An x-ray study of orpiment has been made using the equi-inclination Weissenberg method. The monoclinic character of this crystal is confirmed by x-ray evidence. The dimensions of the reduced cell are:

a=11.47Å
b=9.57
c=4.24
β=90°27'.

This cell contains 4 As2S3:. Its axes have the same orientation as the morphological axes adopted by Mohs, stevanović, Palache, and Modell, but the lengths of the axes are difierent. The matrix of the transformation from the Mohs-stevanović-Paiache-Modell axes to the Buerger axes is

||200||
||101||
||0023||

The space group of orpiment is unequivocally determined as P21n (C2h5) for this cell.

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