CHABAZITE  , a

mineral species belonging to the group of zeolites . It occurs as white to flesh-red crystals which vary from transparent to translucent and have a vitreous lustre . The crystals are rhombohedral, and the predominating form is often a rhombohedron (r) with interfacial angles of -85° 14'; they therefore closely resemble cubes in appearance, and the mineral was in fact early (in 1772) described as a cubic zeolite . A characteristic feature is the twinning, the crystals being frequently interpenetration twins with the principal axis as twin-axis (figs . 1, 2) . The appearance shown in fig . 1, with the corners of small crystals in twinned position projecting from the faces r of the main crystal, is especially characteristic of chabazite . Such groups resemble the interpenetrating twinned cubes of fluorspar, but the two minerals are readily distinguished by their cleavage, fluorspar having a perfect octahedral cleavage truncating the corners of the cube, whilst in chabazite there are less distinct cleavages parallel to the rhombohedral (cube-like) faces . Another type of twinned crystal is represented in fig . 2, in which the predominating form is an obtuse hexagonal pyramid (t); the faces of these flatter crystals are often rounded, giving rise to lenticular shapes, hence the name phacolite (from (Paths, a lentil) for this variety of chabazite . The hardness of chabazite is 42, and the specific gravity 2•o8-2•I6 . As first noticed by Sir David Brewster in 183o, the crystals often exhibit anomalous optical characters: instead of being uniaxial, a basal section may be divided into sharply-defined biaxial sectors .

Heating of the crystals is attended by a loss of water and a change in their optical characters; it is probable therefore that the anomalous optical characters are dependent on the amount of water present . Besides phacolite, mentioned above, other varieties of chabazite are distinguished . Herschelite and seebachite are essentially the same as phacolite . Haydenite is the name given to small yellowish crystals, twinned on a rhombohedron plane r, from Jones's Falls near Baltimore in Maryland . Acadialite is a reddish chabazite from Nova Scotia (the old French name of which is Acadie) . Chemically, chabazite' is a complex hydrated calcium and sodium silicate, with a small proportion of the sodium replaced by potassium, and sometimes a small amount of the calcium replaced by barium and strontium . The composition is however variable,and is best expressed as an isomorphous mixture of the molecules (Ca, Nae)Al2(SiO4)2+41120 and (Ca,Na2)Al2(Si3Os)2d-8H20, which are analogous to the felspars . Most analyses correspond with a formula midway -between these extremes, namely, (Ca,Na2) Al2 (SiO3) 4+6H20 . Chabazite occurs with other zeolites in the amygdaloidal cavities of basaltic rocks; occasionally it has been found in gneisses and schists . Well-formed crystals are known from many localities; for example, Kilmalcolm in Renfrewshire, the Giant's Causeway in Co . Antrim, and Oberstein in Germany . Beautiful, clear glassy crystals of the phacolite (" seebachite ") variety occur with phillipsite and radiating bundles of brown calcite in cavities in compact basalt near Richmond, Melbourne, Victoria .

Small crystals have been observed lining the cavities of fossil shells from

Iceland, and in the recent deposits of the hot springs of Plombieres and Bourbonne-les-Bains in France . Gmelinite and levynite are other species of zeolites which may be mentioned here, since they are closely related to chabazite, and like it are rhombohedral and frequently twinned . Gmelinite forms large flesh-red crystals usually of hexagonal habit, and was early known as soda-chabazite, it having the composition of chabazite but with sodium predominating over calcium (Na2,Ca)Al2(SiO3)46H20 . The formula of levynite is CaAl2SiaO1o+5H20 . (L . J .