Micromorphology and internal structure of apographitic impact diamonds: SEM and TEM study

Micromorphology, internal structure and phase composition of perfect apographitic crystals of impact diamond — paramorphoses were investigated using goniometry, SEM and TEM. X-ray and electron diffraction analyses of the paramorphoses evidence that they are all crystalline and textured. They consist of up to 50 vol.% lonsdaleite and diamond each with minor graphite. The paramorphoses mimic the morphology of the original graphite crystals. On the (0001) planes of the paramorphoses striations caused by twinning as well as bulges on (112¯0) planes together with various micro- and nano-features of growth and etching are observed. Twin striations on (0001) planes of the paramorphoses create triangular-shaped ledges. The walls of the ledges correspond to {112¯1} and {1.12¯.15} planes. The growth and etch figures are strictly oriented along (112¯0). Etch pits with trigonal or hexagonal symmetry are characteristic for diamond whereas rectangular shaped etch pits and growth plates are typical for lonsdaleite. Different types of etch and growth figures allow – at least qualitatively – discrimination between diamond and lonsdaleite. The presence of lonsdaleite and diamond already can be expected from the morphological characteristics observed in SEM images. The orientation of the growth figures and etch pits on the (0001) planes (surfaces) with respect to the hexagonal contours of the paramorphoses suggests that the (0001) planes of the precursor graphite were oriented parallel to (101¯0) planes of lonsdaleite and to (111) planes of diamond. The orientation relationship of the three phases establishing the paramorphoses are confirmed by electron diffraction data from TEM foils cut oriented from the paramorphoses. The internal structure of the paramorphoses indicates that shock compression initiated polysynthetic twinning of the precursor graphite followed by the phase transformation to lonsdaleite and/or diamond. The twinning plane in graphite is (112¯1), and twinning occurs by a rotation of the graphite layers at 20°48′ relative to the basal plane. These twin-related bands in the paramorphoses are predominantly composed of lonsdaleite (0.3–1.0 μm wide) and those composed of diamond are 1.5–2.5 μm wide. The observed bulges on the (0001) planes of the paramorphoses and the orientation relationship of graphite, lonsdaleite and diamond structures suggest a martensitic transformation of graphite to lonsdaleite and/or diamond.

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► The perfect apografitic impact diamonds from Popigai crater (Russia) were studied.
► Their micromorphology, internal structure and phase composition were investigated.
► We present the characteristics of growth and etch forms on the (0001) of crystals.
► The presence of lonsdaleite and diamond can already be estimated from SEM images.
► The characteristics of polysynthetic twins in impact diamond are presented.