Chemical controls on the variation of the pyroelectric effect in tourmaline are described for the first time. The experimentally-observed differences between the pyroelectric coefficients of different tourmalines have been related quantitatively to their Fe content. The incorporation of Fe in the tourmaline lattice linearly decreases the pyroelectric coefficient within the composition range studied between 0.01(1) and 14.6(2) wt% Fe expressed as FeO. Thus, the pyroelectric coefficient of tourmalines may be predicted directly from the chemical composition derived by routine analytical techniques such as electron probe microanalysis. Knowledge of the iron oxidation state is not required.
Crystal-chemical analysis of the relationships between pyroelectricity and chemistry indicates that the pyroelectric coefficient is influenced differently by the occupancies of the three X, Y and Z cation sites in the tourmaline structure. The octahedral Y site occupancy directly influences the pyroelectric coefficient. This effect is due to the preferential occupancy of the Y site by Fe cations. In the absence of Fe, crystal-chemical analysis suggests that the addition of Fe and Mg cations to the smaller Z octahedral site causes an increase in the pyroelectric coefficient. However, due to the unavailability of a suite of samples in which the Z site contains ions other than Al this proposed trend has not been experimentally determined. The chemistry of the cations in the 9-fold coordinated X site and the population of this site do not significantly influence pyroelectricity in tourmaline.
The crystal structures of two elbaite-schorl tourmalines were examined in detail in order to develop atom models which describe the variation in the pyroelectric effect. This structural analysis was also used to determine cation and anion site partitioning in tourmalines. High resolution single crystal X-ray diffraction data were collected from chemically-distinct elbaite and schorl endmembers at l00K and 296K. In addition, single crystal neutron data were collected from the same schorl at around 298K. These structures are the first reported for a schorl at two temperatures. The elbaite structures are separated by 196K. This temperature interval is the widest reported for elbaite, as two elbaite structures have been previously determined over a l00K temperature interval by Fortier (1975). The structural formulae obtained from electron probe microanalysis and subsequent site occupancy refinements of these samples are:
(Na.57Ca.01 #.42)∑=1.00 (Al.2.07Li.84Mn.06Zn.03)∑=3.00 Al6
(BO3)3 Si6O18 (O0.83(OH)3.00F. 17)∑=4.00
(Na.73Ca.0.2K.01 #.24)∑=1.00 (Fe1.86Mg.55Al.42Ti.08Mn.02Zn.02Li.20)∑=3.00
(Al5.81Fe.19)∑=6.00 (BO3)3 Si6O18 (O0.65(OH)3.00F.35)∑=4.00
Structure refmements and Mossbauer spectroscopy confirm that the Z site in this schorl is partially filled by divalent Fe. In both elbaite and schorl, the H3 and boron sites are fully occupied and fluorine partitions entirely to the O1site. A possible hydrogen atom bonded to the O1 ion was not located.
The change in pyroelectric charge of schorl and elbaite are modelled by the shifts of rigid, undeformed ions within the crystal lattice over a 196K temperature interval. Average pyroelectric coefficents are calculated from these charge shifts. The ion shifts of schorl and elbaite over this temperature interval are small and range from 10-3 to l0-4Å. All ion shifts contribute to the observed macroscopic polarisation in schorl and elbaite, as there are no individual ions with statistically larger shifts than those of other ions over this temperature interval. These results contradict previous work which indicates that the O1 ion shift is larger than for other ions, and thus, is the only contributor to the rigid ion shift model of pyroelectricity in tourmaline (Donnay, 1977).
The deformation of individual coordination polyhedra have been analysed for a temperature increase between l00K and 296K. This analysis shows that the 9 - anion X polyhedron and the Y octahedron produce net charge shifts which are the same sign as the macroscopic pyroelectric charge developed in a tourmaline crystal as it is heated. Temperature-induced distortions of the Y polyhedron change with variation of the Y site occupancy due to the elbaite to schorl, Li+ + Al3+<-->2Fe2+, substitution. These results correlate with the observed decrease in pyroelectric coefficient with Fe content, since Fe ions preferentially partition to the Y octahedra. Therefore, the structural data confer with crystal-chemical analysis that the Y site occupancy is a major determinant of relative pyroelectric coefficients in tourmalines.
Donnay, G (1977) Structural mechanism of pyroelectricity in tourmaline. Acta Crystallographica, A33, 927-932.
Fortier, S (1975) "The Relationship of Pyroelectricity and Crystal Structure in Tourmaline", PhD Thesis, McGill University, Montreal, PQ, Canada, Canadian Theses on Microfiche No. 29352, 76 pp.