Abstract
Polytypes of illite from the Broadlands-Ohaaki geothermal system in New Zealand have been studied with transmission electron microscopy and electron diffraction. The illite is predominantly a one-layer polytype with 1.0-nm interlayer spacing. On the scale of a few millimeters, illite occurs as ordered crystals, disordered crystals, and crystals with regions of ordered and disordered stacking. Some crystals are composed entirely of either the one-layer or the two-layer polytype; others show regions of two-layer stacking in a one-layer host. Long period stacking sequences (3- or 4-layer) are less common and occur as lamellar intergrowths in 1M mica. No morphological differences were observed among the discrete crystals of 1Md, 1M, and 2M. Textures indicate that regions with two-layer stacking can be produced from 1Md mica even though there is a general trend with increasing temperature of 1Md to 1M to 2M. Experimental data on the rate of transformation of 1M to 2M1 muscovite combined with estimates of the mole fraction of 2M mica and temperatures of illite crystallization were used to place constraints on the duration of hydrothermal activity in the Broadlands-Ohaaki system.
