File:Aragonite (Spain) 1.jpg

English: Aragonite from Spain. (3.4 centimeters across)

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 6000 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. The above specimen is a side view of a cyclic-twinned, pseudohexagonal aragonite mass.

Aragonite is slightly harder than calcite, at H=3.5 to 4, occurs in many colors, 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.

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Photo gallery of aragonite:

www.mindat.org/gallery.php?min=307