ar shapes, hence the name phacolite
(from [Greek: phakos], a lentil) for this variety of chabazite.

[Illustration: FIG. 1. FIG. 2. Twinned Crystals of Chabazite.]

The hardness of chabazite is 4-1/2, 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 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