Matrix of the Diamond

Page of 85

Text size:



	22 THE MATRIX OF THE DIAMOND tion angle of 39°. In converging polarised light an axis appears on cleavage fragments which are parallel to Three cleavages or partings, each well developed, also occur in chrome-diopside, so that it readily cleaves into almost cubical rhombs. It has an excellent cleavage parallel with , another, less perfect, parallel with , and another parallel with OP. This last is the salite parting, and not a true cleavage. It is probably, as Tschermak,¹ Vom Rath,² and others have shown, due to the interposition of thin twinning lamella; parallel to the base, and the mineral might be called a salite. Yon Koksharow³ speaks of the ' zusammensetzung flitchen' in chrome-diopside from the Urals as often confounded with true cleavage faces. The ordinary augitic prismatic cleavage is not apparent microscopically on these basal parting sections, but the pinacoidal cleavages are seen as fine lines crossing each other at right angles and parallel to the sides of the section. A cleavage fragment gave the angles and The face of the best cleavage, parallel to has a pearly lustre, and is nearly or quite free from stria;. The orthopinacoidal cleavage plates are striated vertically, and show in convergent light a single axis identical with that seen in cleavage plates of diallage. The low extinction angle, 39°, is typical of a pure lime-magnesia diopside, free from iron, for both iron and magnesia, as Tschermak⁴ "Wiik,⁵ Herwig⁶ and Doelter⁷ have shown, tend to increase the extinction angle. The so-called omphacite, occurring in eclogites, peridotites, olivine bombs, and serpentines, seems to be but another name for

Page of 85