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ENCYCLOPEDIA ARTICLE: Silica minerals

 

 

 

Optical and physical properties of silica minerals
Mineral	Specific gravity	Optic sign	Mean index	Nα or Nω*	Nβ*	Nγ or Nε*	Cleavage	Color	Hardness (Mohs scale)	Twinning
Stishovite	4.35	Uniaxial (+)	1.81	1.799		1.826	N.R.^†	Pale gray	N.R.	N.R.
Coesite	3.01	Biaxial (+)	1.59	1.594	1.596	1.597	None	Colorless	8	{021}
Low (α) quartz	2.65	Uniaxial (+)	1.55	1.544		1.553	{1011}, {0111}, {1010}^‡	Colorless, white to variable	7	c axis {1120}, {1122}, and others
Low (α) cristobalite	2.32	Uniaxial (−)	1.48	1.487		1.484	None	Colorless, white, or yellowish	6½	{111}
Low (α) tridymite	2.26	Biaxial (+)	1.47	1.471–1.479	1.472–1.48	1.474–1.483	None to poor prismatic	Colorless, white	7	{110}