A study of phoenicochroite—its structure and properties

The poorly described species phoenicochroite occurs at several localities in Arizona. Material from the Potter-Cramer mine provides new information on the species. Phoenicochroite is associated with crocoite, cerussite, mimetite, and vauquelinite in the oxidized zone of galena-bearing veins. Analysis gives PbO 80.88, CrO₃ 18.08; total 98.96, corresponding to Pb₂O(CrO₄). The mineral is dark, cochineal red with a yellowish-orange streak; $H=212$; p = 7.01 (meas.) 7.075 (calc.). It occurs in anhedral cleavable translucent masses. Good cleavage on {$2¯01$} is easily seen; poorer cleavages are on {001}, {010}, and {011}. Phoenicochroite is monoclinic: space group C2/m, a= 14.001, b = 5.675, c=7.137Å β= 115°13’; Z=4. Strongest lines from x-ray powder photographs are: 3.380(10), 2.979(10), 6.43(5), 6.34(5), 2.831(5), 1.862(5), 2.475(4), 2.263(4) Å. The indices of refraction are α = 2.38_D, β = 2.44_D, and γ=2.65_D; X=bY˄c= —2°. Optically (+) with 2V_z (meas.) = 58°; 2V_z (calc.) = 60°. Dispersion is moderately strong with ρ>v. Only one relatively poorly developed crystal suitable for goniometry was found. Forms recognized were {010}, {100}, {$2¯01$}, and {$2¯11$). The axial ratio is 2.467:1:1.258.

The mineral is isostructural with lanarkite and the structure contains independent chromate tetrahedra; Pb atoms are in special position on mirror planes. The additional oxygen is associated with and tetrahedrally coordinated by the Pb atoms.