Surface microtopography of illite crystals from different modes of occurrence

By means of transmission electron microscopy (TEM) and the gold-decoration technique, the surface microtopography of illite crystals from various envi ronments was investigated. On the crystal surfaces, gold decoration success fully revealed growth steps of similar to 1 nm in height. The illite sample s investigated exclusively exhibit growth spirals of either polygonal or ci rcular form, with varying separation of steps. The spiral morphological cha racteristics were critically analyzed in relation to their mode of occurren ce. Crystals from massive metasomatic specimens (aggregation texture with v ery low porosity) are characterized by circular growth-spirals with a narro w separation of steps, whereas those from veins and druses (aggregation tex ture with high porosity) are characterized by polygonal growth-spirals with a wide separation of steps. Hexagonal and lath-shaped crystals were contro lled by polygonal spiral-growth; on the other hand, irregular platy crystal s are characterized by circular or malformed circular spiral-growth. The mo rphology of illite crystals seems to reflect the spiral-growth process unde r the various conditions.