The rocks (p. [36]-39) are arranged according to their reaction to dilute hydrochloric acid applied to a scratched surface. (The acid reacts more readily to powdered material produced by scratching the rock.) After the reaction to acid has been determined, the texture and components of the rock should be noted. Because rocks grade into one another, clear distinctions are not always possible.
MINERAL IDENTIFICATION KEY
| I. METALLIC LUSTER, STREAK COLORED | ||||
|---|---|---|---|---|
| C—color S—streak | H—hardness Cl—cleavage F—fracture | Remarks | Name and composition | |
| A. Hardness not more than 2.5 | ||||
| C—lead gray S—black | H—2.5 Cl—cubic; perfect in 3 directions F—subconchoidal or even | Very heavy; occurs as crystals, grains, or masses; easily identified by color and cleavage | Galena ([31]) PbS | |
| C—copper red S—metallic, shiny | H—2.5 Cl—none F—jagged | Very heavy; apt to have green coating; distorted or wirelike forms; malleable | Native copper Cu | |
| B. Hardness greater than 2.5 but not greater than 6.5 | ||||
| C—yellow-brown to black S—yellow-brown | H—5.5 (may be as low as 1) Cl—none F—uneven | In earthy masses; coloring material in many sandstones, conglomerates, and soils; often mixed with and difficult to distinguish from goethite and other iron minerals | Limonite ([29]) FeO(OH)·H₂O | |
| C—brassy yellow S—greenish black | H—6 Cl—poor F—conchoidal to uneven | As compact masses, grains, cubes, and in 8- and 12-sided crystals; commonly associated with coal, and with lead-zinc ores of northwestern Illinois | Pyrite ([28]) FeS₂ | |
| C—pale brassy yellow to silver white S—greenish gray | H—6 Cl—poor F—uneven | As fibrous, radiating, tabular, and cocks-comb crystals or compact masses; usually lighter colored than pyrite, but difficult to distinguish from pyrite; associated with coal, and with lead-zinc ores of northwestern Illinois | Marcasite FeS₂ ([28]) | |
| II. NONMETALLIC LUSTER, STREAK WHITE | ||||
| A. Hardness not greater than 2 (can be scratched by fingernail) | ||||
| C—usually white but may be almost any color | H—2 Cl—perfect in one direction, less perfect in two others | Commonly found in Illinois as twinned or needle-shaped crystals in weathered shales containing pyrite and calcium carbonate | Gypsum ([26]) CaSO₄·2H₂O | |
| C—white or a shade of green | H—2 | As needle-shaped crystals or powdery coating on pyrite or marcasite; has an astringent taste | Melanterite FeSO₄·7H₂O | |
| B. Hardness greater than 2 but not greater than 3 (Can be scratched by a penny) | ||||
| C—colorless, silver white, gray, brown | H—2-2.5 Cl—perfect in one direction | In scales or “books”; in splits into thin sheets; common in sandstones, shales, and in igneous and metamorphic rocks | Muscovite (white mica) ([23]) (OH)₂KAl₂ AlSi₃O₈ | |
| C—brown or black | H—2.5-3 Cl—perfect in one direction | As scales or “books”; splits into thin sheets; common in igneous and metamorphic rocks but not in sedimentary rocks such as sandstone or shale | Biotite ([23]) (black mica) (OH)₂K(Mg,Fe)₃ AlSi₃O₈ | |
| C—colorless, white, gray, and various tints | H—3 Cl—perfect in three directions, not at right angles (rhombohedral) | Common mineral; effervesces vigorously in cold acid; occurs in many crystal forms and as fibrous, banded, and compact masses; chief mineral in limestones | Calcite ([24]) CaCO₃ | |
| C—white, gray, red, or almost any color | H—3 Cl—perfect in one direction, less perfect in two other directions | Very heavy; commonly in tabular crystals united in diverging groups, as laminated or granular masses; associated with fluorite in southern Illinois | Barite BaSO₄ | |
| C. Hardness greater than 3 but not greater than 5 | ||||
| (Cannot be scratched by penny; can be scratched by knife) | ||||
| C—white, gray, light yellow | H—3.5 Cl—in one direction F—uneven | Relatively heavy; effervesces in acid; associated with fluorite and barite in southern Illinois but is not abundant | Witherite BaCO₃ | |
| C—white, pink, gray, or light brown | H—3.5 Cl—perfect in three directions, not at right angles (rhombohedral) | In grains, rhombohedral crystals and cleavable or granular masses; effervesces slowly in cold acid when powdered, more vigorously in warm acid; principal mineral in rock called dolomite | Dolomite CaMg(CO₃)₂ | |
| C—colorless, white, gray, grayish black | H—3.5 | In fibrous or compact masses or may be in orthorhombic crystals as a coating on galena; very heavy; effervesces in acid; formed by alteration of galena | Cerussite PbCO₃ | |
| C—brown to gray S—usually white but may tend toward brown when weathered | H—3.5 Cl—in three directions not at right angles (rhombohedral) slightly curved surfaces | In fibrous or botryoidal masses or rhombohedral crystals; effervesces in hot acid | Siderite FeCO₃ | |
| C—yellow, yellow-brown to almost black S—light yellow to brown | H—3.5 Cl—parallel to dodecahedral faces; in six directions | In crystals, in fibrous or layered masses; associated with galena in northwestern Illinois, with fluorite and galena in southern Illinois | Sphalerite ZnS ([30]) | |
| C—colorless, white, yellow, purple, green, blue | H—4 Cl—perfect, parallel to octahedral faces; in four directions | In cubes and cleavable masses; many colors; mined in Hardin and Pope counties | Fluorite ([25]) (Fluorspar) CaF₂ | |
| C—white, tinted yellow, blue, or green | H—5 | As crystalline incrustations or in earthy or compact masses; associated with fluorite-sphalerite ores in southern Illinois, with galena and sphalerite in northwestern Illinois | Smithsonite ZnCO₃ | |
| D. Hardness greater than 5 but not greater than 7 | ||||
| C—white, green, brown, black | H—5-6 Cl—in two directions intersecting at about 60° and 120° | In long, slender 6-sided crystals; cleavage angle important in differentiating from pyroxenes; common in metamorphic and some igneous rocks | Amphibole Group (Mg,Fe,Ca)₇(Si₈O₂₂)(OH)₂ (may also contain Na or Al) | |
| C—gray, dark green, black, dark brown, bronze | H—5-6 Cl—in two directions intersecting at about 90° | Crystals short, stout, and 8-sided; cleavage angle important in differentiating from amphiboles; common in igneous and some metamorphic rocks | Pyroxene Group (Mg,Ca,Fe)₂(Si₂O₆) | |
| C—white, gray, pink, light blue, green | H—6 Cl—in two directions nearly at right angles | As crystals, cleavable masses and grains; common in igneous and metamorphic rocks, also in stream gravel and sand; many varieties | Feldspar Group ([22]) K, Na, Ca, Ba (Al, Si)₄O₈ | |
| C—white when pure; may be colored by impurities | H—7 Cl—none F—conchoidal | Finely crystalline variety of quartz; botryoidal or concretionary masses; lining in geodes | Chalcedony SiO₂ | |
| C—colorless, white, or almost any color | H—7 F—conchoidal | Most abundant mineral; occurs in 6-sided crystals capped by pyramids, in grains or masses; principal mineral in sandstone, also abundant in igneous and metamorphic rocks; is a variety of silica | Quartz ([21]) SiO₂ | |
| C—red | H—7 F—conchoidal | A variety of quartz usually colored red by hematite inclusions; common in glacial and river sand and gravel found along Lake Michigan shores and in the Mississippi River | Jasper SiO₂ | |
| C—many; arranged in bands | H—7 F—conchoidal | Cloudy banded variety of silica; widely used as semi-precious stones. Onyx and silicified wood are forms of agate; found in glacial gravels and upper Mesozoic sediments in southern Illinois | Agate SiO₂ | |
| E. Hardness greater than 7 (cannot be scratched by quartz) | ||||
| C—red, brown, yellow, green, black, white | H—7.5 Cl—poor F—even | Irregular grains or masses; sometimes as 12-, 24-, and 36-sided crystals; abundant in glacial sands and Lake Michigan beach sands; common in metamorphic rocks | Garnet Group (Ca,Mn,Fe,Mg)₃(Al,Cr)₂(SiO₄)₃ | |
ROCK IDENTIFICATION KEY
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
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]) 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])