The iron-rich blueschist facies minerals " 2. Howieite

SUMMARY. The mineral howieite has a stability range that extends down to pressures at, or even below, those normally considered to represent blueschist facies meta-morphism. Its structure, lying somewhere between achain and a sheet silicate, can accommodate almost complete iron for manganese substitution as well as the more extraordinary ferrous iron for ferric iron substitution, that is tolerated by changes in the hydroxyl and water content of the unit cell. The ideal formula is NaMe2 Si12(O,OH)44, where M is principally manganese and iron but may include limited amounts of aluminium and magnesium. Four new localities for howieite have been discovered by the author and the howieites from the eight localities that have been analysed here help complete the compositional spectrum. The most curious crystallographic feature of howieite is that although the structure seems capable of considerable chemical variation, it is only those howieites of approximate NaMn3Fes(A1Mg)~Si 12(O,OH)44 composition that are well crystallized, often growing to over IO mm in length. HOWtEITE is the second of the new minerals discovered by Agrell at the Laytonville Quarry, Mendocino County, California, and named as such in Agrell et al. (1965). Since that time it has been discovered at eight other localities, four of them found within the present study (see Table I). TABLE I. t towie i te localitie