Intercalation of sulphonates in the lamellar structure of Zn-Al-NO3−layered double hydroxides precursor phases

Abstract

Natural diamond grows by at least three different modes: (a) Faceted growth on {111}planes giving octahedra; (b) Fibrous growth in directions giving cubes, or other shapes (such as spherulitic ballas); (c) Cuboid growth on crinkly surfaces approximating to {100}, which also produces cubes [1]. Combinations may also occur: growth by (a) followed by (b) gives coated stones [2], and (a) and (c) growth modes may occur together to give combined morphologies. Twinning of (a) gives spinel-type contact twins (called macles) [3] and twinning of (b) gives fluorite-type interpenetrant cubes [4]. Subsequent dissolution may greatly modify the morphology: for example, octahedral diamonds grown bymode (a) may turn into rounded rhombic dodecahedra [5]. The principal non-destructive techniques for studying diamond morphology areX-ray topography, scanning electronmicroscopy and cathodoluminescence microscopy, supported by various forms of spectroscopy. Here we report a study of a fourth mode of growth: namely, (d) faceted growth on {100} planes. Flat cube facets are a common occurrence for synthetic diamonds grown by highpressure high-temperature techniques, especially at the lower range of temperatures at which diamonds are formed. Although cube facets have been seen on natural diamonds before [6], they are so rare that some have denied that they are a genuine growth form. It is true that they have been seen only on small diamonds (< 1 mm). Their absence from larger diamonds may simply be the result of their growth rate being sufficiently large, compared with the growth rate for octahedral facets, that they grow out. Of the ten specimens studied, which all came from the Premier Mine, South Africa, and which showed more than one cube facet, some were sharp-edged truncated octahedra, while others were twins. We also report a new finding: a diamond which has grown by fibrous mode (b), followed by octahedral growth (a). This succession of events (the reverse of that giving rise to coated diamonds) underlines the necessity to re-examine the geological conditions for which fibrous growth can occur.