Scratch tests, polarised light and X-rays: how mineralogists tell crystals apart
A mineralogist's first tools are almost childlike: scratch one stone against another, check its colour and shine, see how it breaks. The advanced ones fire X-rays through powdered crystal. Between them lies a four-century story that began when Nicholas Steno noticed in 1669 that quartz crystals always meet at the same angles.
Mineralogy studies the chemistry, crystal structure and physical properties of minerals, along with how they form and where they occur. Early writing came from Babylonia, Greece and Rome, China, India and the Islamic world; Pliny the Elder described many minerals, and the Persian scholar Al-Biruni wrote a Book of Precious Stones. In the sixteenth century the German Georgius Agricola published treatises on rocks and metals.
Modern mineralogy grew out of crystallography. Steno's constant angles were confirmed by Rome de l'Isle in 1783, and Rene Just Hauy, called the father of modern crystallography, showed crystals are periodic. In 1814 Jons Jacob Berzelius began classifying minerals by chemistry, and James Dana's System of Mineralogy, first published in 1837, eventually introduced the chemical scheme still standard today. William Nicol's polarising prism of 1827 to 1828 let Henry Clifton Sorby identify minerals in thin rock slices by how they bend light. Max von Laue demonstrated X-ray diffraction in 1912, and the Braggs, father and son, turned it into a way to read crystal structures.
Hands-on identification starts with physical properties. The Mohs scale ranks hardness from talc at 1 to diamond at 10 by what scratches what, though it is nonlinear, and calcite and kyanite are harder in some directions than others. Minerals also differ in how they break: cleanly along cleavage planes, or in shell-like curves, splinters or jagged edges. Under a polarising microscope, crystals outside the cubic system split light into two rays travelling at different speeds, and a bright rim called the Becke line helps estimate refractive index to within about 0.003.
Symmetry imposes strict limits: there are 32 possible crystal classes and 230 space groups. Powder X-ray diffraction can separate quartz from look-alike polymorphs such as tridymite and cristobalite, and since 1960 instruments have largely replaced dissolving samples in acid. About 100 new minerals are found every year, and the field now overlaps strongly with materials science.
Source: Mineralogy
