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Summary. Dickite from a new occurrence at Barkly East, Cape Province, is described and compared with a previously unreported dickitc from Postmashurg and a dickite from Ouray, Colorado. The Barkly East dickite crystals are elongated in the direction of the a-axis ; the ratio of length to width of crystals varies from 3:1 to 16:1. Optical, electron microscopic, X-ray, and thermal data and a chemical analysis are given. The thermogram of the fraction smaller than 2 g e.s.d. 1 is characterized by a single peak at 600 ~ C., while that of fractions larger than 2 tt e.s.d. shows a double endothermal effect. This two-stage dehydroxylation of dickite on heating is attributed to a particle size effect.

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A B ST R A C T: One of the alteration products of a deuteric altered beryl is a highaluminous chlorite-swelling chlorite regular mixed-layer clay mineral. An attempt was made to calculate the structural formula, and the results of X-ray, differential thermal and electron diffraction analyses are given.During an investigation of the alteration products of beryl from the Dernburg pegmatite, Karibib, South West Africa (Roering & Heckroodt, 1964), a mineral with a 30 A spacing was detected in the fine fraction. The responses of reflections, attributed to this mineral, to treatment with organic liquids indicated that the mineral is of an expanding lattice type. The fine fraction was subsequently beneficiated in an attempt to obtain a pure sample for a detailed investigation.The altered beryl occurs in a pegmatite located in quartzites of the Damara System. It has been mined for beryl, amblygonite, and tantalite. Replacement bodies consisting of lithium-bearing mica and albite also occur, with or without cleavelandite-rich borders. The altered beryl occurs where these cleavelandite-rich borders abut against the beryl in the central zone of the pegmatite. From field relationships it would appear that this alteration was caused by deuteric pegmatite solutions intimately associated with the formation of the lithium mica-bearing replacement zones and their cleavelandite-rich margins. PREPARATION OF SAMPLETwo small specimens of highly altered beryl were crushed in an iron pestle and mortar, and any contaminating iron was removed by means of a strong magnet. The crushed sample was dispersed in a 'Waring Blendor' for 20 min to facilitate the separation of the clay particles adhering to other mineral grains and to disintegrate the clay aggregates. The fraction less than 5 t~ was then obtained by sedimentation methods. The water of sedimentation was removed by means of a membrane filter.

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