![]() ![]() ![]() A thin slice, under the microscope, would likely reveal incipient crystallites too It is important to note that obsidian is largely glass, and so lacks the pale quartz and feldspars that define most granitic rocks. Given the silica-rich bulk chemistry of obsidians, it may seem surprising that this rock is a) black, unlike the white, grey or pink tones typical of granites,Īnd b) exhibits distinct magnetism (seen in some but far from all granites). Scattered crystals (phenocrysts) of feldspar may be visible in theĭense groundmass, or incipient crystallization in spherulites, but theseįeatures are absent here. As seen in Figure 1, obsidian is black and glassy. The immediate sources are stratovolcanoes, which display styles of volcanism more explosive than the gentle effusions of basalt noted in the Reykjanes peninsula in 2021-2022 (see Lake Myvatn in northeast Iceland (Saemundsson and Gunnlaugsson, 2014, p.67 Thordarson and Hoskuldsson, 2014, pp.182-184). An occurrence of "textbook" material with conchoidal fracture is found in the Krafla volcanic system, northeast of In Iceland, it is of localised occurrence, related to the more siliceous volcanic centres. Obsidian in Iceland: Obsidian is a glassy volcanic rock of broadly rhyolitic (granitic) composition. "Rock of the Month # 259, posted for January 2023" -ġ. Smaller: 7 x 7 x 4 cm, 284.4 grams, magnetic susceptibility (n=3) averages 23.03x10 -3 SI units. Larger: 10 x 7 x 5 cm, 587.5 grams, magnetic susceptibility (n=3) averages 9.63x10 -3 SI units. Both pieces readily attract a pen magnet. The material is very fine grained to glassy, with subconchoidal fracture and a satiny to subvitreous lustre. ![]() Two pieces of obsidian from a moraine near the south coast of Iceland. ![]()
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