The formula for calcium carbonate is simple enough – CaCO3 – but the mineral itself appears in a bewildering variety. Hiding behind its hexagonal ditrigonal scalenohedral class (whew!) are over 300 forms and more than 1000 combinations. Little wonder that some of our collections are tough to identify without other tests (See "Field Tests" in a recent newsletter.)

We all know that calcite stands for number 3 on Mohs scale between gypsum and fluorite, that it’s easily scratched with the knife, that it fizzes readily in dilute acid. It’s also kissin’ cousin to aragonite which forms the shells of many clams and brachiopods. Calcite has more forms and habits than any other mineral and add to that a wide spectrum of colors from clear to black. But the streak is always white or a pale color.

A variety called Iceland spar is strongly doubly refractive which means that a clear cleavage piece, when laid on a mark on paper will display two separate images. This property was put to good use in pre-Polaroid days in polarizing microscopes – Nicols prisms. A suitably pure piece was sliced in a special way, one piece mounted in the tube and the other under the rotating stage. Thin sections of rock when placed on the stage displayed useful colors and shapes.

The different varieties of calcite may be grouped like this: ordinary calcite; limestones, marbles, chalk and marl, and spring, stream and cave deposits. These groups deserve fuller discussion but for now the notes will be limited to one or two.

This mineral is the basis of limestone, itself of many kinds. Worldwide, most limestone is marine in origin – laid down in warm shallow waters. While fossils play quite a role in many deposits, drawing calcium carbonate out of seawater, much limestone just precipitates out. This is because cold seawater – any water – holds vast amounts of this compound in solution. But if this water warms up in shallow basins like the Persian Gulf (or our teakettles!), the mineral comes out as very fine grains and crystals; this may be the start of chalk. Even fish disturbing such saturated areas may leave a milky trail of precipitating calcite.

A famous example of this fine "lithographic" limestone (yes it once was used to make printing blocks) is in Solenhofen, Germany. It’s so fine grained that the imprints of even leaves and insects are preserved in exquisite detail. We’ve all heard about Archaeopteryx, the earliest (Jurassic) bird.

A famous limestone comes from the Ordovician (age) near Winnipeg. This Tyndall rock is used throughout our parliament buildings and hundreds elsewhere. The mottling is considered to be dolomitized feeding trails of animals.

Under suitable conditions calcite transforms to marble, either sugary or very fine in texture. Its use ranges from underfoot as chips in terrazzo to the glories of Michelangelo’s "David" in Florence. And why not? Nearby are the 200 quarries of famous Carrara marble. After 200 years over a million tonnes a year are still produced, mostly for buildings.

During WW2, to curry favor with Hitler, Mussolini presented him a birthday gift for his "Eagle’s Nest" at Berchtesgarden – a huge slab of red marble for the lintel of the fireplace. Ugly, its only virtue was its richness in ammonite (Jurassic?) fossils. Hitler hated it!