SUMMARY. A study has been made of topaz crystals showing a sectoral texture related to the growth of the crystal, and showing anomalous optical properties within the sectors. The growth surfaces responsible for the development of most of the sectors have been identified. The fluorine/hydroxyl sites in topaz are symmetrically equivalent in the solid crystal, but at a growth surface this equivalence may be lost, resulting in a reduction in the crystal symmetry and the ordering of fluorine and hydroxyl, the ordering scheme being retained once the crystal has formed. The reduction in symmetry expected to be produced by this ordering is in general agreement with the actual reduction in symmetry indicated by the optical properties. Heating of the topaz to about 950 ~ results in the almost complete disappearance of the optical anomalies due to disordering, which may be related to the loss of hydroxyl from the crystal. Anomalous pyro-and piezoelectric phenomena may also be the result of ordering. Published optical and X-ray determinative curves for fluorine in topaz may give erroneous results for ordered topaz.THE crystal structure of topaz, AI2SiO4(F,OH)2 was determined independently by Pauling (i928) and by Alston and West (I928). The crystal structure was described as belonging to the orthorhombic centrosymmetric space group Pbnm and the structure has been successfully refined in that space group (Ribbe and Gibbs, 1971 ). However, topaz has some physical properties that must be considered anomalous if the crystal structure as described is correct and complete, as these properties indicate lower symmetry for topaz than that determined.Topaz normally has the optic orientation ~ = a, fl = b, ~ = c, OAP(o i o) and Bxa = [oo I ]. However, optical anomalies in topaz have long been known to occur (see Dana, 1892 ). A study of topaz showing sectoral texture in thin section was made by Rinne (I926) who found that not only did the orientation of the optic axial plane vary in different sectors, but 1 On leave from Institute of Mineralogy, Petrology and Economic Geology, Faculty of Science, Tohoku University, Sendal 980, Japan.
Copyright the Mineralogical Societyalso that the optic axial angle and the direction of the acute bisectrix could vary. In an X-ray study by Rinne, in spite of the wide variation in the orientation of the optical indicatrix in different sectors, no variation of the orientation of the crystallographic axes was detected. Rinne also reported that the anomalous optical effects disappeared after heating the sections to 950 ~ for three to five hours.Observations. Sectoral texture has been observed in a group of five orange-coloured uniterminal prismatic topaz crystals from Ouro Preto, Brazil (Department of Geology, University of Manchester, specimen number M9oo, distinguished as A, B, C, D, and E). The crystals were each about 8 mm across and 30-40 mm long. The prism faces of all the crystals show striations parallel to the prism axis about 89 mm apart. Thin sections were cut as nearly as possible perpendicular...
