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Ca6(Al,Si)2(SO4)2[B(OH)4](O,OH)12.26H2O
Hexagonal, P31c, a = 11.16, c = 21.21 Å, Z = 2
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| Figure 24-6. Crystal drawing of charlesite from Franklin, showing the form f {1014}. Drawing from Hurlbut and Baum (1960). For discussion of form-designation see Dunn et al. (1983a). | ||
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Charlesite was described from Franklin by Dunn et al. (1983a), but is not known from Sterling Hill. The mineral had been previously considered by Hurlbut and Baum (1960) to be ettringite, to which it is structurally related.
Charlesite occurs as euhedral to subhedral crystals, tabular on [0001]. The only discernible form is {1014}; the crystals have a dipyramidal habit (Figures 24-6, 24-7, and 24-8) and are up to 6 mm in diameter, although most are considerably smaller.
Charlesite is colorless, but crystals may appear to be opaque white due to a surficial alteration. Charlesite is transparent with a hardness of approximately 2½, a density of 1.77 g/cm3, perfect {100} cleavage, and vitreous luster. Optically, it is uniaxial, negative, with w = 1.492 and e = 1.475. Charlesite is weakly fluorescent with a light green or light violet color in shortwave ultraviolet; the fluorescence is weaker in longwave. Bostwick (1992) reported pale blue fluorescence in shortwave. Identification should be made with caution; local material has not been studied with respect to the presence of ettringite. Surface alterations are composed of gypsum in part.
Charlesite is a calcium aluminum sulfate boron hydroxide hydrate mineral of the ettringite group. The only extant analysis, by Jun Ito (Hurlbut and Baum, 1960) yielded: CaO 27.3, Al2O3 5.1, SiO2 3.1, SO3 12.8, B2O3 3.2, H2O 48.6, total = 100.1 wt. %.
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| Figure 24-7. Tabular, hexagonal crystal of charlesite from Franklin. Field of view is 0.1 mm in maximum dimension. | ||
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Charlesite occurred in the Franklin Mine, immediately above the 800 level, about 15 feet from the hanging wall and close to the north side of the Palmer Shaft pillar. A sketch map of the occurrence was given by Hurlbut and Baum (1960) and detailed descriptions of the studied specimens by Dunn et al. (1983a).
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Figure 24-8. Tabular, hexagonal crystal of charlesite on etched willemite from Franklin; note second-generation crystal in different orientation. Field of view is 0.1 mm in maximum dimension. |
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Charlesite occurs within a zone of highly recrystallized minerals, some of which are hydrated. The associated minerals are ganophyllite in tabular pseudohexagonal crystals, clinohedrite, datolite, prehnite, grossular, pennantite, willemite, pectolite, xonotlite, hancockite, roeblingite, manganaxinite, and others. Charlesite is the last mineral to form in this assemblage; colorless clear crystals, partially etched (Figures 24-7 and 24-8), occur within cracks in willemite. Dunn et al. (1983a) implied the existence of a number of different assemblages, but it is now clear that they are all closely related and from one occurrence.
Charlesite was named in honor of Professor Charles Palache, in recognition of his contributions to mineralogy, in particular to the mineralogy of Franklin and Sterling Hill. He is the father of Franklin mineralogy, and his memorial is given by Frondel (1956). His activities are in part discussed in the section entitled “Cultural aspects of Franklin and Sterling Hill.”
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| Copyright © 1995 by Pete J. Dunn |
Website
by Herb Yeates
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Link
to homepage
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This
page created: January 11, 2001 |
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