Abstract
Chloritoid specimens with contrasting metamorphic histories have been studied with high-resolution and analytical transmission electron microscopy. The observations show that chloritoid, an orthosilicate, can intergrow coherently with several sheet silicates and that a wide variation in microstructures can result from different metamorphic histories.
Chloritoid crystals in garnet- and staurolite-grade, aluminous metapelites from the Green Mountains massif near Jamaica, Vermont, experienced a complex polymetamorphic history including both prograde and strong retrograde events. They contain narrow lamellae of chlorite (generally 8.5 to 20 nm wide), stilpnomelane (generally 3.6 to 14.4 nm wide), as well as larger chlorite, paragonite, and berthierine lamellae (80 to 400 nm wide). The intergrown minerals exhibit stacking disorder and mixed layering. In many cases, almost perfect intergrowth on (001) is observed, with structural continuity across many of the grain boundaries. In such intergrowths, b of chloritoid (C2/c) is parallel to a of chlorite (C2/m) and b of stilpnomelane. All of the minerals can intergrow readily on (001) because of the dimensional similarity between the unpolymerized tetrahedral layer of chloritoid and the tetrahedral sheets of the other minerals. Although the intergrowth textures do not provide conclusive evidence, it is suggested that these coexisting layer structures formed by retrograde replacement of the chloritoid, rather than by parallel growth or prograde reaction of chlorite to form chloritoid. The nucleation of arrays of very narrow chlorite lamellae throughout the chloritoid may have resulted from supersaturation with respect to Mg during retrograde metamorphism.
In contrast to the higher-grade Green Mountains samples, chloritoids from the Taconic Range, Vermont, grew at the peak of a single prograde metamorphic event and are texturally simple, with no intergrown sheet silicates. Although much of the chloritoid nucleated and grew in a muscovite, paragonite, and chlorite matrix, it did not replace the sheet silicates by direct topotactic reaction. The reaction that defines the chloritoid isograd in the Taconic Range occurs by a dissolution and reprecipitation mechanism.
