LOCAL CHARGE-COMPENSATION IN HYDROXYL-DEFICIENT UVITE
MATTHEW C. TAYLOR*, MARK A. COOPER & FRANK C. HAWTHORNE
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2
* Department of Earth Sciences, University of California, Riverside, California 92521, U.S.A.
Abstract
The crystal structure of a sample of OH-deficient gem-quality uvite (tourmaline),
0.17Na0.40Ca0.43[Mg1.70Al1.28Cr.02](Al5.16Mg0.78Cr.06)Si5.998O6(BO3)3OH3.311O0.689 [Cr* = Cr + Ti + V + Fe]
has been refined to an R index of 1.8% based on 1134 observed (5 ) reflections measured with
MoK X-radiation. Unit-cell parameters are a 15.917(2), c 7.189(1) Å, V 1577.3(4) Å3, R3m, Z = 3,
D(calc) = 3.036 g cm-3. The crystal used in the collection of the intensity data was analyzed by electron
microprobe, and H2O was determined by Karl Fischer titration. F was not detected. The OH content
of 3.31 apfu (anions per formula unit) is significantly less than the ideal monovalent anion content of 4
apfu. The H atom associated with the O(3) ( OH) anion was located on a difference-Fourier map, whereas no
significant density was found in the vicinity of the O(1) site, in accord with the OH content determined
by chemical analysis. There is significant Mg-Al disorder over the Y and Z sites, and site populations were
determined from mean bond-length - constituent-cation-radius relations. The O(1) site-population is O0.69(OH)0.31,
and of key importance is the mechanism by which short-range bond-valence requirements are satisfied around
both O2- and OH- at this site. The O(1) site lies on a 3-fold axis and the O(1) anion is bonded
only to three Y cations. Where O(1) = O2-, the anion is bonded to 2 Al + Mg; where O(1) = OH-, the
anion is bonded to 3 Mg. The individual bond-lengths needed for complete bond- valence
satisfaction around O(1) average to give the observed Y-O(1) bond-length in this crystal, supporting
these assignments.
Keywords: tourmaline, uvite, hydroxyl-deficient, ordering, short-range order, crystal structure.