Pyralspite Garnets in Volcanic Rocks

Abstract

A pyralspite garnet from a cavity in rhyolite at the Thomas Range, Utah, U., S., A., was analysed as shown in Table 1., It is high in Mn content, being mainly composed of the spessartine and almandine molecules., Pyralspites occurring as porphyritic minerals in volcanic rocks are poorer in Mn than those from cavities and lithophysae in volcanic rocks (Fig., 1)., Pyralspites from granites are generally poorer in Mn than those from pegmatites (Fig., 2)., According to a crystallo-chemical consideration, pyralspite poor in Mn is stable under a narrower range of physico-chemical conditions than those richer in Mn., The physico-chemical conditions under which porphyritic minerals and granites were formed, were more favourable for the generation of pyralspite than those under which cavities, lithophysae, and pegmatites were formed., Therefore, pyralspite poor in Mn could be generated as porphyritic minerals and in granites, but not in cavities, lithophysae, and pegmatites.,