BUSTAMITE IN A ZONED CALC-SILICATE POD FROM THE SILLIMANITE-MUSCOVITE ZONE, SOUTHWESTERN NEW HAMPSHIRE

Bustamite, a Ca-Mn-Fe pyroxenoid with a three-tetrahedron repeat along Si tetrahedral chains, together with quartz, calcite, calcic pyroxene, and grossular-rich garnet, occurs in the core of a calc-silicate pod that also contains anorthite and zoisite, all enclosed in feldspathic schist of the upper member of the Silurian Warner Formation, Monadnock Mountain 7 ½’ quadrangle, New Hampshire. Anorthite and zoisite are rimmed by garnet and do not come into contact with bustamite or pyroxene. Electron microprobe analyses gave the following compositions:

bustamite Ca_2.46 Mn­_0.27 Fe²⁺_0.27 Si₃ O₉

pyroxene Ca Fe²⁺_0.59 Mn­_0.13 Mg_0.23 Si₂ O₆

garnet Ca_2.5 Fe²⁺_0.14 Mn­_0.28 Al_1.74 Fe³⁺_0.26 Si₃ O₁₂

The bustamite, distinct from wollastonite, is quite typical of high-Ca bustamites which have about 5/6 of their octahedral sites occupied by Ca and the remaining positions equally occupied by Mn and Fe²⁺.

The most likely bustamite-producing reaction is calcite + pyroxene + quartz = bustamite + CO₂. Hydrous phases such as actinolite and biotite are increasingly abundant toward the rim of the pod, whereas bustamite, calcite and pyroxene decrease toward the rim, suggesting that an influx of more water-rich fluids from the surrounding schist promoted progressive decarbonation reactions in the core. Water was readily available from prograde metamorphism in the enclosing schists, which contain mineral assemblages of the sillimanite-muscovite zone. Garnet and biotite compositions in nearby pelitic schists yield peak-temperature estimates of 635-670°. A pressure of 6.3 kbar is indicated by the composition of garnet in equilibrium with cordierite and sillimanite in a nearby rock of unusual bulk composition. Although this is the first report of bustamite from New Hampshire, the abundance of calc-silicate pods in the Warner, Littleton, and Rangeley Formations over a large area suggests the possibility of additional occurrences.