Summary: Two monoclinic minerals formerly classified as ganophyllite have been differentiated on the basis of single-crystal X-ray data. True ganophyllite occurs at the Harstig mine, Pajsberg, Sweden (a, 16·60 ± 0·05 Å; b, 27·04 ± 0·08 Å; c, 50·34 ± 0·15 Å; β, 94° 10′ ± 10′; space group A 2/a), at the Benallt mine, Caernarvonshire, Wales, and in Aroostook County, Maine. Bannisterite, a new mineral, occurs at Franklin, New Jersey (a, 22·20 ± 0·07 Å; b, 16·32 ± 0·05 Å; c, 24·70 ± 0·08 Å; β, 94° 20′ ± 10′; space group A2/a) and at the Benallt mine, Wales. Both minerals have similar pseudocells: a, 5·53 Å; b, 3·3 Å; c, 25 Å; β, 94° The chemical analysis of ganophyllite from the Harstig mine, Sweden, is: SiO₂, 39·67 %; Al₂O₃, 7·95; Fe₂O₃, 0·90; MnO, 35·15; CaO, 1·11; MgO, 0·20; PbO, 0·20; K₂O, 2·70; Na₂O, 2·18; Li₂O, trace; H₂O, 9·79; total, 99·85. The chemical analysis of bannisterite from Franklin Furnace, New Jersey, is: SiO₂ 46·20 %; Al₂O₃, 4·74; MnO, 23·02; FeO, 6·40; ZnO, 4·67; CaO, 1·52; MgO, 1·99; Na₂O, 0·29; K₂O, 1·21; H₂O, 9·74; total, 99·78.

Both ganophyllite and bannisterite show a structural resemblance to stilpnome-lane from Deer Isle, Maine, in projection on selected zones. All three have a micaceous cleavage parallel to {001} in the orientations here taken. Stilpnomelane is triclinic, pseudotrigonal, and pseudomonoclinic. In the triclinic cell, a = b, 22·05 Å ± 0·06 Å; c, 17·70 ±0·06; α, 124° 49′; β, 95° 58′; γ, 120° 00′ (angles ± 5′). In the trigonal subcell, a′, 3·2 Å; c′, 36·4. Small distorted trigonal subcells for ganophyllite have a′, 3·38 Å; c′ 37·7, and for bannisterite have a′, 3·264 and c′, 37 Å.