SONOLITE Chapter15. Nesosilicates

SONOLITE

Mn₉(SiO₄)₄(OH)₂
Monoclinic, P2₁/b, a = 4.849, b = 10.611, c = 14.162 Å,
a = 100.61^o, Z = 2.




	Figure 15-17. Prismatic highly-modified sonolite crystal from Sterling Hill. Field of view is 1.6 mm in maximum dimension.

Sonolite was first described from Franklin and Sterling Hill by Cook (1969). It was subsequently described, with much compositional data, by Dunn (1985a) and by White and Hyde (1982b).

Crystal structure

The crystal structures of two local sonolite specimens previously described by Dunn (1985a) were refined by Grice and Dunn (1994); the unit-cell given above is from that study. The results indicated that Mg has a preference for the M(3) and M(1)_c atomic sites and that Zn shows a preference for sites M(1)_c, M(1)_n, and M(3) in almost equal proportions.

Description




	Figure 15-18. V-shaped twinned crystals of sonolite from Sterling Hill. Field of view is 1.2 mm in maximum dimension.

Most sonolite is massive in the ores. Such material is not commonly recognized; the general appearance and color is too similar to that of some brown tephroite and the other manganese humites to permit facile visual identification. However, the common exsolution of willemite in tephroite is wholly lacking in sonolite and is a partially useful discriminant. Fine crystals are known from secondary seams at Sterling Hill; they exhibit the forms {110} and {111} in the conventional monoclinic setting (personal communication, B. D. Sturman). Representative SEM photomicrographs of some of these crystals are shown in figures 15-17, 15-18, and 15-19. The writer has verified Franklin sonolite crystals with a habit such as is illustrated for leucophoenicite in figure 15-24 (bottom left drawing).




	Figure 15-19. Cluster of highly-modified sonolite crystals from Sterling Hill. Field of view is 0.8 mm in maximum dimension.

The color varies from light brown to dark brown and uncommonly pink; the luster is vitreous; and there is no discernible fluorescence in ultraviolet. Sonolite is biaxial, negative, 2V = 85^o, with a = 1.752, b = 1.765, and g = 1.777 (personal communication, B. D. Sturman). X-ray diffraction is the best method for verification.

Composition

Sonolite is a manganese silicate hydroxide mineral of the manganese-humite group. Three microprobe analyses of sonolite are presented in Table 1; 19 others were given by Dunn (1985a). The analysis of specimen #149037 is typical of the composition of sonolite from primary ores from Franklin; they have near end-member compositions with but minor solid solution of other cations.

The analysis of specimen #143755 is typical of the composition of secondary crystals of sonolite from Sterling Hill; this specimen was used in the studies by Dunn (1985a) and Grice and Dunn (1994) and was also studied by B. D. Sturman, as noted above. The chemical composition of these crystals is remarkably similar in several respects to that of the alleghanyite secondary crystals (Francis, 1985a) discussed above.

The third sonolite analysis (#13333) is of reaction rims of dark-brown sonolite on tephroite crystals from Sterling Hill. Because the cation ratios of the underlying tephroite are similar (Dunn, 1985a), these sonolite rims likely formed by hydration of the tephroite cores. Similar material was restudied by Grice and Dunn (1994).

Unlike secondary occurrences of willemite and some other species, secondary crystals of the Mn-humites admit more, not less, solid solution of octahedral cations than primary material.

Occurrence and paragenesis




	Figure 15-20. Massive sonolite (black) between tephroite (gray) and calcite (white) from Sterling Hill. Specimen is 15 cm in maximum dimension. Smithsonian Institution, #R19123. Photo by the author.

Sonolite occurs at both Franklin and Sterling Hill as both primary and secondary material. At Franklin sonolite occurs as an interstitial, granular constituent of the common calcium-poor ore, associated with willemite and franklinite; large 10 cm masses are known. At Franklin, the associated minerals variously include manganosite, willemite, sonolite, hetaerolite/franklinite exsolution intergrowths, jerrygibbsite, leucophoenicite, and also zincite which commonly has abundant hetaerolite exsolution. Specific assemblages were noted by Dunn (1985a). Sonolite is associated with willemite, andradite, and calcite in veins in ore, but the predominant occurrence is as grains in granular franklinite-willemite ore specimens.

Euhedral secondary crystals (Figures 15-17, 15-18, and 15-19) occur in seams and varied vein assemblages at Sterling Hill. The crystals occur on calcite-rich franklinite/willemite ore with no associated zincite; there is minor secondary sphalerite associated. The sonolite crystals are complexly formed, of prismatic habit, and at least a hundred specimens were preserved.

Sonolite also occurs between the 430 and 800 levels, and especially on the 700 level, in the east limb at Sterling Hill, as locally abundant dark-brown reaction rims on zincian tephroite crystals which have abundant willemite exsolution (Figures 12-33, 12-34, and 15-9). The tephroite crystals are up to 15 cm in size, but most are 1-3 cm; the reaction rims are up to 3 cm thick, although most are thinner, being several mm thick. The associated minerals are zincite, franklinite, willemite, and calcite. Other assemblages from Franklin and Sterling Hill were listed by Dunn (1985a).

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