New biopyriboles from Chester, Vermont; I, Descriptive mineralogy

Four new magnesium-iron chain-silicate minerals have been identified from a metamorphosed ultramafic body near Chester, Vermont. They occur with anthophyllite, cummingto-nite, and talc between the chlorite and actinolite zones at the boundary of the body. The cell parameters of the minerals are diagnostic: (1) jimthompsonite is orthorhombic, Pbca, a = 18.6, b = 27.2, с = 5.30A; (2) clinojimthompsonite is monoclinic, C2/c, a = 9.87, b = 27.2, с = 5.32A, β = 109.5°; (3) chesterite is orthorhombic, A2₁ma, a = 18.6, b = 45.3, с = 5.30A; (4) an unnamed mineral is monoclinic, A2/m, Am, or A2, a = 9.87, b = 45.3, с = 5.29A, β = 109.7°. The physical and optical properties are close to those of low-Ca amphiboles. The cleavage angles (37.8° and 44.7°) are lower than those of amphiboles, and intergrowths of the minerals with anthophyllite and cummingtonite are petrographically distinctive. The minerals are biopyriboles and are chemically intermediate between anthophyllite and talc. The ideal chemical composition for jimthompsonite and clinojimthompsonite is (Mg, Fe)₁₀Si₁₂O₃₂(OH)₄, and that of chesterite is (Mg, Fe)₁₇Si₂₀O₅₄(OH)₆. The new minerals might easily be confused with amphiboles if their electron microprobe analyses were considered alone.