Laffittite was found in a museum specimen labelled getchellite, from the Getchell mine, Humboldt County, Nevada. The mineral occurs in grains about 1 mm in maximum length as dark red crystals associated with orpiment, getchellite and realgar.

Laffittite, AgHgAsS3, is monoclinic, space group Aa, a = 7.732(3), b = 11.285(7), c = 6.643(4)Å, β = 115.16(3)°, Z = 4. The strongest lines in the X-ray powder pattern are 3.512(100)(200), 3.202(78)(031,131), 3.015(69)(002), 2.685(67)(231), 1.898(29)(l33,233, 2.136(18)(231,331), 3.633(16)(111), 1.889(14)(411).

Single crystals were hydrothermally synthesized at 350°C under 10,000 psi pressure for 140 hours. The starting material was a fused mixture of Ag2S, HgS and As2S3 having a molar ratio of 1:2:1.

The crystal structure of laffittite is a superstructure based upon the PbS structure. Metal-sulfur bonds form a hexagonal ring of one formula unit. The hexagonal rings link to each other by sharing all edges forming a sheet parallel to (100). The sheets link up and down by Hg-S and Ag-S bonds to form a three-dimensional network. As has the usual trigonal coordination by 3S (As-S = 2.26Å, avg.). Ag has (3 + 1)S coordination: Ag-S = 2.51, 2.52, 2.60 and 2.89Å. The Hg coordination group is a distorted tetrahedron: Hg-S = 2.42, 2.51, 2.71, and 2.73Å. The structure is closely related to that of marrite, PbAgAsS3.

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