Zussmanite

Zussmanite (K(Fe2+,Mg,Mn)13(OH)14) is a hydrated iron-rich silicate mineral. Zussmanite occurs as pale green crystals with perfect cleavage. Zussmanite (K(Fe2+,Mg,Mn)13(OH)14) is a hydrated iron-rich silicate mineral. Zussmanite occurs as pale green crystals with perfect cleavage. It was first described in 1960 by Stuart Olof Agrell in the Laytonville quarry, Mendocino County, California. Zussmanite is named in honor of Jack Zussman (born 1924), Head of the University of Manchester’s Department of Geology and co-author of Rock-Forming Minerals. In the Laytonville quarry, Zussmanite occurs in metamorphosed shales, siliceous ironstones and impure limestones of the Franciscan Formation. It is a location of high pressure and low temperatures where blueschist facies metamorphic rocks occur. This is also the locality in which Deerite and Howieite were first discovered. This type of locality also produces micas, which have a similar structure as zussmanite. The locality in which zussmanite occurs is one of ultra high to high pressure and low temperatures. This Barrovian type of metamorphism is usually distinguished by the P/T range rather than the ranges in pressure and temperatures (Miyashiro et al.,1973). The three principal Barrovian types are low P/T type, medium P/T type, and high P/T type. The high P/T type, referred to as glaucophanic metamorphism, is characterized by the presence of glaucophane and forms glaucophane schists (Miyashiro et al.,1973). Glaucophane schists, commonly referred to as blueschist-facies, result from metamorphism of basaltic rocks and are usually located in folded geosynclinal terranes (Deer, Howie, Zussman et al.,1992). Glaucophane schists are characterized by low temperature (100–250 °C) high pressure (4-9 kbar) metamorphism (Deer, Howie, Zussman et al.,1992). Zussmanite is commonly found with stilpnomelane and quartz, usually forming abundant porphyroblasts up to 1 mm in size, in the newly discovered locality in Southern Central Chile ( Massonne et al., 1998). 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) + ​1⁄2 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.