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The crystal structure of synthetic pääkkönenite, Sb 2.07 As 0.93 S 2.00 , as determined. This study revealed that, in spite of the strong orthorhombic pseudosymmetry shown by Weissenberg photographs, the structure is monoclinic (space group C2/m), with a = 10.75(2), b = 3.959(3), c = 12.49(2) Å, β= 115.25(8)°, V = 481(1) Å 3 , and Z = 4. The crystal structure was solved by direct methods using intensity data collected from a twinned crystal. The refinement led to an R index of 4.58%. The observed pseudosymmetry was shown to arise from {001} twinning. The structure can be described as a sequence of sheets stacked perpendicular to the a axis and mutually displaced by ½b. Each sheet consists of Sb 4 S 4 As 2  ribbon-like units similar to the Sb 4 S 6  chains of stibnite but linked together by As-As bonds. The structural relationships with stibnite are discussed to explain the formation mechanism of pääkkönenite and the presence of twinning. X-ray powder data for synthetic pääkkönenite are also reported.

Crystal structure and twinning of Sb 2 AsS 2 , the synthetic analogue of pääkkönenite