ARAGONITE
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 (see samples shown below), the result of 6 orthorhombic prisms growing
parallel to each other. The sample below looks like a 6-sided
crystal (hexagonal), but it's not.
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.
Aragonite (3.4 cm across) showing pseudohexagonal crystal form. From
Larimer County, Colorado, USA.
Aragonite (2.1 cm across) - pseudohexagonal, cyclic twinned individual from
Spain.
Aragonite (3.4 cm across) - side view of cyclic twinned individual shown above,
from Spain.
"Star Aragonite" (4.6 cm across) -
cluster of radiating pseudohexagonal, cyclic twinned aragonite masses.
From Tazouta Mine, southeast of Sefrou, Middle Atlas Mountains, northern
Morocco.
Aragonite ("flos-ferri") from Eisenerz, Styria, Austria.
(Wayne State University collection, Detroit, Michigan,
USA)