Zussmanite - Composition

Composition

The blueschist facies phyllosilicate mineral occurs as a result of subduction of oceanic crustal rocks and oceanic-continental margin sediments along convergent plate boundaries. The ideal formula for zussmanite is KFe13Si17AlO42(OH)14 with possible substitutions of sodium (Na) for potassium (K), in extremely small amounts (Lopes et al.,1969). The possible iron (Fe2+) substitutes are mainly magnesium (Mg) with trace amounts that could include: manganese (Mn), aluminium (Al), iron3+ (Fe3+) and titanium (Ti) (Lopes et al.,1969). Zussmanite was discovered in combination with deerite and howieite, two new minerals discovered in the Franciscan formation, Mendocino County, California. Deerite and howieite have been found at other locations while zussmanite has only been found at this type locality, making it a rare occurring mineral. Experiments have revealed that zussmanite is stable up to 600 °C at pressures between 10 kb and 30 kb and that the end members of zussmanite are orthoferrosilite, biotite and quartz. The example of the reaction is KFe13(OH)14 (zussmanite) yields 10FeSiO3 (orthoferrosilite) + ½ K2Fe6Si6Al2O20(OH)4 (biotite) + 4SiO2 (quartz) + 6H20 (water) (Dempsey et al.,1981). The manganese analogue of zussmanite, coombsite, has been found in manganese-rich siliceous rocks in the Otago Schist in New Zealand.

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