Barytocalcite is an anhydrous barium calcium carbonate mineral with the chemical formula BaCa(CO3)2. It is trimorphous with alstonite and paralstonite, that is to say the three minerals have the same formula but different structures. Baryte and quartz pseudomorphs after barytocalcite have been observed. Barytocalcite crystallizes in the monoclinic crystal system, typically as massive to druzy accumulations of transparent white to yellow to grey aggregates of slender prismatic crystals. It has a Mohs hardness of 4 and a specific gravity of 3.64 to 3.71. It was first described in 1824 for an occurrence in the Blagill Mine in North Pennines, Cumbria (Cumberland), England, and named for its composition. Structure The mineral crystallises in the monoclinic crystal system. Most sources put it in the prismatic class 2/m, with space group P21/m, but Webmin puts it in the sphenoidal class 2 with space group P21. The structure is similar to that of dolomite, but with two crystallographically different CO3 groups. The cation layers are stacked in an ABCABC... stacking pattern, repeating every 3 layers. The Ca ion is coordinated to 7 oxygens. Unit cell All sources give unit cell parameters whose values rounded down are a = 8.1 Å, b = 5.2 Å, c = 6.5 Å and β = 106°, with two formula units per unit cell (Z = 2). The primary source has a = 8.092 Å, b = 5.2344 Å, c = 6.544 Å, β = 106.05° and Z=2. Appearance Barytocalcite often forms oriented growths on baryte, and calcite, witherite and baryte can be epitaxial on barytocalcite. Crystals are normally short to long prismatic and striated. They are transparent to translucent, colourless, white, greyish, greenish or yellowish with a white streak and a vitreous to resinous lustre. Optical properties Barytocalcite is a biaxial (-) mineral with refractive indices nα = 1.525, nβ = 1.684 and nγ = 1.686. The maximum birefringence (the difference in refractive index between light travelling through the crystal with different polarizations) is δ = 0.161. The optic angle 2V is the angle between the two optic axes in a biaxial crystal. The measured values of 2V for this mineral is 15°. It is also possible to calculate a theoretical value of 2V from the measured values of the refractive indices. The calculated value for barytocalcite is 10°. If the colour of the incident light is changed, then the refractive indices are modified, and the value of