1911 Encyclopædia Britannica/Chabazite

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 φακός, a lentil) for this variety of chabazite.

EB1911 - Twinned Crystals of Chabazite.jpg
Fig. 1. Fig. 2.
Twinned Crystals of Chabazite.

The hardness of chabazite is 41/2, and the specific gravity 2.08-2.16. As first noticed by Sir David Brewster in 1830, 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, Na2) Al2(SiO4)2 + 4H2O and (Ca, Na2) Al2(Si3O8)2 + 8H2O, which are analogous to the felspars. Most analyses correspond with a formula midway between these extremes, namely, (Ca, Na2)Al2(SiO3)4 + 6H2O.

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 Plombières 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)46H2O. The formula of levynite is CaAl2Si3O10 + 5H2O.  (L. J. S.)