KORNERUPINE-GROUP MINERALS IN GRENVILLE GRANULITE-FACIES
PARAGNEISS, READING PRONG, NEW JERSEY
DAVIS A. YOUNG
Department of Geology, Geography, and Environmental Studies, Calvin College, Grand Rapids,
Michigan 49546, U.S.A.
Abstract
Prismatine, the boron-rich member of the kornerupine group, has recently been identified in the
Reading Prong in New Jersey from granulite-facies gneisses consisting of the assemblage
orthoclase mesoperthite + calcic oligoclase + prismatine + biotite + rutile ± almandine ± sillimanite
± graphite ± ilmenite. The prismatine crystals, as much as 6 cm long, are typically altered to
muscovite rich mixtures. The prismatine probably crystallized during isochemical metamorphism of originally
boron-bearing clay-rich sediments. The conditions of peak metamorphism were 670-740 C and between
6.2 and 8 kbar. No evidence remains of the boron-bearing mineral precursor of prismatine, but it is
reasonable to assume that tourmaline may have been present at lower grades. A possible mechanism for
forming prismatine is the reaction: 1.55 Ti- rich biotite + 1.0 tourmaline + 1.22 almandine + 5.23 sillimanite
3.0 prismatine + 2.55 (orthoclase + albite) + 0.46 rutile + 0.39 B2O3 + 2.05 H2O.
Keywords: prismatine, kornerupine, boron, granulite-facies
metamorphism, Reading Prong, New Jersey.