ROCK IDENTIFICATION KEY
Reading edition
SAMPLE
Scratch with a knife and apply dilute acid (HCl)
If rock does not scratch, go directly to I, II, or III
No effervescence or very slight effervescence
I Coarse-grained (p. 37)
II Fine-grained (p. 38)
III Organic (p. 39)
Slight effervescence
gray, light gray, white, or brown: Dolomite
Vigorous effervescence
Composed of pebbles that effervesce
Rounded pebbles: Limestone conglomerate
Angular pebbles: Limestone breccia
Composed of crystals of calcite, fossil shells, or oolites: Limestone
Composed of banded layers of crystalline calcite; commonly found in caves, forming stalactites and stalagmites: Travertine
Large amount of insoluble residue left on acid-treated surface
Individual grains seen with unaided eye: Calcareous sandstone
Individual grains not seen with unaided eye: Calcareous shale
Composed of porous or cellular mass of calcite; commonly found near springs and waterfalls: Tufa
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I COARSE-GRAINED ROCKS
A. Rock consists of interlocking grains or crystals, easily seen; too hard to scratch with a knife
1. Crystals aligned in one direction
a) Crystals in parallel bands with layers of quartz and feldspar separated by mica and other minerals
Gneiss (6)
b) Crystals in thin parallel bands; tends to split into thin sheets parallel to banding; some varieties may be scratched with a knife
Schist (6)
2. Crystals not aligned in any particular direction
a) Light gray, pink, red, or tan with only a few dark minerals; feldspar and quartz principal minerals
Granite (1)
b) Dark to medium gray; composed of feldspar and dark minerals with little quartz
Gabbro (2)
c) Dark green to black; essentially dark minerals, may have some feldspar; quartz generally lacking
Peridotite (5)
d) Light color; similar to granite in texture but lacks quartz; composed of feldspar and some dark minerals
Syenite
e) Large, easily seen crystals set in a fine- to extremely fine-grained background; any color
Porphyry (3)
f) Essentially quartz; grains may be identifiable; specimens break through rather than around grains
Quartzite (9)
B. Rock composed of individual rock particles or fragments, non-interlocking crystals, cemented or not cemented together; may or may not be scratched with a knife
1. Particles or fragments not uniform in size; a mixture of pebbles, sand, and smaller materials
a) Solid rock consisting of particles or fragments generally rounded and cemented together
Conglomerate (7)
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b) Solid rock consisting of particles or fragments, generally angular and cemented together
Breccia
c) Fragments ranging in size from clay to large boulders; may be compacted, but not cemented; much clay generally present; may effervesce
Glacial till
d) Loose particles of many sizes, not cemented together; some particles may effervesce
Gravel
2. Rock particles or fragments, about the size of grains of sugar (2 to .05 mm)
a) Loose particles consisting largely of quartz
Sand
b) Solid rock consisting largely of quartz; can be separated easily into individual particles; granular; breaks around rather than through grains
Sandstone (8)
II FINE-GRAINED ROCKS
A. Cannot be scratched easily with a knife; crystals or particles not easily seen with the unaided eye; very hard, difficult to break; may contain a few crystals or particles large enough to see; granular
1) Dense; brittle; splintery or conchoidal fracture; sharp edges and corners when broken; often associated with limestone; usually white or gray; very dense, dull varieties called flint
Chert (27)
2) Light gray, pink, red, or tan varieties common; boulders or fragments in the glacial drift
Felsite
3) Dark gray, greenish, black, or maroon varieties common; may have small mineral-filled cavities; occurs as boulders or fragments in the glacial drift
Basalt (4)
4) Essentially quartz; grains may be identifiable; specimens break through rather than around grains
Quartzite (9)
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B. May or may not be scratched with a knife; fairly uniformly fine grained
1) Soft; feels slippery or soapy when wet; may disintegrate in water; gives off an earthy odor when breathed upon
Clay
2) Loose; gritty; particles smaller than table salt
Silt
3) Solid rock; often in thin beds or sheets; separates into silt; mica flakes may be present; may contain fossils; may effervesce slightly
Siltstone
4) Solid rock; breaks into thin platy sheets; may feel slippery when wet; black to gray; may contain fossils; shows thin laminations; may effervesce
Shale (10)
5) Solid rock: does not break into thin platy fragments; may effervesce slightly
Mudstone
6) Solid rock; usually gray or black; splits into platy sheets or slabs; harder than shale
Slate
7) Powdery; white or light brown; commonly associated with chert and quartz from which it forms
Tripoli (19)
III ORGANIC ROCKS (DARK COLORED)
A. Soft; spongy when wet; very lightweight when dry; forms in swampy places
1) Fine mass with coarse plant fragments; dark gray to black
Peat (13)
2) Plant fragments small and not easily recognized; fine-grained; black to dark gray; earthy
Muck
B. Hard but can be scratched with a knife
1) Black; contains bands of shiny and dull material; burns well
Coal (14)
2) Dark gray to black; does not contain shiny bands; splits into thin sheets; burns poorly or not at all
Bituminous shale
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