Abstract

Ferdowsiite is a member of a mineral association subsidiary to the principal metamorphic minerals of the Barika ore deposit in NW Iran. The empirical formula of ferdowsiite (based on 32 apfu, 16Me + 16S) is Ag7.97Pb0.08 Sb4.75As3.15Bi0.01S16.04. The crystal-structure formula is Ag8(Sb5.49As2.51)∑8S16; the simplified formula is Ag8(Sb5As3)∑8S16. The slightly plumbian variety of ferdowsiite has the idealized formula Pb2xAg4−xSb3−xAsS8 with x =~0.2. Ferdowsiite is monoclinic, space group P21/n, a 8.677(2) Å, b 5.799(1) Å, c 13.839(3) Å, β 96.175(4)°, V 692.28(10) Å3, Z = 1 for the formula Ag8(Sb5As3)∑8S16. Calculated powder X-ray data are listed; main diffraction lines are [dcalc (Irel) (hkl)]: 3.225 (96)(113); 3.205 (100)(2̅12); 2.8995 (78)(020); 2.7559 (90)(301); 2.7073 (79)(1̅05); 1.9401 (22)(206); 1.9226 (22)(4̅04). Plumbian ferdowsiite has the same space group, P21/n. It has a 8.746(7) Å, b 5.805(4) Å, c 13.929(10) Å, β 96.236(11)°, unit cell volume 703.05 Å3. The mineral is greyish black, opaque, with dark grey streak and metallic luster. It is brittle with irregular fracture; no cleavage or parting was observed. Calculated density is 5.3 g/cm3. In reflected light ferdowsiite is greyish white. Internal reflections are not observable. Pleochroism is distinct, white to grey. Bireflectance is distinct; reflectance values are [λ(nm), Rmin,Rmax]: 470, 34.9, 37.3; 546, 33.4, 35.9; 589, 32.4, 35.1; 650, 30.7, 33.5. Anisotropism is distinct and rotation tints vary between dark brown and bluish grey. Ferdowsiite is associated with large corroded crystals of arsenquatrandorite and is partly replaced by guettardite. It also corrodes smithite. Ferdowsiite is often associated with tetrahedrite/tennantite and forms worm-like symplectites with it. This symplectite is partly replaced on the surface by guettardite. Ferdowsiite is one more member of the group of ABX2-type sulfosalts of silver, combining antimony and arsenic, with a crystal structure different from other members of this group. The closest crystallographic and structural affinity is to the ternary sulfosalt diaphorite.

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