Guide to Rocks and Minerals of Illinois

Igneous Rocks

Granite

Granite is one of the most widespread intrusive (originating deep within the Earth) igneous rocks. Granite consists chiefly of feldspar and quartz with small amounts of biotite, muscovite, or hornblende. Most granite is light-colored, but it can be white, gray, yellow, pink, or deep red. The texture ranges from medium-grained to coarse-grained.

Granite pebbles or boulders are the most common igneous rocks found in glacial deposits in Illinois. These pebbles and boulders are not native to the state but were brought here by the glacial ice sheets that advanced southward across Canada to cover much of the northern United States during Pleistocene time.

Granite is used extensively as building stone. Native Precambrian granitic rock lies at great depths beneath the state and has been recovered from oil test drillings along the western, northern, and southern margins of Illinois.

Gabbro

Gabbro is another intrusive igneous rock, but it is heavier and darker than granite. Gabbro is composed mainly of feldspar and dark iron-bearing minerals such as amphibole or pyroxene that give the rock a dark color. It is coarse-grained and contains very little or no quartz.

Mineral crystals in gabbro are especially tightly interlocked, making the rock very difficult to break. Weathered gabbro is a rusty color on the surface, because the iron minerals in gabbro weather in the same way that a piece of metallic iron becomes coated with rust when left outdoors.

Like other igneous rocks found near the surface in Illinois, gabbro was carried into Illinois by the glaciers and deposited as glacial debris.

Porphyry

Porphyry is an igneous rock identified by its texture rather than its mineral content, which is variable. Large, distinct crystals—called phenocrysts—of minerals are embedded in a matrix of fine-grained rock. The phenocrysts, commonly feldspar, formed before the main molten mass of the rock crystalized.

Any igneous rock may have a porphyritic variety, such as granite porphyry, rhyolite porphyry, or basalt porphyry although porphyries are most likely to form in association with fine-grained igneous rocks.

Porphyry is found in Illinois only in glacial drift.

Basalt

Worldwide, basalt is one of the most widely distributed extrusive volcanic rocks. This rock is composed of pyroxene, feldspar, magnetite (an iron ore), and in some instances olivine, biotite, and hornblende. The dark green, gray, or black color is due to the dark minerals that make up much of the rock. The minerals in basalt are fine-grained and closely packed together. Phenocrysts of olivine, pyroxene, and hornblende may be present. Basalt is easily identified by its dark color and fine-grained texture.

The glaciers brought basalt into Illinois along with other igneous rocks.

Lamprophyre

Lamprophyre is the only igneous rock native to Illinois that crops out at the surface. It is found as dikes (irregular veins) or sills (thin roughly horizontal sheets) that were formed when molten rock from deeper in the Earth intruded into cracks and fissures in the bedrock of southeastern Illinois in Hardin, Pope, Gallatin, and Saline Counties.

Lamprophyre ranges from very fine-grained to medium-grained rock and has an even texture. It is dark gray to greenish gray, depending on the minerals present, and weathers to a distinctive brown to tan color. In general, lamprophyre is composed of biotite, hornblende, and pyroxene; little or no feldspar or quartz is present.

Rocks and Minerals of Illinois Table of Contents

Updated 11/02/2011 SLD