the plane of symmetry varies between 38 deg. and 47 deg. in the two
extremes of the series. Crystals are usually prismatic in habit with a
rectangular cross-section as shown in the figure: the angle between the
prism faces m, parallel to which there are perfect cleavages, is 92 deg.
50'.

[Illustration]

Several varieties, depending on differences in structure and chemical
composition, have been distinguished, viz. coccolite (from [Greek:
kokkos], a grain), a granular variety; salite or sahlite, from Sala in
Sweden; malacolite; diallage; violane, a lamellar variety of a dark
violet-blue colour; chrome-diopside, a bright green variety containing a
small amount of chromium; and many others. Belonging to the same series
with diopside and hedenbergite is a manganese pyroxene, known as
schefierite, which has the composition (Ca, Mg) (Fe, Mn) (Si03)2.

Diopside is the characteristic pyroxene of metamorphic rocks, occurring
especially in crystalline limestones, and often in association with
garnet and epidote. It is also an essential constituent of some
pyroxene-granites, diorites and a few other igneous rocks, but the
characteristic pyroxene of this class of rocks is augite. Fine
transparent crystals of a pale green colour occur, with crystals of
yellowish-red garnet (hessonite) and chlorite, in veins traversing
serpentine in the Ala valley near Turin in Piedmont: a crystal of this
variety ("alalite") is represented in the accompanying figure. These, as
well as the long, transparent, bottle-green crystals from the Zillerthal
in the Tyrol, have occasionally been cut as gem-stones. Good crystals
have been found also at Achmatovsk near Zlatoust in the Urals,
Traversella near Ivrea in Piedmont ("traversellite"), Nordmark in
Sweden, Monroe in New York, Burgess in Lanark county, Ontario, and
several other places: at Nordmark the large, rectangular black crystals
occur with magnetite in the iron mines. (L. J. S.)

DIOPTASE, a rare mineral species consisting of acid copper
orthosilicate, H2CuSiO4, crystallizing in the parallel-faced hemihedral
class of the rhombohedral system. The degree of symmetry is the same as
in the mineral phenacite, there being only an 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 replaced by the faces of a rhombohedron s. The faces are
striated parallel to the e