Important Rocks & Minerals

QUARTZ
(primary mineral)

• translucent, often milky
• irregular surface, not planar
• very hard

Composition: SiO2

Found in: many rocks, esp. light-color (felsic), sandstones.

Muscovite

Biotite

MICAS
(primary minerals)

• platy, sheet-like
• black (biotite) or clear (muscovite)
• soft, can be peeled

Composition: (Al, Si + O) with K

Found in: many igneous and metamorphic rocks.

FELDSPARS
(primary minerals)

• angular, sharp crystal faces/edges
• tan to brown, pink, whitish

Composition: (Al, Si + O) with K

Found in: many igneous and metamorphic rocks.

FERROMAGNESIAN MINERALS
(primary minerals)

• angular, sharp crystal faces
• black or very dark green

Composition: (Al, Si + O) with Mg, Fe

Found in: dark (mafic) igneous rocks.

Hematite

Goethite

IRON OXIDES >>HEMATITE, GOETHITE
(secondary minerals)

• dull, seldom crystalline
• dull red (hematite) to yellow (goethite)
• usually soft, dense masses

Composition: Fe(O,OH)Found in: sedimentary rocks; saprolite.

Kaolinite

Vermiculite

CLAY MINERALS >>KAOLINITE, VERMICULITE
(secondary minerals)

• dull; seldom crystalline
• whitish to grey
• soft

Composition: (Al, Si, O)

Found in: sedimentary rocks; saprolite.

GRANITE
(igneous rock)

• Light colored (mafic); fine to large crystals.
• Uniform texture (no banding).

Composition: Largely quartz, feldspar; some mica.

Found in: Intrusive bodies in Piedmont (Stone Mt).

BASALT
(igneous rock)

• Dark colored (mafic); fine to very fine crystals.
• Uniform texture (no banding).

Composition: High in ferromagnesian minerals (hornblende, augite).

Isolated occurrence in Piedmont (seafloor remnants deposited during orogeny).

SCHIST
(metamorphic rock)

• Intermediate to dark colored; fine to coarse crystals.
• Finely banded (foliated), light and dark.

Composition: Commonly mica, with quartz, feldspar.

Formed from metamorphosed sediments; common in Piedmont, Blue Ridge.

GNEISS
(metamorphic rock)

• Intermediate to light color; medium – coarse crystals.
• Coarsely banded, fine dark bands/coarse light bands.

Composition: Quartz, feldspar, mica.

Formed from metamorphosed granites or sandy marine sediments; common in Piedmont, Blue Ridge.

LIMESTONE
(sedimentary rock)

• Whitish to light grey; few crystals; may contain fossils.
• May be coarsely or finely banded, or massive.

Composition: Calcite, with mixture of clay minerals.

Formed in off-shore marine environments; common in Valley & Ridge, parts of Coastal Plain.

SANDSTONE
(sedimentary rock)

• Various colors (grey, brown, tan, reddish).
• Feels sandy when rubbed.
• May or may not be banded.

Composition: Quartz with some oxides.

Formed in near-shore marine deposits; common in Valley & Ridge, parts of Coastal Plain.