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(K,Na)2(Ca,Mn,Fe,Mg,Zn)28Zn5Al4Si40O112(OH)16
Hexagonal, P3c1, a
= 9.777, c = 33.293 Å, Z = l
Minehillite was first described from Franklin by Dunn et al. (1984a). It has not been found at Sterling Hill.
The crystal structure was solved by Dai et al. (1995) and is related to that of reyerite. Dai et al. reported the structure to consist of a “stacked sequence of three types of layer units: 1) an infinite sheet of edge-sharing Ca-(O,OH) polyhedra, 2) a single sheet of SiO4 tetrahedra connected in oval and pseudohexagonal-shaped six-membered rings, and 3) a complex slab built of SiO4 tetrahedra and AlO6 octahedra into which alkali elements and zinc are accommodated.”
Minehillite occurs as massive aggregates of mostly warped, lamellar crystals, up to 1.0 cm in size, and twinned by a 60o rotation about c. It is colorless, but may appear white in the aggregate or locally black due to inclusions of native lead. The luster is vitreous, but pearly on the perfect {0001} cleavage. The density is 2.93 g/cm3. Minehillite fluoresces medium dull-violet in shortwave ultraviolet and duller violet in longwave. Optically, minehillite is uniaxial, negative, with w = 1.607 and e = 1.604. It is easily distinguished from margarosanite by its indices of refraction and by the intense fluoresence of the latter; prehnite is likewise distinguished by its fluorescence.
Minehillite is a potassium calcium zinc aluminum silicate hydroxide mineral and is closely related to reyerite (Dunn et al., 1984a). A microprobe analysis is presented in Table 15; others were given by Dunn et al. (1984a) and agree closely. Solid solution among octahedral cations is limited.
Minehillite was found in the northern end of the west limb of the Franklin Mine in an area near the Palmer Shaft pillar, where Pb-silicates were discovered (Dunn, 1985b). It is associated with margarosanite, wollastonite, allanite, microcline, diopside, grossular, vesuvianite, and calcite. It replaces most of these minerals and is likely a secondary, low-temperature, hydrothermal, replacement mineral. It forms as reaction rims and occurs as aggregates on microcline.
Minehillite does not accept Pb when margarosanite is replaced; platelets of native lead are present in many specimens as inclusions. They form gray to black rims indicating the extent of minehillite replacement of the pre-existing assemblage. The presence of Pb, however, does not imply that conditions for the formation of this assemblage were more strongly reducing than the overall conditions at Franklin.
The presumption of Dunn et al. (1984a) that there were numerous host assemblages is apparently in error; subsequent study indicates there was likely but one very varied assemblage. Minehillite specimens are moderately common; much of the material was preserved by collectors and miners by serendipity because of the strong fluorescence of the associated wollastonite and margarosanite.
Minehillite was named for Mine Hill, the original outcrop of the Franklin deposit in Franklin, New Jersey.
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| Copyright © 1995 by Pete J. Dunn |
Website
by Herb Yeates
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