PREHNITE

Ca₂Al₂Si₃O₁₀(OH)₂
Orthorhombic

Prehnite was first reported from Franklin by Palache (1935); it had been studied at the end of the last century by C. H. Warren when the minerals from the Parker Dump were first encountered and a number of new species described. There have been no additional published studies. It has not been reported from Sterling Hill.

Description

	Figure 18-42. White prehnite from Franklin, associated with andradite (dark gray) and franklinite (black). Specimen is 11 cm in maximum dimension. Smithsonian Institution, #C5172-3. Photo by the author.

Franklin prehnite in its several habits has an appearance wholly atypical for the species. Here it occurs as pearly, lamellar, platy crystals and aggregates (Figure 18-42), which vary substantially in grain size. It also occurs as spherical (2-4 cm) radial sprays of prismatic crystals. It is commonly white to grayish white to colorless, but some tight radial sprays have pink stains, the two colors forming pink-and-white concentric bands. Cleavage is imperfect; the density is 2.89-2.96 g/cm³ (Palache, 1935); and the luster varies from pearly to vitreous. Optically, prehnite is biaxial, positive, with a = 1.617, b = 1.625, and g = 1.643. Some prehnite is fluorescent in ultraviolet (Bostwick, 1982) with an orange-pink color in shortwave. Local prehnite is best distinguished from similar species using X-ray and optical methods.

Composition

Prehnite is a calcium aluminum silicate hydroxide mineral. An analysis by Lawson Bauer of the radial, pink-stained aggregates yielded: SiO₂ 40.77, Al₂O₃ 25.46, CaO 23.08, ZnO 2.36, MgO 0.66, FeO 1.00, MnO 1.06, H₂O 5.69, total = 100.08 wt. %. If it is free from admixture of other phases, it suggests substitution of Al for Si at near the known limits and minor substitutions of Mn, Mg, Fe, and Zn for one eighth of the octahedral cations. Bauer also analyzed the more common Franklin prehnite and found 4.11 wt. % MnO, with FeO and MgO both < 0.3 wt. %.

Occurrence and paragenesis

The main assemblage for Franklin prehnite is from the Parker Mine and consists of large massive specimens composed mostly of prehnite, pectolite, margarosanite, manganaxinite, franklinite, and andradite, with minor amounts of willemite, mica, hancockite, xonotlite, and clinohedrite.

A less abundant occurrence of Franklin prehnite is one consisting of spherules, up to 4 cm in diameter, of radiating prismatic crystals. This material closely resembles the common habit of much pectolite from zeolite assemblages elsewhere. The few specimens seen are associated with an unanalyzed brown mica. A notable feature of these spherules is concentric bright-pink bands, sometimes irregular, which are apparently the result of staining by an unknown agent. Some prehnite may be mislabeled as wollastonite or pectolite in older collections.

Copyright © 1995 by Pete J. Dunn			Website by Herb Yeates
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This page created: January 14, 2001