Description:
<jats:title>Abstract</jats:title><jats:p>The present study provides an example of the accurate identification of polytypes of trioctahedral 1:1 layered silicates from single-crystal X-ray diffraction data collected with the aid of a four-circle diffractometer equipped with an area detector. Single crystals of the mineral cronstedtite from the Nagybörzsöny gold ore deposit, northern Hungary, were studied. The chemical composition of some crystals was determined by electron probe microanalysis (EPMA). The precession-like images of the reciprocal space (RS) sections created by the diffractometer software and presented in the study were used to determine the OD (ordered-disordered) subfamilies (Bailey’s groups A, B, C, D) and particular polytypes. With one exception, all crystals studied belong to subfamily A. The rare polytype 1<jats:italic>M</jats:italic>, <jats:italic>a</jats:italic> = 5.51, <jats:italic>b</jats:italic> = 9.54, <jats:italic>c</jats:italic> = 7.33 Å, β = 104.5°, space group <jats:italic>Cm</jats:italic> is relatively abundant in this occurrence. Another polytype 3<jats:italic>T, a</jats:italic> = 5.51, <jats:italic>c</jats:italic> = 21.32 Å, space group <jats:italic>P</jats:italic>3<jats:sub>1</jats:sub> was also found. Both polytypes occur separately or in mixed, mostly 1<jats:italic>M</jats:italic> dominant crystals. Some 1<jats:italic>M</jats:italic> polytype crystals are twinned by order 3 reticular merohedry with a 120° rotation along the <jats:bold>c</jats:bold><jats:sub>hex</jats:sub> axis as the twin operation. A rare 1<jats:italic>M</jats:italic>+3<jats:italic>T</jats:italic> mixed crystal with 1<jats:italic>M</jats:italic> part twinned also contains a small amount of subfamily C. A possible presence of the most common 1<jats:italic>T</jats:italic> polytype of this subfamily cannot be confirmed because of overlap of the characteristic reflections with those of 3<jats:italic>T</jats:italic>. Several completely disordered crystals produced diffuse streaks instead of discrete characteristic reflections on the RS sections. The EPMA revealed Fe, Si, traces of Mg, Al, S, and Cl. One black crystal originally considered to be cronstedtite was identified as (111) twinned sphalerite. Some crystals of cronstedtite are covered partially by a honey-brown crust or small crystals of siderite.</jats:p>