Then they found two new radioactive elements, polonium and radium, after boiling down pitchblende. The uranium occurs as pitchblende in a hydrothermal vein deposit in Precambrian granite. The initial uranium mineralisation consists of quartz, carbonate, fluorite and pitchblende. Veins of this type contain dolomite, calcite, pitchblende, minor sulphides and selenides (especially clausthalite). Uranium was identified in 1789 by the German chemist Martin Heinrich Klaproth in pitchblende ore. The oldest type of mineralisation consists of quartz, calcite, fluorite, pitchblende and minor hematite. One ton of pitchblende typically yields about one seventh of a gram of radium. Primary mineralization consists of pitchblende and coffinite, with weathering producing secondary mineralization. There are two ore horizons containing disseminated pitchblende and coffinite. The veins contain pitchblende, autunite and uranophane, with accessory marcasite and barite. Below the water table, hypogene ores include uraninite (pitchblende), Co-Ni sulfides and selenides. Curie's systematic studies included two uranium minerals, pitchblende and torbernite (also known as chalcolite). These minerals are of little commercial value as most uranium is extracted from pitchblende. The chief ore mineral is pitchblende, which occurs with adularia and ankerite. Working with the mineral pitchblende, the pair discovered a new radioactive element in 1898.