Guide to Rocks and Minerals of Illinois

Metamorphic Rocks

Gneiss and Schist

Gneiss is a metamorphic rock composed of roughly parallel bands of different minerals. It is medium- to coarse-grained and is generally light in color. The names given to gneiss emphasize a distinctive texture or indicate a dominant mineral composition. For example, biotite gneiss emphasizes a mineral, and granite gneiss indicates the rock's composition.

Schist is much like gneiss, but schist is fine-grained and has a thinly layered, micaceous structure that commonly makes the rock break with a wavy, shiny surface. Some common types of schistose rocks are talc schist, chlorite schist, and hornblende schist. As the names indicate, they are characterized by their dominant mineral. Mica schist may be formed by the metamorphism of either sedimentary or igneous rocks.

Gneiss and schist are not native to Illinois but are found in the glacial drift.

Quartzite

Quartzite is a metamorphic rock that originally was quartz sandstone. Quartzites are produced by intense heat and/or pressure, probably aided by hot silica-bearing solutions. The quartz grains may be so closely interlocked that individual grains are no longer recognizable. The rock fractures conchoidally (with a curved surface) through both the grains and cement, so the broken surface, unlike that of sandstone, is smooth and may even exhibit a glassy luster like quartz.

Color depends upon the amount and kind of impurities that are present. An all-quartz quartzite is white or gray, but iron or other elements may change the quartzite's color to shades of purple, yellow, brown, or red. Quartzite is a very resistant, hard rock and cannot be scratched by a knife.

Quartzite is abundant as boulders and pebbles in the glacial drift of Illinois, having been brought into the state during the Ice Age.

Rocks and Minerals of Illinois Table of Contents

Updated 11/29/2011 SLD