Herapathite, or iodoquinine sulfate, is a chemical compound whose crystals are dichroic and thus can be used for polarizing light.
According to Edwin H. Land, it was discovered in 1852[1] by William Bird Herapath, a doctor in Bristol. One of his pupils found that adding iodine to the urine of a dog that had been fed quinine produced unusual green crystals. Herapath noticed while studying the crystals under a microscope that they appeared to polarize light.
Prof. Ferdinand Bernauer invented a process to grow single herapathite crystals large enough to be sandwiched between two sheets of glass to create a polarizing filter. These were sold under the Bernotar name by Carl Zeiss. This method has since been lost.
Herapathite's dichroic properties came to the attention of Sir David Brewster, and were later used by Land in 1929 to construct the first type of Polaroid sheet polarizer. He did this by embedding herapathite crystals in a polymer instead of growing a single large crystal.
For hypothetical crystal structure see A.M. Marks.
Crystal structure recently reported in Science Magazine.[2]