beryl, emerald, Pakistan, gems, pegmatitic emerald

An electron microprobe was used to study a suite of nine emerald samples representing four separate occurrences of gem-quality emeralds from northwestern Pakistan and neighboring Afghanistan. The study was undertaken to characterize the chemistry of the emeralds of the region, to compare the emeralds with emeralds from other locations, and to investigate local variations in composition. The results of the study could help unravel the complex and unusual parageneses of these deposits, and ultimately could aid in refinement of exploration techniques. The study also includes data for (1) green vanadian beryls from the Mohmand emerald district, (2) green chromium-rich tourmalines from the Swat emerald district, and (3) minerals in the carbonate host rock at Mingora. The electron microprobe technique allows nondestructive chemical analysis of discrete spots across a single crystal and is useful in the study of minerals. This technique can determine transition metals responsible for color and can identify compositional zoning within single grains. The principal disadvantages of the technique are that elements lighter than fluorine (atomic number 9) cannot be determined and that different oxidation states of elements such as iron cannot be distinguished. Thus, we do not report data for beryllium, a major constituent of emerald; lithium and water, which can occur in emeralds in amounts up to several weight percent; and boron, a major constituent of tourmaline. Snee and others report on the beryllium content of these emeralds in chapter 5 of this volume.