Abstract: A chalky white, fibrous mineral, in the amygdales of Tertiary basalts from the Gobbins area, Co. Antrim, has been discovered to be a new mineral. It consists of fibrous dusters of lath-shaped crystals showing straight extinction with negative elongation. Its refractive index is ɛ 1.489, ω 1.494 (both ± 0.003) and specific gravity is 2.194 (meas.), 2.147 (calc.). The mineral is tetragonal with ɛ = c (elongation); a = 10.145, c = 9.788 Å. The strongest powder lines (in Å, intensity in brackets) are: 7.10, 4.10, 3.19(100); 3.10, 2.69(80); 5.07(50); 2.64(40). A spectrographic analysis shows major Si, Al, minor Ca, Na, Cu, and trace Fe. Two chemical and one probe analysis give the ideal formula Na4(Ca,Mg,K2)Al6Si10 O32.12H2O. The name gobbinsite (approved by the IMA Commission on New Mineral Names) is suggested after the locality. The type specimen (I7881) is preserved in the Ulster Museum.
Rotation and Weissenberg photographs of gobbinsite and garronite show doubled reflections, which are explained on the basis of submicroscopic twinning on (101). Relationship between gobbinsite and garronite is discussed and evidence is presented for the occurrence of minute amounts of a merlinoite-type mineral that forms regular intergrowths with gobbinsite and garronite.