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DIALLAGE
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DIALLAGE , an important See also:mineral of the See also:pyroxene See also:group, distinguished by its thin foliated structure and bronzy lustre. The chemical See also:composition is the same as See also:diopside, Ca Mg (SiO3)2, but it sometimes contains the molecules (Mg, Fe") (Al, Fe"')2 SiO6 and Na Fe"' (SiO3)2 in addition, when it approaches to See also:augite in composition. Diallage is in fact an altered See also:form of these varieties of pyroxene; the particular See also:kind of alteration which they have undergone being known as "schillerization." This, as described by Prof. J. W. See also:Judd, consists in the development of a See also:fine lamellar structure or parting due to secondary twinning and the separation of secondary products along these and other planes. of chemical weakness (" See also:solution planes ") in the crystal. The secondary products-consist of mixtures of various hydrated oxides—opal, See also:gothite, See also:limonite, &c.—and appear as microscopic inclusions filling or partly filling cavities, which have definite outlines with respect to the enclosing crystal and are known as negative crystals. It is to the reflection and interference of See also:light from these See also:minute inclusions that the See also:peculiar bronzy sheen or " See also:schiller " of the mineral is due. The most pronounced lamination is that parallel to the orthopinacoid; another, less distinct, is parallel to the basal See also:plane, and a third parallel to the plane of symmetry; these planes of secondary parting are in addition to the See also:ordinary prismatic cleavage of allpyroxenes. Frequently the material is interlaminated with a rhombic pyroxene (See also:bronzite) or with an See also:amphibole (smaragdite or uralite), the latter being an alteration product of the diallage.
Diallage is usually greyish-See also:green or dark green, sometimes See also: It does not occur in distinct crystals with definite outlines, but only as lamellar masses in deep-seated igneous rocks, principally See also:gabbro, of which it is an essential constituent. It occurs also in some peridotites and serpentines, and rarely in volcanic rocks (See also:basalt) and crystalline See also:schists. Masses of considerable See also:size are found in the coarse-grained gabbros of the See also:Island of See also:Skye, Le Prese near Bornio in See also:Valtellina, See also:Lombardy, See also:Prato near See also:Florence, and many other localities. The name diallage, from BcaXXayi7, " difference," in allusion to the dissimilar cleavages and planes of fracture, as originally applied by R. J. Haiiy in 18or, included other minerals (the orthorhombic pyroxenes See also:hypersthene, bronzite and bastite, and the smaragdite variety of See also:hornblende) which exhibit the same peculiarities of schiller structure; it is now limited. to the See also:monoclinic pyroxenes with this structure. Like the minerals of similar See also:appearance just mentioned, it is sometimes cut and polished for ornamental purposes. (L. J. Additional information and CommentsThere are no comments yet for this article.
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