Exploring the Rocks of Iowa

Iowas landscape varies widely from place to place across the state. A state shape by glaciers, rivers, and seas during its geologic past has lead to a variety of minerals to be found in Iowa.

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Agate

Streak: White

Hardness: 7

Chemical Formula: SiO2

Agate is a common form of chalcedony, a microcrystalline form of quartz. It’s banded, colorful appearance has lead to its widespread use throughout human history.

Most varieties of agate are named for its visual appearance, such as fire agate which has inclusions of red or brown hematite. Agate can also vary in its banding appearance, some agates have a dendritic habit or lace-like patterns with eyes, swirls, and other patterns.

Because this mineral is a form of quartz, it’s hardness lends itself to uses not commonly associated with such visually appealing minerals. Agate has been used to create precision pendulums, mortars and pestles, and fine knife-bearing edges for laboratory balances.

Photo: Agate by James St. John is licensed under the Creative Commons Attribution 2.0 Generic license.

Large example of pink dolomite with three quartz crystals growing from it

Calcite

Streak: White

Hardness: 3

Chemical Formula: CaCO₃

Calcite is found commonly in ore deposits, as a cementing medium in sandstone, or in small quantities in igneous rocks.

These Calcite crystals come in a variety of sizes. Calcite can be microscopic or grow to several feet in length. It’s quite common to see excellent examples of calcite in “dogtooth” shaped crystals. These crystals have a pyramid shape. Calcite does not vary much in color. It is generally white or a pale amber.

Photo: Calcite, Dolomite by Robert M. Lavinsky is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.

Barite

Barite is a heavy non-metallic mineral. It generally comes in white, gray, or blue-gray coloring. Historically, Missouri has been a large Barite producer. Barite has been used in the past as a paint pigment, a source of barium for chemicals, and as a filler in rubber, textiles, and plastics.

The most common specimens of Barite that you may see for sale are bladed and opaque. These can be found in Washington County, near Potosi and Old Mines. Smaller crystals have also been found in Morgan and Moniteau Counties.

Photo: Baryte, Calcite by Robert M. Lavinsky This file is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.

galena mineral sample

Galena

Streak: Lead-gray

Hardness: 2.5

Chemical Formula: PbS

Galena is the natural mineral form of Lead Sulfide, the most important ore of lead and silver. Galena is one of the most abundant sulfide minerals, a group which contains other popular minerals such as pyrite and cinnabar.

When fresh or tested on a streak plate, galena is opaque and a bright metallic gray. Galena can be found as a cubic, octahedral, or dodecahedral crystalline structure or a combination of these.

Galena deposits are found worldwide with notable deposits in the Lead Belt in the United States, the Sullivan Mine of British Columbia, and in the Driftless Area of Illinois, Iowa, and Wisconsin.

There is historically evidence to support the use of galena as far back as 6500 BCE as lead beads. Galena was also used in Ancient Egypt in cosmetic eye kohl.

Keokuk Geodes

Keokuk Geode from Missouri

Keokuk geodes are found in the south-eastern most corner of Iowa. They can also be found in Missouri and Illinois. These geodes get their name from the Iowa city of Keokuk, located at the center of the collecting area.

These geodes can be found easily in this area where they often wind up in stream channels where water has eroded the rock.

In a Keokuk geode, you can usually find beautiful white quartz crystals. The outer wall of these geodes is made up of chalcedony. When you find a keokuk geode it is not uncommon to find other minerals inside your geode. Pyrite, dolomite, and kaolinite have all been found inside keokuk geodes.

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Celeste
Author: Celeste

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