alstonite, a barium and calcium carbonate mineral, CaBa(CO3)2, with minor amounts of strontium. It is colourless or light gray or pink in appearance and is also transparent or translucent. Its crystal structure is orthorhombic and is identical to that of aragonite, with barium and calcium in nine-fold coordination but ordered within layers; the layers can in turn be arranged in various stacking sequences, resulting in a complex structure. Alstonite is found in association with calcite, barite, and witherite as low-temperature hydrothermal deposits. For detailed physical properties, see carbonate mineral (table).