Calcite CaCO3

calcite

Calcite CaCO3

Although a rather usual mineral throughout much of Western Ohio, Calcite geodes are a highly uncommon occurrence. Unlike many midwestern states where Quartz geodes are widespread and abundant, geodes of any type are a rare find throughout the state.

The Calcite in these geodized brachiopods formed in two or more generations. The first generation of Calcite forms a coating of drusy appearing crystals completely lining the hollow interior of the fossil. While rare doubly terminated crystals are occasionally found, most first generation Calcite crystals are well formed clear white to grayish brown, singly terminated scalenohedrons or scalenohedrons with rhombohedral modifications.

Typically, first generation crystals are no larger than 1/4 inch. These crystals are generally nondescript and relatively uninteresting in comparison to Calcite specimens from other more famed localities

Doubly Terminated Scalenohedral Calcite

Although the crystal faces are slightly imperfect the form is still easily recognizable. To the right, a fine example of the sequence of mineralization in these geodized brachiopods. This brachiopod was first mineralized with scalenohedral, clear, Calcite.

Second, a blade of Celestine grew on the first generation Calcite. Sometime later, a larger second generation pink scalenohedral Calcite grew through the pocket and encompassed the Celestine.

In general, most second generation Calcite crystals, at this locality, are larger and far more aesthetic and diverse in form. Brachiopods with larger complete and undamaged second generation crystals make very attractive specimens.

Usually forming more colorful crystals, both scalenohedral and Scalenohedral with rhombic modification 2nd generation Calcite crystals have been observed in the Georgetown road cuts. Several distinct second generation Calcite habits have been identified in the geodized fossils at Georgetown. However, the more striking second generation Calcite specimens are those with multiple growth, twinned or phantomed crystals.

Quite commonly, the second generation Calcite crystal will grow across the cavity and is difficult to save as a presentable specimen. Brachiopods from separate strata tend to form different habits of second generation Calcite. Geodized brachiopods in the same strata typically have similar second generation Calcite growth. Although typically not in specimen quality, brachiopods with more than one habit of second generation Calcite have been recovered at this locality.

Unfortunately, about 1/3 of all mineralized brachiopods have been mostly to completely filled with second generation Calcite. On average, complete brachiopods with lighter tan colored shells are most likely to have been mineralized with second generation Calcite.