Common Accessory Minerals of the Philadelphia Area

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Rock-forming mineral

A mineral that (often) forms the bulk of a rock

Acessory mineral:

a mineral that does not form the bulk of a rock, but occurs in small amounts within the rock

Andalusite occurrence

Metapelites (metamorphosed mudstone)

Andalusite identification

Dull, brown-gray, log-like; sometimes you can see chiastolite

Kyanite occurrence

Metapelites (metamorphosed mudstone)

Kyanite identification

blue/grey, bladed

Sillimanite occurrence

Metapelites (metamorphosed mudstone)

Sillimanite identification

Light-colored, semi-translucent needles

Staurolite occurence

Metapelites (metamorphosed mudstone)

Staurolite identification

Rusty-brown, high-relief, cylindrical; sometimes forms a cross

Tourmaline occurrence

igneous and metamorphic rocks

Tourmaline identification

Long, slender, dark-colored adamantine, columnar, basal parting; triangular cross-section

Serpentine occurrence

Metamorphosed olivine (mafics/ ultramafics)

Serpentine identification

Green, mottled, undulating, sometimes fibrous (“chrysotile”)

Talc occurrence

Metamorphosed ultramafics

Talc identification

Scratched by fingernail, greasy/pearly-luster, white/greenish

Chlorite occurrence

Low-grade metamorphics

Chlorite identification

Makes schists green

Magnetite occurrence

Metamorphic and igneous rocks

Magnetite identification

Black, octahedral, magnetic