Aragonite
Just like Calcite, Aragonite is composed of calcium carbonate, but it belongs to a
different crystal system. Aragonite is orthorhombic and Calcite is trigonal. We often get
Aragonite crystals that are trillings, three individuals intergrown in a regular fashion.
Superficially they look like six-sided barrels, but if you look carefully you note, the
faces don't line up properly. The trillingsare not quite 120 degrees from each other,
creating a nick in one of the faces. These crystals occur either individually or as
intergrown aggregates in clay, and specimens from Spain and Morocco are cherished
collectors items.
Aragonite is named for the type locality, Molina de Aragon in Guadalajara, Spain,
where the first described specimens came from. This is still a productive locality, even if
the first material was collected in the 18th Century. No, 'all the good localities' don't go
away over night!
Aragonite properties
It is interesting to evaluate the properties of Aragonite against those of Calcite. They
have the same composition, but differ in crystallography, and have widely different
properties. Aragonite is harder and has a higher specific gravity, is denser, than Calcite.
That is conceptually not too difficult to understand - the atoms are packed a little closer
in Aragonite. But Aragonite is also far more soluble in water than Calcite, neither
dissolves easily, but both dissolve a little. 'More soluble' means that Aragonite dissolves
faster, needs less time to form a saturated solution, but what is really weird, it also
forms a solution with far more dissolved calcium carbonate per unit volume! If you
leave Aragonite in a vat of water, the water will at equilibrium contain approximately
15% more dissolved calcium carbonate per liter, than if you leave Calcite in the vat.
Aragonite occurs in several different geological - and biological! - environments. It is
fairly common as minute acicular crystals in basalts, associating zeolites, and in various
hot spring deposits. The classic hot springs in Karlovy Vary (Karlsbad) in Bšhmen
(Czech Republic) produce banded 'Sprudelstein' and aggregates of Aragonite globules
called 'Erbsenstein' i.e. 'pea stone.'
Biological Aragonite
Aragonite is biologically interesting, forming skeletal structures of many organisms.
The skeletons of humans and other vertebrates are composed of phosphates (Carbonate-
hydroxyl-apatite if you must know), but many shells and other exoskeletons of
invertebrates are composed of Aragonite. Most mollusk shells and modern corals are
composed of Aragonite. Nacre, 'mother of pearl', is one of the more spectacular forms
of biologically deposited Aragonite.
Scanning electron micrograph of the sheet nacre in a pearl oyster, Pinctada
margaritifera. Each tablet is approximately 5 µm across. The exterior of the
shell is towards the top of the phot.
Nacreous structures are aragonitic laminar structures consisting of polygonal to rounded
tablets arranged in broad, regularly formed, parallel sheets. The tablets are typically 5-
10 micrometer across. Nacres are called "columnar nacre" or "sheet nacre" depending
on the stacking mode of the tablets. Columnar nacre is composed of tablets of rather
uniform size, stacked in a more or less regular net, with coinciding centers, containing
calcified organic material that determines the nucleation site of the overlying tablet.
Columnar nacre is found in some gastropods and cephalopods. In sheet nacre (see the
figure), found in some bivalves and Monoplacophora, deposition takes place over most
of the inner surface of the shell, and the tablets are stacked in a Òbrick wallÓ pattern,
spanning the interface between underlying tablets.
Aragonite associate minerals
My records indicate that Aragonite from a range of deposits occurs on specimens that
also carry one or more of the following minerals: Amicite, Andradite, Andradite,
Ankerite, Antigorite, Aurichalcite, Azurite, Barbertonite, Brushite, Calcioferrite,
Calcite, Copper, Dolomite, Fluorite, Galena, Groutite, Gypsum, Huntite, Magnetite,
Malachite, Merlinoite, Mimetite, Montmorillonite, Novacekite, Opal, Phillipsite, Pyrite,
Quartz, Rapidcreekite, Rhodochrosite, Saleeite, Sphalerite, Stilbite, Sulfur, Tenorite,
Thomsonite, Vanadinite var. Endlichite, and Wšhlerite
Specimen Handling
Aragonite is for all practical purposes stable in a normal household environment. It is
not harmed by light, changes in temperature in the normal comfort range, or known to
decompose. Aragonite specimens can be brittle and should be handled with care like any
other mineral specimen - be particularly careful with thin, pointed and acicular crystals.
Aragonite is slightly soluble in water, but a gentle rinse will normally not harm a
specimen.
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