Fluorite (also called fluorspar) is a halide mineral composed of calcium fluoride, CaF2. It is an isometric mineral with a cubic habit, though octahedral and more complex isometric forms are not uncommon. Crystal twinning is common and adds complexity to the observed crystal habits.
The word fluorite is derived from the Latin root fluo, meaning “to flow” because the mineral is used as a flux in iron smelting to decrease the viscosityof slags at a given temperature. This increase in fluidity is the result of the ionic nature of the mineral. The melting point of pure calcium fluoride is 1676 K.
In 1852 fluorite gave its name to the phenomenon of fluorescence, which is prominent in fluorites from certain locations, due to certain impurities in the crystal. Fluorite also gave the name to its constitutive element fluorine.
Fluorite has a very low dispersion, so lenses made from it exhibit less chromatic aberration than those made of ordinary glass. However, naturally-occurring fluorite crystals without optical defects were only large enough to produce microscope elements.
With the advent of synthetically-grown fluorite (calcium fluoride crystal), it could be used instead of glass in some high-performance telescopes and camera lens elements. Its use for prisms and lenses was studied and promoted by Victor Schumann near the end of the 19th century.
In telescopes, fluorite elements allow crisp images of astronomical objects even at high magnifications. Canon Inc. produces synthetic fluorite crystals that are used in their more expensivetelephoto lenses.
Exposure tools for the semiconductor industry make use of fluorite optical elements for ultraviolet light at wavelengths of about 157 nanometers. Fluorite has a uniquely high transparency at this wavelength. Fluorite objective lenses are manufactured by the larger microscope firms (Nikon, Olympus, Carl Zeiss and Leica). Their transparence to ultraviolet light enables them to be used forfluorescence microscopy. The fluorite also serves to correct optical aberrations in these lenses. Nikon has previously manufactured at least one all-fluorite element camera lens (105 mm f/4.5 UV) for the production of ultraviolet images.
Hydrofluoric acid (HF) is a solution of hydrogen fluoride in water. It is a valued source of fluorine and is a precursor to numerous pharmaceuticals such as fluoxetine (Prozac) and diverse materials such as PTFE (Teflon).
Hydrofluoric acid is a highly corrosive acid, capable of dissolving many materials, especially oxides. Its ability to dissolve glass has been known since the 17th century, even before hydrofluoric acid had been prepared in large quantities by Carl Wilhelm Scheele in 1771. Because of its high reactivity toward glass and moderate reactivity toward many metals, hydrofluoric acid is usually stored in plastic containers (although PTFE is slightly permeable to it).
Hydrogen fluoride gas is an acute poison that may immediately and permanently damage lungs and the corneas of the eyes. Aqueous hydrofluoric acid is a contact-poison with the potential for deep, initially painless burns and ensuing tissue death. By interfering with body calcium metabolism, the concentrated acid may also cause systemic toxicity and eventual cardiac arrest and fatality, after contact with as little as 160 cm2 (25 square inches) of skin.