Some physical properties of silica minerals

phase symmetry specific gravity hardness

● quartz

● (alpha-quartz)

● hexagonal;

● trigonal

● trapezohedral

2.651 7

● high quartz

● (beta-quartz)

● hexagonal;

● hexagonal

● trapezohedral

2.53 at 600 degrees Celsius 7

low tridymite monoclinic? 2.26 7

high tridymite orthorhombic 2.20 at 200 degrees Celsius 7?

low cristobalite tetragonal 2.32 6–7

high cristobalite isometric 2.20 at 500 degrees Celsius 6–7

keatite tetragonal 2.50 ?

coesite monoclinic 2.93 7.5

stishovite tetragonal 4.28 ?

vitreous silica amorphous 2.203 6

opal

● poorly

● crystalline or

● amorphous

1.99–2.05 5 1/2–6 1/2