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Clues for the Identification of Those Pesky Rock Forming Minerals (Follow along on p. 62)

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. 1. PLAGIOCLASE (2 types to identify) (Ca,Na)(Si,Al) 4 O 8 Have striations on cleavage planes

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Clues for the Identification of Those Pesky Rock Forming Minerals (Follow along on p. 62)

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  1. Clues for the Identification of Those Pesky Rock Forming Minerals (Follow along on p. 62) Colby Geology

  2. A. FELDSPARS (two varieties to identify) All have two directions of cleavage. 1. PLAGIOCLASE (2 types to identify) (Ca,Na)(Si,Al)4O8 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

  3. Labradorite has labradorescence or chatoyance. LABRADORITE

  4. A. FELDSPARS (two varieties to identify) All have two directions of cleavage. 2. ORTHOCLASE (pink, tan or white) KAlSi308 Has stringers that go through the mineral - looks superficially like striations. Look at display so that you know what striations are. ORTHOCLASE

  5. B. FERROMAGNESIANS (iron and magnesium silicates) All have two directions of cleavage. 1. AMPHIBOLES (2 types to identify) Cleavage planes intersect forming 124o and 56o cleavage angles. Crystals usually long and thin. a. HORNBLENDE (black) Cleavage Angles

  6. b. ACTINOLITE (greenish gray or grayish green) Crystals usually in radiating masses (check with me if not visible!) ACTINOLITE

  7. 2. PYROXENES (1 to identify) Cleavage planes intersect forming 88o and 92o cleavage angles. Crystals usually short and stubby. a. AUGITE (greenish black) AUGITE

  8. C. MICAS (3 to identify) All have one direction of cleavage, producing platy appearance. 1. MUSCOVITE (white-silver) 2. BIOTITE (brown) 3. CHLORITE (green) - may not look like the other micas in terms of plateyness.

  9. CHLORITE

  10. QUARTZ (SiO2) • Hardness is 7. • Can be many colors due to impurities. • Displays conchoidal fracture. ROSE QUARTZ AMETHYST

  11. CONCHOIDAL FRACTURE

  12. E. OLIVINE Olivine is olive green. Granular appearance. May be able to see conchoidal fracture of grains with hand lens.

  13. F. CALCITE (CaCO3) 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

  14. DOUBLE REFRACTION

  15. G. GARNET Reddish-brown color (but can be others). Has hacklyfracture, no cleavage. Roundish crystals with 12-24 flat faces readily visible in most cases.

  16. H. STAUROLITE Crystals usually twinned ("fairy crosses").

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