NASONITE Chapter16. Sorosilicates and cyclosilicates

NASONITE

Pb₆Ca₄(Si₂O₇)₃Cl₂
Hexagonal?, P6₃/m, a = 10.08, c = 13.27 Å, Z = 2




	Figure 16-18. Crystal drawing of nasonite from the Parker Shaft area in Franklin. Drawing is from Palache (1935) who provided crystallographic data.

Nasonite was originally described from Franklin by Penfield and Warren (1899). The relation to the apatite group was defined by Frondel and Bauer (1951). Analytical data were provided by Dunn (1985b).

Crystal structure

The crystal structure of nasonite was described by Giuseppetti et al. (1971); the space group was confirmed by Loiacono et al. (1982). Nasonite is structurally related to the apatites. The structure has Si₂O₇ groups, Pb in irregular coordination polyhedra, and calcium atoms surrounded by six oxygen atoms at the vertices of trigonal prisms forming columns along the three-fold axis. However, Brès et al. (1987) noted proof of non-hexagonal symmetry, and more work remains to be done.

Description

Nasonite occurs as massive material, granular aggregates, and small 1-2 mm euhedral crystals. The habit of the crystals, unlike those of ganomalite, is markedly prismatic. The morphology was well described by Palache (1910, 1935) (Figure 16-18) and Frondel and Bauer (1951). Crystals of nasonite vary considerably in their height:width ratio; some are stubby (Figure 16-19); and long steeply tapering prismatic crystals, some arranged like jackstraws, have also been found.




	Figure 16-19. Cluster of both symmetrical and distorted nasonite crystals from Franklin. Field of view is 2.5 mm in maximum dimension.

Nasonite is white to colorless, with adamantine to greasy luster, basal and prismatic cleavages, and a density of 5.55 g/cm³. Optically, it is uniaxial, positive, with w = 1.946 and e = 1.969. Much nasonite is fluorescent in shortwave ultraviolet, with a pale yellow to white color. It is differentiated by its high luster and mineral associations; barite has better quality cleavages and a lower luster.

Composition

Nasonite is a lead calcium silicate chloride mineral. Most analyses conform to the theoretical composition, but there is some limited substitution of Ca for Pb. A representative analysis is given in Table 8. Nasonite is the only halogen-bearing lead silicate; the Cl content is relatively constant.

Occurrence and paragenesis

In the original occurrence nasonite was associated with euhedral crystals of glaucochroite; in this assemblage it is also associated with cuspidine, pennantite, and barite. The best of these nasonite samples, described by Palache (1910, 1935), is preserved in the Smithsonian Institution.

Nasonite occurs with more species than any of the other lead silicates from the Parker Mine area. Although it occurs with high-temperature minerals, it is found as the last-formed mineral in such assemblages. Additionally, it occurs as a filler in interstices in other silicates. The most commonly associated minerals are barysilite, manganaxinite, willemite, and andradite. Less common ones are barite, glaucochroite, ganomalite, datolite, cuspidine, prehnite, johannsenite, clinohedrite, margarosanite, and hardystonite. Nasonite occurs as intimate mixtures, especially with datolite, and also with prehnite and other minerals.

Name

Nasonite was named in honor of Frank L. Nason of the New Jersey Geological Survey who, in addition to having other roles, was a consultant geologist for the New Jersey Zinc Company.

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

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