Art. LII.—On a New Form of Iron Pyrites. (With Illustrations.)
[Read before the Wellington Philosophical Society, September 17, 1870.]
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Iron Pyrites has long been known as a dimorphous mineral, occurring crystallized in the tesseral and rhombic systems; the latter variety, called marcasite, occurs as a right rhombic prism—∞ P 106° 5′, brachydome ⅓ P ∞ 136° 54′, brachydome P ∞ 80.20 and a macrodome P ∞ 64° 54′ combined. The other form is common in cubes ∞ O ∞, octahedron O, and several semitesseral forms, the pentagonal dodecahedron ∞02/2; the hemihedral form of the tetrakishexa-hedron, rarely tetrahedral; a common combination is that of the pentagonal dodecahedron with the octahedron, the faces of the latter replacing the trigonal angles of the dodecahedron. Macles are common, but are not material to the present purpose.
Plate XXVI., fig. 7., is a new form from the Chatham Islands. The lustre, specific gravity, and hardness, are the same as the common varieties; the system is oblique, nearly isomorphous with felspar, but having the clinodiagonal longer; the faces, which are smooth and brilliant, are ∞ P prism (P), OP clinopinacoid (M), P hemipyramid (a), nP∞ hemidome (d), (nP∞) clinodome (n).
The thick lines show where the crystal is cut off.