Fluorite

Fluorite is a mineral that mostly occurs in hydrothermal veins. The chemical composition is very simple: CaF₂. It is not very common mineral because fluorine (F) makes up only 0.06% of the mass of the crust and there are other minerals also that want fluorine into their crystal structure. Most notable of them are tourmaline, amphiboles, micas, and apatite, but all of them can replace fluorine with something else. This is something that fluorite can not do — it needs fluorine to build up the crystal structure. The composition is usually almost pure CaF₂, but some minor substitutions of Ca with Sr, Y, or Ce are possible.

Fluoritic (variety antozonite) sand grains picked from a sand sample collected near Wölsendorf, Bavaria, Germany. The width of the view is 15 mm.

However, it is fairly common in certain environments. Fluorine tends to concentrate in late magmatic fluids. Therefore, it usually forms as a component of pegmatites, greisens, and hydrothermal veins with lots of other minerals with unusual composition. Fluorite may sometimes occur in granite and syenite or even in sandstone as a cementing agent (although rarely). Fluorite also may occur in clastic sediments but it isn’t very common detrital mineral.

It was chosen by a German mineralogist Friedrich Mohs in 1812 to be one of the minerals in his hardness scale of minerals (it is number four, harder than calcite but softer than apatite).

This mineral does not have its own color. Almost any color tone is possible, but it is usually violet, blue, green, yellow, or colorless.

The color may be variable but some tones are very characteristic (as shown by the sand grains or the crystal druse below) and help to identify the mineral. Crystals are common, they form beautiful cubes. Anhedral (without well-developed crystal faces) fluorite may resemble quartz but fluorite is significantly softer mineral. Fluorite is the principal ore of fluorine.

Even almost black version of this mineral exists, it is named antozonite. The crystal structure of antozonite is partly damaged due to alpha radiation which comes from the mineral itself (fluorite can be radioactive if some of the Ca is substituted by radioactive elements). Antozonite emits strong odor when crushed. It is also called stink-fluss (stink-flow).