HYALOPHANE

(K,Ba)Al(Si,Al)₃O₈
Monoclinic, a = 8.601, b = 13.005, c = 14.412 Å, b = 116.01^o, Z = 8

Hyalophane, a barium potassium aluminum silicate mineral of the feldspar group, was first recognized by Bauer and Palache (1926), and additional data were provided by Palache (1935). This material was also studied by Vermass (1953), and optical data were given by Roy (1965). Frondel et al. (1966) investigated the Ba-bearing feldspars from Franklin, provided critical reviews of previous analyses not given here, and gave X-ray and optical data. Unit-cell parameters were determined by Gay and Roy (1968), Goetz (1975), and Goetz et al. (1975) for a number of specimens; Gay and Roy included some studied by Frondel et al. (1966); and the data in the heading are for a specimen with Cn₁₂. Hyalophane has not been reported from Sterling Hill.

Description

Franklin hyalophane occurs in anhedral crystals and granular aggregates; grain size varies substantially. Hyalophane is white to pale brown; red or blue specimens may be colored by impurities or natural staining. The luster is vitreous, varying to somewhat pearly. The density varies with composition, from 2.64 g/cm³ for Cn₁₂ to 2.80 g/cm³ for Cn₃₂.

Hyalophane cleavages are perfect to imperfect. Optically, hyalophane is biaxial, negative, with indices of refraction for samples with Cn₁₈ and Cn₃₂ respectively: a = 1.529, 1.541, b = 1.532, 1.545, and g = 1.535, 1.547. Some specimens have a weak red fluorescence in shortwave ultraviolet. Hyalophane is best differentiated from other feldspars by chemical analysis; the density is helpful.

Composition

	Table 17. Chemical analyses of hyalophane and celsian.

Hyalophane is a potassium barium aluminum silicate of the feldspar group. Local material varies substantially in composition, from barian orthoclase through hyalophane to celsian with Cn₅₂. Older analyses are suspect; some modern ones are provided in Table 17.

Occurrence and paragenesis

Hyalophane occurs with calcite, andradite, franklinite, hendricksite, and a number of other minerals from the calcium-silicate units at Franklin. Few specific assemblages have been recorded. Next to microcline, hyalophane is the most abundant feldspar at Franklin. A white to colorless feldspar, associated with bustamite and vesuvianite (cyprine), was reported by Larsen and Shannon (1922). The writer’s analysis of this material shows it to be hyalophane with 9.8 wt. % BaO, 0.4 wt. % Na₂O, and 12.6 wt. % K₂O, with calcium absent. Hyalophane is also associated with barian muscovite (Dunn, 1984).

Copyright © 1995 by Pete J. Dunn			Website by Herb Yeates
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