THE CRYSTALLOGRAPHY OF LENGENBACHITE, A MINERAL WITH THE NONCOMMENSURATE LAYER STRUCTURE

The studied lengenbachite represents a non-commensurate layer structur e. The pseudotetragonal (Q) layer set has subperiodicities a 36.89 ang strom (the layer stacking direction), b 5.84 angstrom, c 5.85 angstrom (layer modulation direction), alpha = beta = 90-degrees and gamma - 9 1.01 angstrom, A-centred subcell; the pseudohexagonal (H) layer set ha s subperiodicities a 36.89 angstrom, b 3.90 angstrom, c 6.38 angstrom, alpha = beta = 90-degrees, gamma = 91.01-degrees, A-centred subcell. The A-centred unit cell (1.0.0/0.2.0/0.0.12 for the Q subcells; 1.0.0/ 0.3.0/0.0.11 for the H subcells) has a 36.89 angstrom, b 11.68 angstro m, c 70.16 angstrom alpha = beta = 90-degrees and gamma = 91.01-degree s. Analysis of lattice intensity data as well as comparison with other published lattice data on lengenbachite and with HRTEM results Of WIL LIAMS & PRING (1988) suggests that our lengenbachite specimen is a uni t-cell (glide) reflection-twinned 2a variety of lengenbachite and unit layers of lengenbachite (H & Q) should have a almost-equal-to 8.5 ang strom and beta almost-equal-to 95.5-degrees. An O-D model for the idea lized polytypic structure of lengenbachite is given and possible reaso ns for the general absence of simple stacking polytypes are discussed. Further O-D phenomena that stem from the layer non-commensurabilities and profoundly affect the reciprocal lattice of lengenbachite (with c ontinuous streaks for many lattice rows) are described and explained i n detail.