It occurs in nature as the mineral strontianite. Associated minerals include strontianite, dawsonite and calcite. The principal minerals are celestite and strontianite. The principal strontium ores are celestine SrSO and strontianite SrCO. In strontianite Z is parallel to the a crystal axis. Synthetic barium chlorapatite, strontianite, and fluorapatite were used as standards for the elements they contain. Yet by 1883, the demand on strontianite had begun to shrink. In strontianite repeated twinning forms cyclic twins with three or four individuals, or polysynthetic twins. In strontianite Y is parallel to the b crystal axis. At the beginning of the 19th century Drensteinfurt obtained some wealth through Strontianite mining. Strontianite (SrCO) is an important raw material for the extraction of strontium. Strontianite is soluble in dilute hydrochloric acid HCl and it is not radioactive. Strontianite is very common at the Francon Quarry, in a great variety of habits. Formerly, strontianite was also mined and used as a whitener for the sugar industry. Most strontianite phosphoresces a strong, medium duration, yellowish white after exposure to all three wavelengths.