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"Rod" Polytypism in Vesuvianite : Crystal Structure of a Low-Temperatur P4nc Vesuvianite with Pronounced Octahedral Cation Ordering

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"Rod" polytypism in vesuvianite : crystal structure of a low-temperatur P4nc vesuvianite with pronounced octahedral cation ordering Autor(en): Armbruster, Thomas / Gnos, Edwin Objekttyp: Article Zeitschrift: Schweizerische mineralogische und petrographische Mitteilungen = Bulletin suisse de minéralogie et pétrographie Band (Jahr): 80 (2000) Heft 2 PDF erstellt am: 26.09.2021 Persistenter Link: http://doi.org/10.5169/seals-60955 Nutzungsbedingungen Die ETH-Bibliothek ist Anbieterin der digitalisierten Zeitschriften. Sie besitzt keine Urheberrechte an den Inhalten der Zeitschriften. Die Rechte liegen in der Regel bei den Herausgebern. Die auf der Plattform e-periodica veröffentlichten Dokumente stehen für nicht-kommerzielle Zwecke in Lehre und Forschung sowie für die private Nutzung frei zur Verfügung. Einzelne Dateien oder Ausdrucke aus diesem Angebot können zusammen mit diesen Nutzungsbedingungen und den korrekten Herkunftsbezeichnungen weitergegeben werden. 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MITE 80, 109-116, 2000 "Rod"" polytypism in vesuvianite: crystal structure of a low-temperature PAnc vesuvianite with pronounced octahedral cation ordering by Thomas Armbruster1 and Edwin Gnos- Abslract Vesuvianites are known to crystallize in three different tetragonal space-groups, PAInnc,PAIn, and P4nc.These different varieties are interpreted as "rod" polytypes. Positional cation disorder on strings parallel to the 4-fold axes, characteristic of vesuvianiles crystallized between 400 and 800 °C, leads to PAInnc symmetry. Low temperature (< 300 °C) vesuvianites have long-range ordered strings leading to either PAIn (centrosymmetric) or PAnc symmetry (non- centrosvmmetric). The crystal structure of a low-temperature vesuvianite (a 15.569(2), c= 11.841(1 À)of CawAl9 0Mg2.2Fe^jSii8O6i)(OH)9 composition, front a rodingitized dolerite dike of the Bela Ophiolite in Pakistan, was refined from single-crystal X- ray data in P4nc symmetry to R 2.92%. The refinement showed that ca. 84% of the strings have a non- cenlrosymmetric arrangement where the apices of the five-fold coordinated Fe3+ square pyramids point in the same direction. In addition, the entire structure relaxes to PAnc symmetry leading to pronounced cation ordering on the octahedral Y sites. Y1 hosts only Al. whereas Y2a and Y2b (symmetry equivalent in PAInnc symmetry) show strongly different bond lengths indicating different occupation by AI. Mg. and Fe% Keywords: vesuvianite, polytypism. long-range ordering, single-crystal X-ray structure refinement, cation ordering, Pakistan, rodingite. Introduction nels. These channels have potential cation positions with the sequence Y'X'X'Y', where X' is Conventionally, a compound is defined as polytypic eight- and Y" is five-coordinated (Fig. 1). Due to if it occurs in several different structural short X'X' and Y'X' distances occupied sites modifications, each of which may be regarded as always alternate with vacancies (Dj.Thus a string or built up by stacking of layers of (nearly) identical "rod" along a 4-fold axis has locally either a structure and composition, where the modifications Y'DX'D or DX'nY' arrangement (Fig. 2). If we differ only in their stacking sequence assume for electrostatic reasons that two adjacent (GlJlNlER et ah. 1984). The same nomenclature vacancies do not occur, each string itself is fully paper also remarks: "a more general definition of ordered. However, adjacent strings are cither polytypism that includes 'rod' and 'block' long-range disordered (space group PAInnc) or polytypes may be become necessary in the future". they follow some specific ordering patterns leading The major reason for the omission of "rods" and to decreased symmetry {PAIn or PAnc). An "blocks" is the fact that well defined examples of easy way to visualize the string arrangement in these types were missing at that time. A subsequent space group PAIn is that the apex of the square extension of the definition suggested by pyramid around Y' points to opposite directions Angel (1986) also includes "rod" polytypes. along the two adjacent 4-fold axes. In space group The structure of vesuvianite is composed of a PAnc apices of square pyramids point in the same polyhedral framework enclosing structural chan¬ direction (Fig. 2).The various arrangements of or- 1 Laboratorium für chemische und mineralogische Kristallographie. Universität Bern, Freiestrasse 3. CH-3012 Bern. Switzerland. <[email protected]> 2 Mineralogisch-Petrographisches Institut. Baltzerstrasse 1, Universität Bern, CH-3012 Bern. Switzerland. TH. ARMBRUSTER AND E. GNOS dered strings may be described in terms of "rod" polytypism. String ordering is frozen in during growth and does not alter with heat treatment of the crystal (Allen and Burnham, 1992). The aspect of polytypism in vesuvianite has mostly been overlooked or ignored. The reasons X'4b are that in contrast to conventional polytypes Y'3a composed of stacked layers, the "rod" polytypes of vesuvianite have the same cell dimensions and the observed space groups have similar symbols (PMnnc, P4ln, and P4«c).The space groups (PMn, P4nc) of the ordered polytypes are actually vasvm the subgroups of disordered polytype (PMnnc). String at Furthermore, the extinctions of systematic X-ray 0, 0 z String at diffractions which enable distinction of the various V2. Z Fig. 2 Arrangement of potential X'4 and Y'3 sites Q forming strings parallel to the 4-fold axes.The Y'3 site is a live-coordinated square pyramid where the tip may point up or down. The X'4 site possesses dodecahedral coordination. Due to short distances of adjacent Y'3, X'4 and X'4, X'4 polyhedra, only two of the available sites are occupied. In space group P4nc the crosshatched polyhedra form a type A string and the polyhedra with Q the weaving pattern form an alternative type B string. The black circles are Ca atoms bonding to the oxygen atoms of the siring sites. space groups are not very pronounced. In other words the decisive reflections are either very weak or absent.Thus powder diffraction methods are not suitable for a distinction of the stacking variants (AREM and Burnham, 1969). Polytypism in vesuvianite is of great importance also from a petrological point of view because polytypes with long-range string ordering (space groups PMn and P4nc) arc characteristic of low crystallization temperature (< 300 °C) whereas high-temperature (» 300 °C) vesuvianites have long-range disordered strings leading to P4lnnc diffraction symmetry (Allen and Burnham. 1992). A consequence of symmetry lowering from P4lnnc to P4ln or P4nc is splitting of various tet- rahedral, octahedral, and seven- to nine-coordinated sites. As an example, in space group PMnnc (high-temperature vesuvianites) there are only two octahedral positions (Yl and Y2). In space A Vh, z group P4nc (low-temperature vesuvianites) there are three symmetry independent octahedral sites Fig. 1 Large, medium, and small sized spheres represent (Yl, Y2a,and Y2b) and in space group P4/n (low- Ca. and cations in oxygen, five-coordinated square temperature vesuvianites) there are four symmetry pyramidal coordination (Y'3 sites), respectively. Upper independent octahedral sites (Yla. Y lb. Y2a. of vesuvianite rods the c-axis. part: Projection along and Y2b).Thus in addition to string ordering, low- Lower part: Perspective view of the vesuvianite rods. vesuvianites exhibit a more For clarity, the two rods on the back of the unit cell have temperature complex for cation Unfortunately been removed. Notice that the close spacing of two adjacent pattern possible ordering. X'4 or X'4 and Y'3 sites does not allow for simultaneous all experimental studies dealing with vesuvianite occupation.Thick dashed lines indicate hydrogen stability have either ignored "rod" polytypism bonding connecting two modules within a string. in vesuvianite or were carried out at temperatures ROD" POLYTYPISM IN VESUVIANITE above 400 °C where the disordered PMnnc variant fecture. Japan, from Muslimbagh. Pakistan has to be expected (e.g. Valley et al.. 1985). (Ohkawa et al., 1994), and from hydrothermal A simplified formula of \esuvianite. taking pocket fillings of the N'chwaning II mine in the into account the various cation coordinations, Kalahari manganese fields. South Africa (Arm- mav be written as XlsX'Y|2Y'ZiiS0„u(0H,F)„. bruster and Gnos. 2000b). There are only two where X and X' are seven to nine-coordinated, Y structure refinement of a non-centrosymmetric has octahedral coordination, Y' has square PMic vesuvianite, one from Saueseter, Drammen, pyramidal coordination (five-fold), and Z represents Norway (Ohkawa et al., 1994) and one from the tetrahedral coordination. X and X' are N'chwaning II mine (ARMBRUSTER and Gnos, commonly occupied by Ca. Y and Y' host elements 2000a). with an average valence of 2.85 (e.g. 11 (AI, Fe")
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