The zeolites form a large family of hydrous silicates which show close similarities in composition and in their associations and mode of occurrence. They are silicates of Al with Na and Ca as the important cations. They are secondary minerals found typically in cavitiies and veins in basic igneous rocks. The zeolite group is compromised of about 25 species. The more common species will be discussed.
NATROLITE, Na2(Al2 Si3 O10).2H20, Monoclinic sphenoidal; prismatic or acicular habit. White to colorless, sometimes tinted red to yellow. Recognized chiefly by its radiating crystal structure. Notable occurrences: Bergen Hill, N.J.; various localities in Nova Scotia.
CHABAZITE, (Ca,Na)2 (Al2 Si4 O12).6H20, Hexagonal scalenohedral; May be recognized by its rhombohedral crystals. Color: white, yellow, pink to peach. A secondary mineral found in association with other zeolites, lining cavities in basalts and diabases. Notable localities include W. Paterson, N.J.; Goble Station, Oregon and localities in Nova Scotia.
ANALCIME, Na(Al Si2 O6).H20, Isometric hexoctahedral; crystals usually trapezohedral in habit, resembling garnet and leucite. Colorless to white; transparent to translucent. Occurrences are the Cyclopean Islands, near Sicily; the Lake Superior Copper District; and Table Mountain, Golden, Colorado.
STILBITE, Ca(Al2 Si7 018).7H20, Monoclinic prismatic; commonly in sheaf-like aggregates. Color: white, peach, yellow, red to brown. Stilbite is a mineral of secondary origin found in cavities in basalts and related rocks. Magnificent specimens come from the Jewel Tunnel, Poonah, India; also from Kilpatrick, Scotland; and Paterson, N.J.
HEULANDITE, Ca(Al2 Si7 018).6H20, Monoclinic Prismatic; Colorless, white, yellow, pink to red; transparent to translucent. Heulandite is a secondary mineral found in cavities in basalt in association with other zeolites and calcite. Notable occurrences are Paterson, N.J.; Nova Scotia; and the Harz Mts., Tyrol, Austria.
LAUMONTITE Ca(Al2 Si4 012.4H20, Monoclinic Drismatic; occurs predominately as small, stout prismatic crystals with oblique terminations. Occurs in veins and cavities in igneous rocks. Color: white to dull gray or chalky. Notable occurrences are the Virginia Quarry, Leesburg, Virginia with beautiful white crystals in association with excellent bottle-green prehnite; Rio Grande do Sul, Brazil; Cochise Co., Arizona.
HARMOTOME, Ba(Al2 Si6 O16).6H20, Monoclinic, occurring often as interpenetrant twins; white to colorless, or tinted gray. Occurs with other zeolites in basalts. This species is rare in U.S. localities. It does, however, occur as small, glassy crystals from West Chester Co., New York. It also occurs at the Bellesgrove Mine, Strontian, Argyllshire, Scotland and at Andreasburg, Harz Mts., Germany.
SCOLECITE, Ca(Al2 Si3 010).3H20, Monoclinic, occurring as finely-fibrous white crystal clusters with a divergent or radiating structure. Occurs with other zeolites in basalts. Occurs at the Jewel Tunnel, Poonah, India and at Bremerton, Washington.
MESOLITE Na2 Ca2 (Al2 Si3 010)3.H20, Monoclinic with an appearance similar to scolecite. Occurs as white, radiating crystal aggregates in cavities in igneous rock. Notable localities include Teigarhorn, Iceland; Baker Co., Oregon; and Melbourne, Australia.
THOMSONITE, NaCa2(Al Si)10 O20.6H20, Orthorhombic; rosettes of whitish hue occur in amygoaloidal basalts on the shore of Lake Superior near Grand Marais, Cook Co., Minnesota. Also occurs at Table Mountain, Jefferson Co., Colorado and at Bernitzgrun, Vogtland, Germany.
Distribution and Associated Minerals
Approximately 25 zeolite species are recognized, but only a handful occur in good specimens. Zeolites are widespread and abundant as low-temperature species, usually among the last minerals to form in the paragenetic sequence. The finest examples occur in cavities in basalts and diabases. Basalts and diabases are prized by the construction industry for the production of concrete. For this reason, specimens continue to be supplied from the quarries near population centers where the formations are exposed.
Although most abundant as fine specimens in the rock types mentioned, zeolites also occur in lesser quantities in the upper portions of ore veins, in small amounts in granitic pegmatites, and in the near-surface portions of some sedimentary rock formations.
Common associated minerals include prehnite, apophyllite, datolite, pectolite, calcite, pyrite, epidote, sphene, actinolite, and quartz.
Sinkankas, J., Mineralogy for Amateurs, (1964), pp. 464-474.
Dana, J., Dana's Manual of Mineralogy, 17th ed., (1966), pp. 506-511.
Berry & Mason, Mineralogy, (1959), pp. 494-499.