The crystal structure of a violet phyllosilicate from the serpentinite massif of the Sobotka Mountains, lower Silesia, Poland, identified as amesite by powder diffractometry, wet chemical analysis, and IR absorption spectrometry, has been determined by single-crystal X-ray diffraction methods. The mineral has a composition close to that of the ideal end-member amesite, but with significant amounts of Cr, Fe2+, and Ni. The formula is [6](Mg3.80Al1.75Cr0.29Fe0.05Ni0.020.09)[4](Si2.15Al1.85)O10(OH)8.

Unit-cell dimensions obtained by least-squares refinement of 25 low- to medium-angle reflections with an automated single-crystal diffractometer gave a triclinic unit cell with parameters a = 5.31(1) Å, b = 9.212(2) Å, c = 14.401(7) Å, α = 102.11(3)°, β = 90.2(1)°, γ = 90.1(1)°, with two serpentine-type layers per unit cell.

The structure has been refined to an agreement factor R = 0.061, demonstrating the existence of ordering of cations in the tetrahedral sheet, also implying ordering of cations in the octahedral sheet. Two stacking modes occur in alternate pairs of layers, one due to a 180° rotation with no shift of neighboring layers, resulting in a change of the octahedral cation configuration, and another due to 180° rotation plus translation of −b/3. The structure does not conform to any of the accepted standard or regular polytypes for the serpentine-like minerals.

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