DIOPSIDE Chapter17. Inosilicates

DIOPSIDE

CaMgSi₂O₆
Monoclinic




	Figure 17-2. Crystal drawing of pyroxene, likely diopside, from Balls Hill in Franklin. Drawing is from Palache (1935) who provided crystallographic data.

Diopside is the dominant clinopyroxene at both Franklin and Sterling Hill; in addition, it is found in moderate abundance in the marble quarries. Because the relations among the clinopyroxenes were not well understood at the time of early investigations, a number of superfluous names were used. The names schefferite, white schefferite, and zinc-schefferite, when applied to white or light brown specimens, refer to material that is diopside. Schefferite was so-called because of its physical and compositional similarity to the original Swedish material (Palache, 1910, 1935). Zinc-schefferite was described by Wolff and Melczer (1900), who analyzed material previously examined by Hillebrand (1900). These names were adopted by Palache (1935). For dark brown or brownish-green material, see the remarks under augite.




	Figure 17-3. Crystal drawings of pyroxene, likely diopside or augite, from Sterling Hill. Those in upper left and upper right are of material called schefferite. Drawing in top center is of a twinned crystal. Drawings are from Palache (1935) who provided crystallographic data.

Description

Diopside from the Franklin and Sterling Hill orebodies is white to light brown; it may be colorless in the Franklin Marble. It has a density of 3.2-3.4 g/cm³, a vitreous luster, and a prominent parting. Optically, the white material of Jenkins and Bauer (1926) is biaxial, positive, 2V = 60^o, with a = 1.673, b = 1.680, and g = 1.700; dispersion is r > v. In ultraviolet, near-end-member diopside from the Franklin Marble has a blue fluorescence in shortwave and a light yellow fluorescence in longwave. Most samples from the orebody have no discernible fluorescence. It is easily distinguished from andradite on the basis of its common parting.

Composition

Diopside is a calcium magnesium silicate mineral of the pyroxene group and is the preferred host for Mg in the calcium-silicate assemblages. The name diopside is best applied only to light-colored specimens. Diopside from the quarries in the Franklin Marble is of near end-member composition, but the platy material from the orebodies contains much Mn, Zn, and Fe and grades into aegirine and augite. Representative analyses of diopside from the orebodies (colorless to light brown) are given in Table 10, and Johnson (1990) provided others from Sterling Hill. There is much variation in composition, and numerous solid solution series extend toward johannsenite, hedenbergite, petedunnite, aegirine, and augite.

Occurrence and paragenesis




	Figure 17-4. Crystal drawing of pyroxene, likely diopside, from Franklin. Drawing is from Palache (1935) who provided crystallographic data.

Diopside is common to many of the high-temperature assemblages of the calcium-silicate units at Franklin and Sterling Hill (Figures 12-19, 12-20, 12-24, 12-25, and 12-28). The more common associated minerals are franklinite, willemite, andradite, and calcite. Rhodonite, bustamite, hardystonite, apatite, and secondary clinohedrite are also associated, as are numerous other minerals. The assemblages for diopside have not been examined in detail and the petrographic relations are unstudied.

At Sterling Hill, diopside is associated with gahnite and calcite. Dark brown material with prominent parting, likely diopside or augite, has been found on the 1300 and 1400 levels at Sterling Hill, associated with calcite, franklinite, rhodonite, andradite, and willemite. Large 1.5 cm unanalyzed crystals are associated with hornblende. Reilly (1983) reported diopside-augite with exsolved rhodonite from Sterling Hill drill-hole #124, 19 feet from its beginning on the 340 level.

In the Franklin Marble, diopside is associated with tremolite, corundum (Figure 22-55), and numerous other silicates found in the quarries.

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