Minehillite, a new zinc layer silicate mineral from Franklin, New Jersey, is hexagonal, space group P63mmc, P63mc or P62c, with a = 9.77(2) and c = 33.01(7)Å. The strongest lines in the X-ray powder diffraction pattern are (d, I/Io, hkl): 2.764,100,209,216; 1.847,90,555,349; 16.1,70,002; 3.07,70,213,1.0.10; 3.14,60,118,207. Microprobe analysis with H2O by the Penfield method yielded: SiO2 50.4, Al2O3 4.5, FeO 0.2, MgO 0.1, CaO 32.3, MnO 0.1, ZnO 8.4, K2O 2.0, Na2O 0.5, H2O 2.83, total = 101.3 wt.%. The tentative chemical formula, based on analogy to reyerite, is: (K,Na)2–3 (Ca,Mn,Fe,Mg,Zn)Σ28 [Zn≈4Al4Si40O112 (OH)4](OH)12, with Z = 1.

Minehillite is colorless with perfect {0001} cleavage, hardness (Mohs) approximately 4, and density 2.93 (meas), 2.94 (calc) g/cm3. The luster on cleavage surfaces is very pearly. Minehillite fluoresces in ultraviolet radiation with a medium violet fluorescence. Optically, minehillite is uniaxial (—) with ω = 1.607 and = 1.604 (both ±0.002). Minehillite occurs associated with diopside, calcite, grossular, vesuvianite, wollastonite, and microcline in a number of varied assemblages from the Franklin Mine. It apparently forms by low-temperature hydrothermal replacement of the associated minerals. Minehillite is named for Mine Hill, the hill upon which the Franklin deposit cropped out.

Minehillite is structurally related to reyerite, gyrolite, and truscottite. Type material is preserved at the Smithsonian Institution.

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