Distribution and characteristics of diamonds from Myanmar

Diamonds occur in headless placers at several locations within Myanmar. Twe nty-six stones from the Momeik area of northern Myanmar and 111 stones from the Theindaw area of southern Myanmar have been studied to characterise th eir morphology, crystal forms, colour, degree of resorption, surface featur es, internal structures, mineral inclusions, and nitrogen content and aggre gation state. Most stones grew originally as octahedra, but now show very h igh degrees of resorption, and highly polished surfaces, reflecting transpo rt in a magma. Etch features are abundant, and breakage and abrasion are co mmon, due to alluvial transport. Brown radiation spots are common, suggesti ng that these diamonds have a long history in surface environments. Cathodo luminescence (CL) images of plates and whole stones commonly display marked oscillatory zoning of yellow and blue bands, outlining octahedral growth z ones. Many other stones show uniform yellow CL. Syngenetic mineral inclusio ns identified thus far are mainly of peridotitic paragenesis and include ol ivine, chromite and native iron. Infrared spectroscopy studies show that si milar to 10% of the diamonds have very low-N contents (Type II diamonds). M ore N-rich diamonds show high degrees of aggregation (Type IaAB). Both type s are consistent with derivation from the upper mantle, rather than from cr ustal metamorphic sources. The primary source of these diamonds is believed to be an alkaline igneous rock (lamproitic rather than kimberlitic) but th ey may have reached their present locations via a secondary collector such as a sedimentary rock. (C) 2000 Elsevier Science Ltd. All rights reserved.