Abstract
The new species, parahilgardite, found in the Choctaw Salt Dome, Louisiana, is so named because of its close similarity with hilgardite. The specimens are invariably made up of one right-hand and one left-hand individual attached to the negative end of the a-axis of a hilgardite crystal. Parahilgardite is triclinic-pedial—the second mineral to be described in this class. a:b:c = 0.5045: 1:0.2783; α = 90°00′,β = 90°00′,γ=91°12′. Its physical properties are identical with those of hilgardite: clear and colorless; cleavage {010} and {100}, H=5, G=2.71. Biaxial positive, 2V=35°, r>v, nX=1.630, nY=1.636, nZ=1.664. The dimensions of the unit cell: a0=11.24Å, b0=22.28Å, c0=6.20Å. The chemical formula expressing the content of the unit cell can be written as: 2[Ca8(B6O11)3·Cl4·4H2O].
