Aragonite
Aragonite
is a common carbonate mineral. It is unfortunately often thought of as
the poor cousin to calcite. But aragonite is an interesting and
attractive mineral in its own right. It forms interesting habits and
can have a soft pretty color. Its modes of formation and relationship
to calcite are both curious and intriguing.
Aragonite is a polymorph of calcite, which means that it has the same
chemistry as calcite but it has a different structure, and more
importantly, different symmetry and crystal shapes. Aragonite's more
compact structure is composed of triangular carbonate ion groups (CO3),
with a carbon at the center of the triangle and the three oxygens at
each corner. Unlike in calcite, the carbonate ions do not lie in a
single plane pointing in the same direction. Instead they lie in two
planes that point in opposite directions; destroying the trigonal
symmetry that is characteristic of calcite's structure. To illustrate
this, imagine the symmetry of an equilateral triangle; a three fold
rotation with three mirror planes that cross in the center. Now join
two of these triangles together at their bases and you have a
diamond-shaped figure with the symmetry of a two fold rotation with
one mirror plane in the middle. This is what the effect of the two
carbonate planes with opposite orientations has on the symmetry of
this structure. Aragonite has an orthorhombic symmetry (2/m 2/m 2/m)
instead of calcite's "higher" trigonal (bar 3 2/m) symmetry.
A very rare mineral called vaterite is also a polymorph of aragonite
and calcite; making them all trimorphs. Vaterite has an hexagonal
symmetry (6/m 2/m 2/m).
Carbonate minerals with this same structure as aragonite belong to
the Aragonite Group of minerals. The structure is responsible for the
similar properties of this group. Dissimilar properties are the
responsibility or result of the differing metal cations in the various
minerals of the group.
Aragonite is technically unstable at normal surface temperatures and
pressures. It is stable at higher pressures, but not at higher
temperatures such that in order to keep aragonite stable with
increasing temperature, the pressure must also increase. If aragonite
is heated to 400 degrees C, it will spontaneously convert to calcite
if the pressure is not also increased. Since calcite is the more
stable mineral, why does aragonite even form? Well under certain
conditions of formation, the crystallization of calcite is somehow
discouraged and aragonite will form instead. The magnesium and salt
content of the crystallizing fluid, the turbidity of the fluid and the
time of crystallization are decidedly important factors, but there are
perhaps others. Such areas as sabkhas and oolitic shoals tend to allow
significant amounts of aragonite to form. Also metamorphism that
includes high pressures and low temperatures (relatively) can form
aragonite. After burial, given enough time, the aragonite will almost
certainly alter to calcite. Sedimentologists are very interested in
aragonite and calcite stability fields because the conversion of
aragonite to calcite after deposition has a distinct effect on the
character of the sedimentary rocks.
Aragonite's most famous crystal habit is the twinned pseudo-hexagonal
prismatic crystals that it produces. Twinning is the result of an
error during the growth of the crystal. It occurs when the atomic
layer stacking, in a sequence such as ABCABCABCABCABCABC etc, makes a
mistake and a C layer instead of a B layer is place next to an A
layer. The result is an ABCABCABCACBACBACBA stacking sequence (can you
pick out the mistake?). Where the mistake occurs, a mirror plane is
produced that was not there before (the left side is the mirror image
of the right side). This has the effect of increasing the apparent
symmetry of the crystal. The error in aragonite's structure causes a
bend in the crystal of exactly 120 degrees. If three bends or twins
occur, then a 360 degree crystal, called a cyclic twin or trilling,
can form. In this case, aragonite can appear hexagonal (six sided).
These crystals can be thought of as a "triple siamese twin"
where one crystal takes up one third (or 120 degrees) of a hexagon.
Individual cyclic twins from Aragon, Spain have been popular but are
being surpassed by the amazing clusters of aragonite twins that are
now available from Morocco in large numbers. Aragon, Spain is where
aragonite was first discovered and from where aragonite gets its name.
Cyclic twins often show notches that separate the twin individuals.
Aragonite also has another popular habit called flos ferri or "flowers
of iron". This is a branching, clumpy habit that can make
delicate tree, coral or worm-like formations that are most unique. A
steep pyramidal habit forms clusters of sharp spiked crystals
sometimes referred to as a "church steeple" habit. Aragonite
is a constituent of many sea creatures' shell structures; a curious
development since calcite is the more stable form of calcium
carbonate. Most bivalve animals and corals secrete aragonite for their
shells and pearls are composed of mostly aragonite. The pearlization
and iridescent colors in sea shells such as abalone are made possible
by several minute layers of aragonite. Other environments of formation
include hot springs deposits, cavities in volcanic rocks, caves and
mines.
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