Synthesis of Grunerite, Fe7Si8O22(OH)2

Abstract

GRUNERITE is a major rock-forming silicate in several metamorphosed sedimentary iron formations and is an important indicator of metamorphic grade in these rocks1. There have therefore been several attempts to determine experimentally the amphibole's thermal stability limits2–6. But these efforts have been frustrated by the inability to synthesize grunerite directly from oxide starting materials. Thus data on the stability range of grunerite must be inferred from field relations and oxygen isotope work7. Flaschen and Osborn2 found that grunerite formed only as a metastable decomposition product of minnesotaite (Fe3Si4O10(OH)2) at temperatures above 480° C and concluded that grunerite did not have a stability range under the conditions investigated. Kopp and Harris8 synthesized grunerite in the system NaOH–SiO2–Fe–H2O and Schurmann5 produced the amphibole by using bulk compositions which contained 2.7 weight per cent CaO. But the chemical composition of the grunerite produced in each case may depart significantly from the ideal Fe7Si8O22(OH)2 formula.