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It is not isomorphous with the orthorhombic barium and strontium sulphates, as might be expected from the chemical formulae. Distinctly developed crystals are somewhat rare, the mineral usually presenting the form of cleavage masses. The hardness is 3-1/2 and the specific gravity 2.9. The colour is white, sometimes greyish, bluish or reddish.
These three directions may be called the principal directions of absorption, relative to this band. 4. In the orthorhombic crystals, by a necessary consequence of crystalline symmetry, the principal directions of absorption of all the bands coincide with the three axes of symmetry. We may thus observe three principal absorption spectra.
ANHYDRITE, a mineral, differing chemically from the more commonly occurring gypsum in containing no water of crystallization, being anhydrous calcium sulphate, CaSO . It crystallizes in the orthorhombic system, and has three directions of perfect cleavage parallel to the three planes of symmetry.
British and Roman camps, coins and ornaments have been dug up and discussed, especially by the Hon. Mr. Stanley of Penrhos. Pen Caer Gybi is Roman. See Edw. ANGLESITE, a mineral consisting of lead sulphate, PbSO , crystallizing in the orthorhombic system, and isomorphous with barytes and celestite. It was first recognized as a mineral species by Dr.
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