File:Aragonite-lined vug (Vinton Member, Logan Formation, Lower Mississippian; Mohawk Dam spillway, western Coshocton County, Ohio, USA) 1 (26621629314).jpg

Aragonite-lined vug in the Mississippian of Ohio, USA.

A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 5200 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.

The carbonate minerals all contain one or more carbonate (CO3-2) anions.

Aragonite has the same chemistry as calcite - it is calcium carbonate (CaCO3). Why is it a different mineral? Aragonite has a different molecular structure - the atoms are packed differently. Different minerals having the same chemical formula are called polymorphs (another good example is graphite & diamond - both C).

The difference in atomic-level packing between calcite and aragonite can be seen at the level of mineral hand samples. Aragonite forms crystals in the orthorhombic class. Many aragonite crystals are acicular (needle-like). Many aragonites form pseudohexagonal crystals, the result of 6 orthorhombic prisms growing parallel to each other.

Like calcite, aragonite is moderately soft (H=3), is often clearish to whitish to yellowish, and easily bubbles in acid. Aragonite is a little bit heavier than calcite, due to closer packing of atoms.

Most modern seashells & coral skeletons are composed of the aragonite. Whitish-colored lime sand beaches in the world are aragonitic. Occasionally, "whitings" are seen in shallow, warm ocean environments. Whitings (cloudy, milky seawater) turn out to be loaded with tiny hair-like needles of aragonite.

In the rock record, aragonitic or aragonite-rich sediments convert to calcite over time. Cenozoic-aged carbonate sedimentary rocks are often aragonitic. Mesozoic- and Paleozoic-aged carbonates are almost always calcitic. Many ancient fossils have had their aragonitic shells dissolved away. Ancient shells that were originally calcitic are often still well preserved.

The sample shown above is an irregularly shaped cavity lined with aragonite crystals. It is hosted in the siliciclastics-dominated Vinton Member of the Logan Formation (note the intraclastic nature of the surrounding host rock, especially below and to the right of the cavity). The Vinton is mostly siltstone, but sandstone and shale also occur. Some carbonate horizons and concretions are also present, which are the likely ultimate source for the calcium carbonate making up the crystals in this vug.

Host rock: Vinton Member, upper Logan Formation, Osagean Stage, upper Lower Mississippian

Locality: Mohawk Dam emergency spillway, northwest of the town of Nellie, western Coshocton County, eastern Ohio, USA (vicinity of 40º 21' 26.42" North latitude, 82º 04' 57.97" West longitude)

Photo gallery of aragonite: