Staurolite-quartzite bands in kyanite quartzite at big rock,Rio Arriba County,New Mexico

Concordant igneous-looking bands of ferruginous bulk composition occur in a highly aluminous Precambrian metasedimentary series composed predominantly of kyanite quartzite. The bands consist of quartz, staurolite, and magnetite (partially martitized) with accessory amounts of muscovite, chlorite (pseudomorphous after biotite), chloritoid, apatite, and monazite. Quartz is found in three types (I–III) differing in appearance as well as in origin. Staurolite, in combination with quartz-II, shows peculiar radial sieve textures caused by mimetic crystallization after preexisting chloritoid rosettes. The chloritoid has been largely consumed, either by a reaction with hypothetical former kyanite to produce staurolite+ quartz with rock composition unchanged, or, possibly, by metasomatic introduction of oxygen (oxidation) to yield staurolite+quartz+magnetite; the remaining chloritoid, however, persisted in stable equilibrium with the other minerals of the rock. The staurolite quartzites are thus considered to represent original sedimentary bands which have undergone several stages of recrystallisation and (possibly) metasomatic modification during their metamorphic history. Their igneous aspect results from annealing crystallisation during a late static, i.e. postdeformational, thermal event of regional metamorphism.Chemical analysis of the staurolite shows no unusual features. For all staurolites plotted there is a positive relationship of the excess H⁺ over 2.0 and the Si⁺⁴-deficiency in the unit cell. This suggests partial substitution of 4 H⁺ for Si⁺⁴.The formation of staurolite in regional metamorphic rocks with excess silica, low alkali contents, and (FeO+MgO)/Al₂O₃ ratios < 1 showing chloritoid at lower grades appears to be governed, in many cases, by the reaction chloritoid+Al-silicate=staurolite+quartz+H₂O.The assemblage chloritoid-staurolite may be stable in regional metamorphism over a limited pressure-temperature range.