Apophyllite on Prehnite
Fairfax County, Virginia

The single Apophyllite crystal on this specimen measures 8cm on edge

Apophyllite KCa₄Si₈O₂₀(F,OH) ^.8H₂O

Phyllosilicates, hydrous calcium potassium fluorsilicate, often with a small amount of iron and nickel

Color and Luster: Colorless, white, gray, greenish, yellowish, reddish; vitreous, pearly on cleavage; streak white

Hardness: 4.5-5

Cleavage: Perfect, one direction

Fracture: Uneven

Crystals: Tetragonal; usually cubelike or tabular, square in cross section; also lamellar, granular, compact

Prehnite Ca₂Al₂Si₃O₁₀(OH)₂

Complex silicate; calcium aluminum silicate, often with some iron

Color and Luster: Light green, oil-green, gray, white, colorless; frequently fading upon exposure; vitreous, waxy; streak colorless

Hardness: 6-6.5

Cleavage: Distinct, one direction

Fracture: Uneven

Crystals: Orthorhombic; crystals rare, usually in botryoidal masses, with ridged surfaces marked by edges of curving crystals; also stalactitic, radial fibrous

Photo by Mary Sutherland

Descriptions taken from
The Audubon Society Field Guide to North American Rocks and Minerals