Barytocalcite

The mineral crystallises in the monoclinic crystal system. Most sources[3][4][5] put it in the prismatic class 2/m, with space group P2₁/m, but Webmin[2] 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.[3] The cation layers are stacked in an ABCABC... stacking pattern, repeating every 3 layers. The Ca ion is coordinated to 7 oxygens.[5]

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[5] has a = 8.092 Å, b = 5.2344 Å, c = 6.544 Å, β = 106.05° and Z=2.

Barytocalcite often forms oriented growths on baryte, and calcite, witherite and baryte can be epitaxial on barytocalcite.[3][4] 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.

Barytocalcite is a biaxial (-) mineral with refractive indices n_α = 1.525, n_β = 1.684 and n_γ = 1.686. The maximum birefringence (the difference in refractive index between light travelling through the crystal with different polarizations) is δ = 0.161.[4] 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 2V changes. This is known as dispersion of the optic axes. For barytocalcite the effect is weak, with 2V larger for red light than for violet light (r>v).[3]
The optical directions X, Y and Z are the directions of travel of light with refractive indices n_α, n_β and n_γ respectively. In general they are not the same as the directions a, b and c of the crystallographic axes. For barytocalcite Z is parallel to the b axis, X is at an angle of 64° to the c axis and Y is at an angle of 26° to the c axis.[3]

Sources disagree about fluorescence. Webmin[2] describes the mineral as non-fluorescent, Dana[3] categorises it as weakly fluorescent and Mindat[4] states that it is fluorescent dull yellow under both longwave and shortwave ultraviolet light.

Barytocalcite has at least one perfect and one imperfect cleavage.[2][3] It is a brittle mineral, and breaks with an uneven to conchoidal fracture. It is quite soft, with a hardness 4, the same as that of fluorite. Calcite CaCO₃ and dolomite CaMg(CO₃)₂ are carbonates where calcium Ca and magnesium Mg replace the barium in the formula for barytocalcite, BaCa(CO₃)₂. These two minerals are even softer than barytocalcite, and much less dense. Barytocalcite has specific gravity 3.7. This is higher than that of calcite (2.7) or dolomite (2.85) because the barium atom is very heavy, with atomic mass 137, which is much more than magnesium at 24 and calcium at 40. Barytocalcite is soluble in dilute hydrochloric acid HCl[4] and it is not radioactive.[2]

The type locality is the Blagill Mine, Nent Valley, Alston Moor District, North Pennines, North and Western Region, Cumbria, England. This mine is no longer operative, and it has been designated a site of Site of Special Scientific Interest so collecting is no longer allowed. In the past lustrous, transparent, prismatic crystals of barytocalcite have been found there[6] in veins in limestone, associated with fluorite, calcite and baryte.[4]