Dark Colored Minerals with Non-Metallic Lusters,
Softer than Glass, and with Cleavage
PHYSICAL PROPERTIES 
MINERAL NAME, FORMULA
 
BLACK to DARK BROWN; PERFECT 1-WAY CLEAVAGE; thin flexible sheets; flakes in small grains; H=2.5. BIOTITE
  K(Fe,Mg)3AlSi3O10(OH)2
GREEN to GREENISH BLACK color; scaly; 1-way cleavage; H=2. CHLORITE
  (Mg,Fe)6Si4O10(OH)8
BLACK, DARK GRAY, DARK GREEN, but light colors are more common; 3-WAY CLEAVAGE, not at right angles; EFFERVESCES (fizzes) IN DILUTE HCl acid; H=3. CALCITE
  CaCO3
DARK GRAY, BLACK, but also light colors; 3-WAY CLEAVAGE, not at right angles; commonly in fine-grained granular masses; REACTS WEAKLY WITH HCl acid (slowly or  only if sample is powdered); H=3-3.5; may occur as curved, pink crystals. DOLOMITE
  CaMg(CO3)2
 


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