DIOPSIDE, an important member of the pyroxene group of rock-forming minerals. It is colourless, white, pale green to dark green or nearly black in colour, the depth of the colour depend ing on the amount of iron present, and is a calcium-magnesium metasilicate, crystallizing in the monoclinic system. Usually some iron is present, replacing magnesium, and when this predominates there is a passage to hedenbergite, a closely allied variety of monoclinic pyroxene ; these are distin guished from augite by containing little or no aluminium. The specific gravity and optical constants also vary with the chemical composition; the specific gravity of diopside is 3.2, increasing to 3.6 in hedenbergite, and the angle of optical extinction in the plane of symmetry varies between 38° and 47° in the two extremes of the series. Crystals are usually prismatic in habit, with a rectangular cross-section, the angle between the prism faces m, parallel to which there are perfect cleavages, being 92° 5o'. The name magnesium diopside is given to the solid solutions of diopside and enstatite commonly found in quartz-dolerites. They are distinguished from diopside by smaller extinction angles and smaller angles between the optic axes.
Belonging to the same series with diopside and hedenbergite is a manganese pyroxene, known as schefferite, which has the com position (Ca,Mg) (Fe,Mn) (SiO3)2.
Diopside is easily prepared synthetically from a melt of its com ponent oxides. The artificial crystals melt sharply at 1,391 ° C., and this temperature is used as a fixed point on the thermometer scale. (C. E. T.) DIOPTASE, a rare mineral species consisting of acid copper orthosilicate, H,CuSiO,, that has occasionally been used as a gem-stone, especially in Russia and Persia. It has a fine colour, but a low degree of hardness and the transparency is imperfect, and it crystallizes in the parallel-faced hemihedral class of the rhombohedral system, in which there is only an axis of triad symmetry and a centre of symmetry. The crystals have the form of a hexagonal prism terminated by a rhombohedron. There are perfect cleavages truncating the polar edges of the primary rhombohedron. From these cleavage cracks internal reflections are often to be seen in the crystal, and it was on account of this that the mineral was named dioptase, by R. J. Haiiy in 1797, from h&oRrTEV€cV, "to see into." The crystals vary from trans parent to translucent with a vitreous lustre, and are bright em erald-green in colour; they thus have a certain resemblance to emerald, hence the early name emerald-copper (German, Kupfer smaragd) . Hardness 5 ; sp.gr. 3.3. The fine crystals from Mount Altyn-Tiibe on the western slopes of the Altai mountains, Asiatic Russia, line cavities in a compact limestone; they were first brought to Europe in 1785. Excellent crystals have also come from the copper mines in the French and Belgian Congos and from South-West Africa.