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CaPb(Al,Fe3+)3(Si2O7)(SiO4)(O,OH)2
Monoclinic, P21/m, a = 8.958, b = 5.665, c
= 10.304 Å,
b = 114.4o, Z = 2.
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Figure 16-3. Stout, prismatic hancockite crystals in parallel growth, from Franklin. Field of view is 1.0 mm in maximum dimension. |
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Hancockite was originally described by Penfield and Warren (1899). Paragenetical information was provided by Hurlbut and Baum (1960). Additional chemical data, obtained on single crystals, was given by Dunn (1985b). Although reported from Norway by Neumann (1985), that material has insufficient Pb to be considered hancockite; a valid second hancockite occurrence, from Sweden, was reported by Holtstam and Langhof (1994).
The crystal structure of hancockite was described and the relation to epidote confirmed by Dollase (1971), who noted the diffuseness of the X-ray reflections from this species. Hancockite has Ca and (Pb,Sr) ordered in the A(1) and A(2) sites, respectively.
Hancockite occurs as superb euhedral 1-2 mm crystals in vugs, as well as massive intergrowths with other species; the latter are much more abundant. Euhedral crystals (Figure 16-3) are composed of the forms {100}, {001}, {101}, {101}, and {111} and are bright and sharp. Hancockite is invariably red, grading to reddish brown, pinkish brown, and brown when mixed with other species; the luster is vitreous to dull; and the density is 4.03 g/cm3. Optically, hancockite is biaxial, negative, 2V = 500, with a = 1.788, b = 1.81, and g = 1.83; pleochroism is strong; absorption is Z > X. There is no discernible fluorescence in ultraviolet.
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Table 8. Chemical analyses of lead-silicate minerals. |
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Hancockite is a calcium lead aluminum silicate hydroxide mineral of the epidote group. The original data are lower in Pb and Fe and higher in Ca and Mn than those subsequently provided by Dunn (1985b); one new analysis is given in Table 8. Hancockite is unique in the epidote group in containing essential lead Pb. The red color may be due to the presence of Mn3+ (Penfield and Warren (1899).
Hancockite was first found on the Parker Dump; later, in 1923, some dumps were partly removed for reprocessing, and it was then found in quantity. The best hancockite crystals (Figure 16-3) occur lining vugs in recrystallized assemblages composed largely of andradite, manganaxinite, mica, willemite, and franklinite, together with other locally rare species, such as xonotlite. Additional associated minerals were reported by Penfield and Warren (1899) to be copper, lead, clinohedrite, vesuvianite, datolite, and barite. Fine samples are prized.
The bulk of the preserved hancockite is massive brick-red dull-lustered material, mixed with shards and remnants of the above-listed species. This material, found in large amounts, is very inhomogeneous at the microprobe level and has not been extensively investigated. Hancockite also occurs disseminated in feldspar. Frondel (1972) reported hancockite replacing manganaxinite, a relation not investigated by this writer. Some massive hancockite is severely altered; prehnite occurs in such specimens.
Hancockite was named in honor of Mr. Elwood P. Hancock, a mineral collector. He was known for his skill in relief-carving of minerals from the surrounding calcite matrix (Figures 7-14 and 7-15). His collection is at Harvard University.
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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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