The chemical composition and the thermal behavior of sodium and hydrogen octosilicate was studied by chemical and thermal analysis, infrared (IR), magic-angle-spinning nuclear magnetic resonance (MAS-NMR) spectroscopy, and X-ray diffractometry. Both compounds are layer silicates with basal spacings of 11.10 and 7.38 Å, respectively. In both forms the ratio of Q4 silicon (connected via oxygen bridges with four silicon atoms) to Q3 silicon (connected with only three other Si atoms) is 1. At least a small part of the Q4 silicon can be substituted by aluminum. Elimination of water coordinated to the cations in sodium octosilicate results in a concomitant structural collapse. Replacement of the sodium ions by protons affects the atomic arrangement in the sheets only to a minor degree, but results in a decrease of the periodicity along the crystallographic c axis. Upon heat treatment an endothermal structural rearrangement occurs at about 360 K as revealed by significant changes of the IR and 29Si MAS-NMR spectra. Reexchange with Na ions largely, but not completely, restores the structure of the parent octosilicate.

The X-ray diffraction pattern of sodium octosilicate was indexed in the monoclinic system with a = 7.345 Å, b = 12.74 Å, c = 11.25 Å and β = 99.3°. Based on conclusions drawn from the results of the present study, the X-ray pattern of hydrogen octosilicate was tentatively indexed in the monoclinic system with a = 7.345 Å, b = 12.74 Å, c = 8.51 Å and β = 119.8°.

Tourmalines of unusual (mushroom) habit are common in granitic pegmatites of Momeik, northeast of Mogok, Myanmar. Here, we examine a sample of elbaite of significantly different habit, consisting of a series of diverging crystals, resembling a sheaf of wheat and ranging in colour from light purplish-red at the base to dark purplish-red at the tip with a thin green cap at the termination. The crystal structures of eight crystals are refined to R1-indices of ∼2.5% using graphite-monochromated Mo-Kα X-radiation; the same crystals were analysed by electron microprobe. 11B and 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectra were collected on four regions of the wheatsheaf crystal, and show ∼0.3 a.p.f.u. [4]B and <0.1 a.p.f.u. [4]Al in the structure. 57Fe Mössbauerspectr oscopy was done on the dark green rim at the termination of the crystal, showing all Fe in this region (∼0.6 a.p.f.u.) to be Fe2+. Detailed electron-microprobe traverses show that the principal compositional variation involves the substitutions [4]B + YAl → Si + YFe*, where transition metals are present, and [4]B2 + YAl → Si2 + YLi, where transition metals are not present, although several other minor substitutions also affect crystal composition. Successive microscopic bifurcation of crystallites causes divergence of growth directions along the c axis, imparting the overall ‘wheatsheaf’ shape to the crystal aggregate. We suggest that such bifurcation is common in pegmatitic elbaite crystals, resulting in their common divergent habit.