What is "icelandite"

Icelandite is a type of volcanic rock, an iron rich, aluminium poor andesite. Icelandites are between rhyodacite and tholeiitic basalt in composition and contain andesine, hypersthene and augite, with a silica (SiO) content greater than 60%.

The name was coined by the British geologist Ian S. E. Carmichael (who later became professor at the University of California, Berkeley) while working around 1960 on his PhD thesis at a Cenozoic volcano near the parsonage Þingmúli in East Iceland. For continental cogenetic series of volcanic rocks it is generally the case that the concentration of iron decreases with increasing silica content, but at Þingmúli the opposite was true, leading Carmichael to the conclusion that the iron-rich intermediate rock deserved its own name, icelandite.