Guide to Rocks and Minerals of Illinois
Keys for Identification of Common Illinois Rocks and Minerals
Two keys, one for rocks and one for minerals, present clues that may aid the collector in identifying rocks and minerals found in Illinois. In outline form, the keys are a guide to some of the easily observable properties that various rocks and minerals display.
The key includes all rocks and minerals in the ISGS school set, Typical Rocks and Minerals of Illinois, plus other relatively common rocks and minerals that can be found in Illinois. Because of the great diversity of rocks and minerals in this state, the keys are not conclusive. Other, more complete guides (such as that in the Manual of Mineralogy) should be consulted when identifying rocks and minerals that are from other states or Illinois specimens that are difficult to identify.
The minerals are arranged in two groups: (1) those with metallic luster and (2) those with nonmetallic luster. Each group is arranged according to increasing hardness. Other characteristics such as color, streak, cleavage, fracture, and composition are listed.
The rocks 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.
Rock Identification Key
I. Coarse-Grained Rocks
1. Crystals aligned in one direction
Crystals in thin parallel bands; tends to split into thin sheets parrallel to banding; some varieties may be scratched with a knife.
Dark to medium gray; composed of feldspar and dark minerals with little quartz.
Dark green to black; essentially dark minerals, may have some feldspar; quartz generally lacking.
Light color; similar to granite in texture by lacks quartz; composed of feldspar and some dark minerals.
Large, easily seen crystals set in a fine-grained to extremely fine-grained background; any color.
Essentially quartz; grains my be identifiable; specimens break through rather than around the grains.
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.
Solid rock consisting of large angular particles or fragments cemented together.
Fragments ranging in size from clay to large boulders; may be compacted, but not cemented; much clay generally present; may effervesce when 10% hydrochloric acid is applied.
Loose particles of may sizes, not cemented together; some particles may effervesce.
Solid rock consisting largely of individual grains of quartz; granular; breaks around rather than through grains.
II. Fine-Grained Rocks
very hard; difficult to break; may contain a few crystals or particles large enough to see; granular.
Light gray, pink, rad, or tan varieties common; boulders or fragments in the galcial drift.
Dark Gray, greenish, black, or maroon varieties common; may have small mineral-filled cavities; occurs as boulders or fragments in the glacial drift.
Essentially quartz; gains may be identifiable; specimens break through rather than around grains.
Loose; gritty; particles smaller than rable salt; commonly gray.
Solid rock; often in thin beds or sheets; separates into silt; mica flackes may be present; may contain fossils; may effervesce slightly.
Solid rock; breaks into thin platy sheets; may feel slippery when wet; black to gray; may contain fossils; shows thin laminations or may effervesce.
Solid rock; does not break into thin platy fragments; may effervesce slightly.
Solid rock; usually gray or black; splits into platy sheets or slabs; harder than shale; occurs in galcial deposits.
Powdery; white or light brown; commonly associated with chert and limestone from which it forms; may contain fossils.
III. Organic Rocks (Dark Colored)
Plant fragments small and not easily recognized; fine-grained; black to dark gray; earthy.
Dark gray to balck; does not contain shiny bands; splits into thin sheets; burns poorly or not at all.
Rocks and Minerals of Illinois Table of Contents
The printed version of Guide to Rocks and Minerals in Illinois can be purchased from the Shop ISGS Web site.
Updated 11/29/2011 SLD