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Encyclopedia Britannica



DIOPTASE

This article appears in Volume V08, Page 289 of the Encyclopedia Britannica.

Encyclopedia Britannica - Main :: DIO-DRO
DIOPTASE , a rare
mineral
  species consisting of acid copper orthosilicate, H2CuSiO4, crystallizing in the parallel-faced herniahedral class of the rhombohedral
system
 . The degree of sym-
metry is the same as in the
mineral
  phenacite, there being orly ar
axis
  of triad symmetry and a centre of symmetry. The crystals have the form of a hexagonal prism m terminated by a rhombohedron r, the alternate edges between these being sometimes re-placed by the faces of a rhombohedron s. The faces are striated parallel to the edges between r, s and in. There are perfect cleavages parallel to the faces of a rhombohedron which truncate the polar edges of r: from the 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. Hauy in 1797, from Sioirrciav, " to see into." The crystals vary from transparent to translucent with a vitreous lustre, and are
bright
 
emerald
 -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 mineral is decomposed by hydrochloric acid with separation of gelatinous
silica
 . At a red heat it blackens and gives off water. The fine crystals from
Mount
  Altyn-Tube on the western slopes of the Altai Mountains in the Kirghiz
Steppes
 , Asiatic Russia, line cavities in a compact limestone; they were first sent to Europe in 1785 by Achir Mahmed, a Bucharian merchant, after whom the mineral has been named archirite. More recently, in 189o, good crystals of similar habit, but rather darker in colour, have been found with
quartz
  and malachite near Komba in the French Congo. As drusy crystalline crusts it has! been found at Copiapo in Chile and in Arizona.
Dioptase 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. (L. J. S.)


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