Morphology of monocrystalline diamond and its inclusions

Since first being discovered in India over 2000 years ago, the most striking and universally recognized shape of an uncut diamond is the octahedron. This shape was revered from early times, in part because of the hardness of the crystal, but principally because of the ability of that crystal to display a spectrum of color in its uncut state. To ancient people, this property gave the crystal mystical powers and a diamond octahedron soon became a talisman believed to protect the wearer from pestilence or ensure survival in battle. The octahedron reached Europe at the beginning of the Roman Empire (27 BC–AD 476) usually set in gold rings and always in an uncut form. For centuries thereafter diamond was owned exclusively by the nobility and broader ownership developed only after new discoveries in Brazil and South Africa in the 18th and 19th Centuries respectively. Since that time, the octahedron along with other important shapes such as the macle (a twin form), the dodecahedron (a secondary form), and even irregular diamonds, have helped create a world-wide diamond jewelry industry with retail sales worth USD 78.13 billion in 2018 (Tacy 2018).

For some diamonds, it was not always an easy journey. Once released from their primary rock, diamonds are affected by processes such as dissolution that can change their morphology (e.g., to rounded dodecahedral shapes) and/or impose new surface features. Additionally, about one percent of them had the responsibility of carrying synchronously formed mineral inclusions, subsequently so beloved by diamond researchers. Diamond is, however, an extremely resilient mineral and can survive considerable physical and chemical challenges. Rough diamonds have a variety of morphologies, surface features, and mineral or fluid inclusions that mean no two rough diamonds are exactly alike. All these features can tell an interesting story of diamond’s geological history deep within Earth—both in the mantle rocks where diamonds grew and during their subsequent volcanic transport. Based on diamond morphology, distinctions can often be made between suites of rough diamonds from different kimberlites (e.g., Harris et al. 1975, 1979). This likely reflects specific mantle regions being sampled during kimberlite eruption, combined with the volatile composition of the kimberlite resulting in specific secondary morphologies. Also, surface features of diamonds are heavily influenced by the composition and temperature of the dissolving fluid/melt, and are another tool that mantle geochemists can use to study carbon- bearing fluids/melts in the deep Earth. Investigating the differences in diamond morphologies and surface features between different kimberlites, therefore, can potentially help geologists understand the compositional variety of kimberlite magmas, getting a step closer to the primary melt composition, which has been a longstanding enigma in mantle geochemistry. This chapter is divided into four sections and leads the reader through the external and internal morphologies, the surface features and finally the characteristics of the mineral inclusion content of this amazing mineral.

Authors: Jeff W. Harris, Karen V. Smit, Yana Fedortchouk, Moreton Moore

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