GANOPHYLLITE

(K,Na,Ca)₂Mn₈(Si,Al)₁₂(O,OH)₃₂(OH)₄.8H₂O Monoclinic, A2/a,
a = 16.59, b = 27.08, c = 50.36 Å, b = 94.17^o, Z = 24

	Figure 18-34. Prismatic ganophyllite crystals on rhodonite from Franklin. Field of view is 1.2 mm in maximum dimension.

Ganophyllite, first found at Pajsberg, Sweden, was described from Franklin by Palache (1910, 1935), Larsen and Shannon (1922, 1924), and Smith and Frondel (1968). The crystal structure was described by Jefferson (1978) and Kato (1980) using Swedish material. Ganophyllite was chemically reinvestigated by Dunn et al. (1983b). It is a layer silicate closely related to bannisterite and is known from Franklin, but not Sterling Hill. The most recent review is by Guggenheim and Eggleton (1988).

Description

Ganophyllite occurs in two distinct crystal habits at Franklin. The first of these, described by Larsen and Shannon (1922), consists of prismatic crystals, which may form radial, cm-sized sprays, or be randomly distributed (Figures 18-34 and 18-36). The second habit consists of pseudohexagonal 1-2 mm platy crystals resembling some phlogopite (Figure 18-35).

	Figure 18-35. Pseudohexagonal platy ganophyllite crystals with crystals of clinohedrite (bottom right-center), and calcite (white), on franklinite ore from Franklin. Specimen is 10 cm in maximum dimension. Smithsonian Institution, #C6247. Photo by the author.

Ganophyllite is light brown, brittle, with perfect {001} cleavage, a density of 2.77 g/cm³, and a vitreous luster. Optical data were given by Larsen and Shannon (1924) and Smith and Frondel (1968); the latter reported a small 2V, with a = 1.571, b = 1.610, and g = 1.611. Dispersion is r < v; pleochroism is yellow-brown in all three principal vibration directions; and the absorption is X > Y = Z. There is no discernible fluorescence in ultraviolet radiation. Ganophyllite is distinguished from similar species in part by its brittleness.

Composition

Ganophyllite is a potassium sodium calcium manganese aluminosilicate hydroxide hydrate mineral. Franklin material has been thoroughly analyzed, and a representative analysis is presented in Table 14. There are no compositional differences for samples of the two crystal habits. Unlike bannisterite, which has broad solid solution of Fe, Mg, and Zn for Mn, ganophyllite is principally an K-Mn-Al silicate with little substitution among the octahedral cations. The substitution of Al for Si is limited, and Mn is apparently limited to 7 of the 8 octahedral cations. The large cations are apparently ordered, and the water content, which is very loosely bonded, may vary, as in some zeolites and smectites. Dunn et al. (1983) suggested that ganophyllite might behave like a zeolite, and this was confirmed by Guggenheim and Eggleton (1986) and Komarneni and Guggenheim (1988). 

Occurrence and paragenesis

Most of the samples studied by this writer were from two major assemblages, each representing one of the two crystal habits. The first and third assemblages reported by Palache (1935) were not observed by this writer. Other occurrences are rare and represented by only a few specimens.

	Figure 18-36. Elongate, prismatic ganophyllite crystals from Franklin. These are commonly associated with manganaxinite and marsturite, which are not shown here. Field of view is 0.8 mm in maximum dimension.

The assemblage described by Larsen and Shannon (1922b, 1924) consists of pink rhodonite crystals with marsturite rims (Figures 17-17 and 17-19), yellow manganaxinite crystals (Figure 16-17), elongate marsturite crystals (Figures 17-15, 17-16, and 17-18) and prismatic sprays of ganophyllite (Figure 18-36); atacamite is found rarely. This assemblage was further described by Dunn and Leavens (1986); the mineral called “bustamite” by Larsen and Shannon is now known to be marsturite. This material was acquired by major museums in the 1920’s, but is not well represented in local collections.

The second assemblage is represented in local Franklin area collections, but is not well represented among the older holdings of the major museums. It might have been acquired in the 1950’s when the Palmer Shaft pillar was removed and abundant amounts of lead silicates were recovered. Thus, this second assemblage might not have been known to earlier investigators. The bulk of the available ganophyllite specimens are from the area of a vein assemblage described by Baum in Hurlbut and Baum (1960) and from which charlesite (Dunn et al., 1983) was reported. Ganophyllite occurs as veins in ore; the examined specimens consist of both fresh and recrystallized material. Veins of fresh ganophyllite have hendricksite wall contacts and contain small amounts of rhodonite and willemite, the latter in very irregular segregations. Clinohedrite is present in small amounts and is in the center of the veins, crystallizing late in the sequence (Figure 18-35).

Altered and recrystallized specimens of this second assemblage show the same vein minerals and additionally show ganophyllite forming directly as rims on decomposed hendricksite. Pre-existing willemite is recrystallized and occurs as colorless to light-green prismatic crystals. In addition to these minerals, the altered assemblage is host to a large number of accessory minerals such as prehnite, thomsonite, roeblingite (Figure 18-43) (Dunn, 1982a), charlesite (Figures 24-7 and 24-8), xonotlite, and many others. These likely formed under hydrated and mobilizing conditions; large cations (Pb,Ba,Ca,Na,K) are quite prominent, crystallizing late in this assemblage.

One other assemblage, consisting of ganophyllite in andradite and calcite, was observed; it might be a replacement of hendricksite. Others may be known.

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