Hemihedrite is a new species discovered at the Florence Lead-Silver mine in Pinal County, Arizona. A second locality is the Rat Tail claim near Wickenburg, Maricopa County, Arizona.

Hemihedrite is named in allusion to its morphology. Crystals exhibit triclinic hemi-hedral symmetry with α = 120o01’, β = 91°40’, γ = 55°55’, a:b:c = 0.8345:1:0.9360. Three twin laws have been found. The refringence is α = 2.105, β = 2.32, γ = 2.65(25°C); optically (+) with 2V(calc.) = 88°. Dispersion is strong and unsymmetric.

Crystals are orange to almost black and have a saffron yellow streak. Hardness on the Mohs scale is 3.

Unit-cell data at 22°C are as follows: P1; a = 9.497 ± .001Å, b = 11.443± .002 Å, c = 10.841 ± .002 Å; α = 120°30’, β = 92°06’, γ = 55°50’. The Delaunay cell is a’= 9.954 Å, b’=10.841 Å, c’ = 9.497 Å; α’ = 92°06’, β = 1O7°58’, γ1 = 123°16’. The most intense X-ray lines are: 4.872 (90) (120), 4.364 (80) (220), 3.301 (100) (122), 3.146 (80) (320), 3.102 (80) (103, 102), 2.924 (55) (233), 2.849 (45) (242), 2.182 (45b) (411, 340). γ’ = 123°16’. Transformation (morphology to Delaunay cell) is 110/001/100.

Chemical analyses combined with structural information lead to the composition ZnF2[Pb6(CrO4)3SiO4]2(with Z= 1), although chemical compositional variation is indicated. Specific gravity is 6.42 (meas. at 24.2°C) and the density (calc.) is 6.50 g/cm3.

Hemihedrite forms in the oxide zone of lead-bearing veins. Associated minerals may include the following: cerussite, phoenicochroite, vauquelinite, willemite, and wulfenite.

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