English: Aragonite ("star aragonite") from Morocco.

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 5600 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).

Unlike calcite, aragonite forms crystals in the orthorhombic class. Many aragonite crystals are acicular (needle-like) or pseudohexagonal. The latter is the result of six orthorhombic prisms growing parallel to each other. The sample seen here is a radiating cluster of pseudohexagonal, cyclic-twinned aragonite masses.

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.

This specimen is supposedly derived from a ferruginous alluvial clay deposit (Quaternary?).

Locality: Tazouta Mine, southeast of Sefrou & southeast of Fez, Middle Atlas Mountains, northern Morocco

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Photo gallery of aragonite:

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