platy appearance MUSCOVITE (white-silver) BIOTITE (brown) CHLORITE (green) - may not look like the other micas in terms of plateyness CHLORITE D QUARTZ (SiO ) Hardness is Can be many colors due to impurities Displays conchoidal fracture ROSE QUARTZ AMETHYST CONCHOIDAL FRACTURE E OLIVINE Olivine is olive green Granular appearance May be able to see conchoidal fracture of grains with hand lens F CALCITE (CaCO ) Fizzes or effervesces in HCl Three directions of cleavage not at right angles (rhombohedral cleavage) Clear pieces may exhibit double refraction RHOMBOHEDRAL CLEAVAGE HCl ACID REACTION DOUBLE REFRACTION G GARNET Reddish-brown color (but can be others) Has hackly fracture, no cleavage Roundish crystals with 12-24 flat faces readily visible in most cases H STAUROLITE Crystals usually twinned ("fairy crosses")