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Mg5(SiO4)2(F,OH)2
Monoclinic, P21/b, a = 4.7190, b =
10.2616, c = 7.8154 Å,
a = 109.147o, Z = 2.
Chondrodite was first described locally by the name maclureite (Seybert, 1822). The name brucite was applied early to such material (Gibbs, 1819); Frondel (1972) described the history of the matter, but erroneously attributed the maclurite of Nuttall (1822) to chondrodite.
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| Figure 15-14. Chondrodite crystals in white calcite of the Franklin Marble, from Franklin. The visible surface is polished. Specimen is 7 cm in maximum dimension. Smithsonian Institution, #132777. Photo by the author. | ||
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Local chondrodite occurs as both euhedral crystals and as subhedral grains and masses up to 10 cm in size. The euhedral crystals, up to 1 cm in size, may have been known to Palache in 1935, but have not been measured or figured. The bulk of local chondrodite is brown to orange-brown to yellow. The luster is vitreous; cleavage is poor; and the density is 3.260 g/cm3. Optically, it is biaxial, negative, with large 2V, and a = 1.594, b = 1.607, and g = 1.619; dispersion and pleochroism are weak or absent. The fluorescence in ultraviolet, for chemically studied samples, is somewhat variable: most specimens fluoresce orange, with slightly more intensity in shortwave than longwave. This variation may be due to the presence of Fe substituting for Mg. Chondrodite is best differentiated from norbergite and other humites using X-ray diffraction methods.
Chondrodite is a magnesium silicate fluorine hydroxide mineral of the humite group. Unlike norbergite, there is some limited solid-solution of octahedral cations for Mg. In the marble, Fe is the major substituent, whereas in the Sterling Hill orebody Mn and Zn are common substituents. Local material has been much studied. The best of these studies are those of Sahama (1953) and Jones (1969), who addressed the OH/F ratios in local material. Hinthorne and Ribbe (1974) used Franklin material to establish the boron-for-silicon substitution mechanism in chondrodite. Dunn (1985a) gave the composition of manganoan and zincian samples occurring in the Sterling Hill orebody and demonstrated the extent of local solid-solution to alleghanyite. Analyses of chondrodite from both the Franklin Marble and the Sterling Hill orebody are given in Table 1.
Chondrodite is common as irregular segregations, isolated crystals (Figure 15-14), and masses and is likely a very common mineral throughout the Franklin Marble. It is associated with spinel, pyrrhotite, graphite, and other minerals and was found on the Buckwheat Dump by Cook (1973). Larsen et al. (1928) noted that chondrodite formed the cores of some norbergite crystals. The writer’s studies do not support the statements by Larsen et al. (1928) and Cook (1973) that most of the known local specimens labeled chondrodite are norbergite. The distinctly brown to orange-brown material is mostly chondrodite, an observation also supported by Kearns (1977) for specimens from the northern parts of the Franklin Marble in New York.
At Sterling Hill, manganoan chondrodite was found as massive material with calcite, franklinite, and zincite and is also found in coexistence with unanalyzed but presumably manganoan humite. Other Sterling Hill occurrences include one with arsenopyrite and calcite on the 900 level and one with calcite and mica on the 1500 level.
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| Copyright © 1995 by Pete J. Dunn |
Website
by Herb Yeates
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This
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