Calcite (twin)

specimen number:
2013521

location:
Berry Materials Corp. Quarry, North Vernon, Indiana, USA

description:
A sharp unusually large lustrous mostly gemmy twin crystal of calcite 4.5 cm across set on matrix with many smaller non-twinned crystals. Collected in the 1980s, superb for the species and locality.

The Berry Materials Corp Quarry is located near North Vernon in Jennings County, Indiana, and is a significant industrial site within the Illinois Basin. Geologically, the quarry exposes a stratigraphic sequence of Middle Devonian carbonate rocks, primarily the North Vernon Limestone and the underlying Jeffersonville Limestone. These sedimentary layers were deposited approximately 380 to 390 million years ago in a shallow, warm epicontinental sea. The mineralization found within the quarry occurs as a Mississippi Valley-Type (MVT) deposit, where hydrothermal fluids migrated through regional joints and solution cavities. Historically, the quarry has been a major producer of crushed stone, aggregate, and agricultural lime for the southern Indiana region for several decades.

In a contact twin, two individual crystals are joined together along a single common plane, known as the composition plane. This plane acts like a mirror, where the crystal lattice on one side is a symmetrical reflection of the other. For calcite, this most famously occurs on the {0001} basal plane or the {0112} rhombohedral plane. When this happens to scalenohedral crystals, like those found at North Vernon, it creates a very distinct heart-shaped or V-shaped habit. Unlike a random cluster where crystals grow in various directions, a contact twin is a single, unified mineralogical individual with a precise geometric relationship.

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