The Zeolites
The zeolites are a group of white minerals, with a pearly luster, light weight, and easy solubility in acids; which, because their contained water is lightly held, readily boil before the blowpipe. They are all secondary minerals, which result from the decomposition of feldspars, when exposed to weathering. They are almost universally found in seams and cavities of disintegrating lavas. From a group of a dozen or so, three are common enough to be considered here. They may be found by watching such places, as where trap rock is being quarried for road material, or being blasted for any reason.
[Analcite]
Na₃Al₂Si₄O₁₃ + 2H₂O
[Pl. 44]
Occurs as trapezohedrons in seams and cavities in lavas; hardness, 5.5; specific gravity, 2.2; colorless, white or pink; luster vitreous; transparent on thin edges.
Analcite usually occurs in the 24-sided form, known as a trapezohedron, as illustrated in figure A, [Pl. 44]; but it may also occur in cubes with the three faces of the trapezohedron on each corner. Small crystals are often colorless, but the larger ones are either white or pink, and are opaque. While the form is the same as that of garnets, the color, lesser hardness, and the occurrence in lavas will serve to distinguish this mineral. If placed in hydrochloric acid analcite dissolves to a gelatinous mass.
It is always found in seams and cavities in lavas, as at Bergen Hill and Weehawken, N. J., Westfield, Mass., in the Lake Superior region, etc.
[Natrolite]
Na₂Al₂Si₃O₁₀ + 2H₂O
[Plate 44]
Occurs as bristling crystals in seams and cavities in lavas; hardness, 5.5; specific gravity, 2.2; colorless; luster vitreous; transparent on thin edges.
Natrolite occurs as beautiful bristling tufts of needle-like crystals, each crystal an orthorhombic prism with a very low pyramid on the end. This mineral is so easily fusible that it can be melted in a candle flame, giving to the flame the characteristic yellow color due to sodium. In hydrochloric acid it dissolves to a gelatinous mass. It is always a secondary mineral in cavities and seams in disintegrating lavas, and the tuft-like manner of growth is so characteristic, that once seen, it will always be recognized.
Natrolite is found at Weehawken and Bergen Hill, N. J., at Westfield, Mass., in the Lake Superior region, etc.
[Stilbite]
H₄(CaNa₂)Al₂(SiO₃)₆ + 4H₂O
[Pl. 44]
Usually occurs in sheaf-like bundles of fibrous crystals; hardness, 5.5; specific gravity 2.2; colorless to white, yellow or brown; luster vitreous; transparent on thin edges.
Stilbite crystals are really monoclinic, but on account of almost universal twinning, appear as if orthorhombic. Like the two foregoing minerals, stilbite is found in the seams and cavities of disintegrating lavas. It is readily recognized by its habit of forming in sheaf-like bundles of fibrous crystals. It may also, but more rarely, occur in radiating masses. In hydrochloric acid it is completely dissolved. It is found in lavas, at Weehawken and Bergen Hill, N. J., in the Lake Superior region, etc.