The vicissitudes of the low-quartz crystal setting or the pitfalls of enantiomorphism

Based on a literature study, the following proposals endeavour to combine the advantages of previous conventions for setting low-quartz crystals: (1) Knowing that symmetry axes are parallel to the edges of the smallest hexagonal cell, place Oz along the 3-axis and Ox, Oy, Ou along the macroscopic 2-axes. (2) Direct the polar axes positively toward the s and x faces, if present, or toward the positive charge developed on extension by piezoelectric test. (3) Match the hand of the coordinate system to that of the structural screw; thus, the right-handed coordinate system (RHCS) for the known right screw (RS) of laevorotatory quartz. (4) Let the coordinate axis that coincides with a 2-axis in the structure be Ox[100]. These rules result in both enantiomorphs obeying Lang's mnemonic rule and being in the r(+) setting, in which {101} symbolizes the major rhombohedron r, defined by I_obs(101) > I_obs(011). Low quartz has only one structure. It should be published only for the RS (P3₁21) and in the RHCS: anyone can visualize the LS enantiomorph by plotting the atomic coordinates in the LHCS. This policy should apply to any enantiomorphous substance with known absolute configuration. If a crystal structure, determined in the RHCS, turns out to have a LS, transform to LHCS and publish the new coordinates, which also fit the standard RS in RHCS, but add, 'Known only as LS' or 'Observed as LS'.