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Non-space-group absences in sapphirine X-SAY studies of a number of sapphirines indicate that non-space-group absences are a characteristic of the diffraction patterns. The absences may be understood most simply when reflections are indexed with respect o a cell derived from that given by Kuzel (196...
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| Language: | English |
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| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.614.2626 http://www.minersoc.org/pages/Archive-MM/Volume_36/36-279-449.pdf |
| Summary: | Non-space-group absences in sapphirine X-SAY studies of a number of sapphirines indicate that non-space-group absences are a characteristic of the diffraction patterns. The absences may be understood most simply when reflections are indexed with respect o a cell derived from that given by Kuzel (1961) [a = 11.26, b = 1l'46, c = 9.95A; fi = 125 ~ 20'] by the matrix [101/010/001]. Accurate cell dimensions determined for this cell by precession-camera techniques with a pale-blue sapphirine from Fiskernaesset, West Greenland are a = 9.77, b = 14.54, c = 10.06 A (all 4-0.01 A), fi = l I0 ~ 20 ' (:h2'). Statistical analyses of the intensities of this sapphirine indicate a centre of symmetry. The approximate formula (neglecting small amounts of Fe 2+, Fe 3+, Ti, Mn, Na, K) is 7MgO.9A12Oa.3SiO ~ (Z = 2) which agrees with the formula proposed by Kuzel (1961). Reflection conditions, which operate for each of the four sapphirines studied (from Fiskernaesset, W. Greenland; SukkertSppen, W. Green-land; Madura, India; and Betroka, Madagascar), are: hkl absent if h+ k is even and l = 4n § or if h § is odd and 1 = 4n; hOl absent if h+l is odd. The conditions are consistent with those obeyed by comparable reflections in the diffraction pattern of a sapphirine from Mautia Hill, Tanganyika studied by McKie (1963), though his specimen was unusual in showing a doubled b-axis and additional diffuse streaks parallel to b*. The non-space-group absences (hkl reflections) can be explained by supposing that the atoms in the unit cell are grouped into pairs whose coordinates are related by the displacements a/2, hi2, c/4, or a/2, b/2, 3c/4. Since the non-space-group absences include OkO reflections with k odd quite irrespective of the presence or absence of a 21 axis, the space group may be either P2/n or P21/n. Preliminary structural investigations of the Fiskernaesset sapphirine show that the oxygens are approximately cubic-close-packed with the diagonals of one cube face parallel to [010] and [101] of sapphirine and the perpendicular ... |
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