VESUVIANITE Chapter16. Sorosilicates and cyclosilicates

VESUVIANITE

Ca₁₉(Al,Mg,Fe,Cu,Mn,Zn)₁₃(Si,Al)₁₈O₆₈(OH,F)₁₀
Tetragonal, P4/nnc, a = 15.532, c = 11.776 Å, Z = 2




	Figure 16-22. Crystal drawing of vesuvianite from Franklin. Drawing is from Palache (1935) who provided crystallographic data.

Vesuvianite was first reported from Franklin by Vanuxem and Keating (1822b), but Palache attributed such early reports to mistaken identifications of uvite in marble. Vesuvianite was later described by Penfield (1899) as red crystals in nasonite. Bright blue vesuvianite, first found by Palache in 1905, was subsequently found in abundance in 1922 and was studied by Shannon (1922), who called it cyprine, and by Lewis and Bauer (1922). Palache and Bauer (1930) reported the occurrence of a purported beryllian vesuvianite, Be-vesuvianite; Palache (1935) called this material beryllium vesuvianite. Arem and Burnham (1969) showed this “beryllian” vesuvianite to have the space group given above for blue Franklin vesuvianite.




	Figure 16-23. Simple prismatic vesuvianite crystal from Franklin. Field of view is 1.0 mm in maximum dimension.

Blue Franklin vesuvianite was analyzed and subjected to a crystal structure refinement by Allen (1985); similar material was used for the crystal structure refinement by Fitzgerald et al. (1986) who obtained the same space group. Other local samples were studied by Allen and Burnham (1992), who classified Franklin material as “high” vesuvianite, and by Groat et al. (1992). Vesuvianite is found in moderate amounts at Franklin and is perhaps less common at Sterling Hill.

Description

Franklin vesuvianite occurs in a wide variety of habits; it is known in fine crystals and in radiating and fibrous crystal aggregates. The crystals figured by Palache (1935) are among the best and are of simple morphology (Figure 16-22). Crystals from another assemblage are shown in figures 16-23 and 16-24.




	Figure 16-24. Prismatic vesuvianite crystal (right) with leucophoenicite (left) from Franklin. Field of view is 0.8 mm in maximum dimension. See figure 15-27.

Franklin vesuvianite has much variance in color: it is red, reddish-brown, brown, green, deep bright green, violet, and bright blue; Sterling Hill vesuvianite is greenish brown. The luster is vitreous, cleavage is indistinct, and the density is 3.385 g/cm³ for reddish brown crystals. Optically, it is uniaxial, negative, with w = 1.713 and e = 1.705 for blue crystals; pleochroism is moderate. There is no discernible fluorescence in ultraviolet.

Composition

Vesuvianite is basically a calcium magnesium aluminum silicate hydroxide mineral; small amounts of other cations are essential, and at Franklin, these can be Fe, Mn, Cu, or Zn. The formula given in the heading is that of Fitzgerald et al. (1986), determined for a blue vesuvianite; the supporting data were modified by Fitzgerald et al. (1992). Some analyses of Franklin vesuvianite are given in Table 7. These show some of the extent of solid solution of cations, including Mn, Mg, and Cu, and the maximum Mn-content (4.1 wt. %); other data were given by Fitzgerald et al. (1992). Some trace elements were reported by Groat et al. (1992). Four anomalous aspects of the composition of Franklin vesuvianites are the concentrations of copper, beryllium, manganese, and zinc.




	Figure 16-25. Vesuvianite (locally called cyprine(abundant light gray) from Franklin, with lath-like crystals of bustamite (white), and andradite (dark gray patches). Thin elongate “crystals” are a precursor mica now replaced in part by vesuvianite. Specimen is 13 cm in maximum dimension. Smithsonian Institution, #R3636-1. Photo by the author. See figure 12-22.

The maximum copper content of blue fibrous vesuvianite was found to be 1.85 wt. % CuO by Steiger in Palache (1910), 1.91 wt. % CuO by Fitzgerald et al. (1986), and 2.17 wt. % CuO by Fitzgerald et al. (1992). Numerous analyses by the writer indicate a maximum CuO content of 2.1 wt. %; those of Groat et al. (1992) have values less than this. Some blue crystals (FeO 1.2, MgO 0.9 wt. %) are overgrown by violet rims (FeO 0.8, MgO 2.2 wt. %); this relation remains unstudied.

The beryllium content of Franklin vesuvianite has been the focus of several investigations. It was first reported by Palache and Bauer (1930), who reported 9.2 wt. % BeO in brown prismatic crystals. Hurlbut (1955) reinvestigated this material, using samples of purported “beryllian” vesuvianite provided by Bauer, and gave analyses by Gonyer showing only 1.56 to 3.95 wt. % BeO. The writer has not performed analyses for beryllium on such samples, but recent, incomplete, microprobe analyses of the samples studied by Hurlbut (1961), and presumably Bauer, (HU- 113952, HU-90372) found them to have analytical totals low by only 2-3 weight percent and near-normal Al₂O₃ values. Groat et al. (1992) reported 0.06 wt. % BeO in one of their Franklin samples. Bauer, it should be noted, was a superb mineralogist and analyst, and this writer does not dismiss the possibility that Bauer had a unique sample, not restudied here or by others. However, in the absence of proof to the contrary, one must conclude that Be is not a major constituent of the preponderance of Franklin vesuvianite.

The manganese content of Franklin vesuvianite is quite limited; no specimen among many studied by the writer has more than 4.1 wt. % MnO (total Mn calculated as MnO). As part of a broader unpublished study of Mn in vesuvianite, the writer analyzed numerous red vesuvianites. In addition to Franklin samples, these were from the Kombat Mine in Namibia and from Pajsberg and Långban in Sweden; some were associated with Mn-oxides. None of the studied samples has more Mn than the above cited Franklin specimen, nor do any of those studied by Groat et al. (1992) or Fitzgerald et al. (1992).

The zinc content of local vesuvianite has been examined by Groat et. al. (1992) who reported a maximum of 2.35 wt % ZnO; Fitzgerald et al. (1992) found 4.46 wt. % ZnO.

Sterling Hill vesuvianite has not been analyzed, but it occurs in a typical calcium silicate assemblage and may be of normal composition.

Occurrence and paragenesis

Franklin vesuvianite occurs in a number of assemblages. Most of the preserved specimen material is the blue vesuvianite (cyprine), which is associated with garnet (presumably andradite-grossular), hyalophane, willemite, hardystonite, barite, calcite, mica, and bustamite (Figure 16-25). This was found in abundance in 1922 near the 850 level, 400 feet south of the Parker Shaft. In this assemblage, vesuvianite varies from blue to green to blue-violet and varies in texture from fine-grained fibrous to coarse-grained fibrous to granular; it also occurs in coarse-grained radial aggregates of prismatic crystals. Mica (presumably phlogopite) is replaced by blue vesuvianite in some specimens, and some caswellite is in part vesuvianite. Green vesuvianite is commonly massive; crystals and fibrous textures were not observed. It is very likely that the preponderance of blue material in local and systematic collections is due to selective retention of these very attractive specimens.

Of substantial significance is the vesuvianite reported to contain Be. This assemblage consists of massive andradite and leucophoenicite, with subhedral crystals of light green willemite, barite, and a very light brown to white unanalyzed apatite-group mineral. Vesuvianite occurs as abundant brown prismatic crystals, as depicted by Palache (1935), which grade into fine-grained material. Gageite occurs as colorless radial sprays of crystals, found most commonly where the silicates contact barite (#C5110-1, HU #113952, HU #90372). The assemblage appears to be a breccia.

The Franklin vesuvianite which has the maximum Mn-content is from a simple assemblage consisting largely of massive brown andradite which contains minor amounts of willemite, franklinite, and calcite. Vesuvianite is found as irregular segregations, up to 5 cm, with a dark red color; much of this material is massive, but fine crystals are found at contacts with calcite.

Among the best crystals of Franklin vesuvianite are those which occur as sharp, clear, light brown, 1-2 mm euhedra, together with dull-lustered, nearly black, dark red leucophoenicite (Figures 15-27 and 16-24), and willemite crystals as a late-stage vein assemblage on a white matrix. This matrix is composed for the most part of white granular grossular (a replacement of an unknown precursor) with minor amounts of willemite and andradite. Vesuvianite occurs on fracture surfaces in this matrix, followed by leucophoenicite and then willemite.

The above Franklin assemblages are but the most notable; vesuvianite also occurs spradically in a number of other associations, many evident only in one-of-a-kind specimens, and mostly unstudied. These include brown crystals on vein surfaces and brown, green, and red massive material. Of particular interest are the red Mn-bearing vesuvianites and a rare association of vesuvianite with tephroite.

Contrary to the assertion of Leavens (1990), vesuvianite is known from Sterling Hill; the assemblage is unstudied. It occurs as greenish brown, isolated, subhedral, 2-3 mm crystals associated with light brown garnet (presumably grossular), calcite, and diopside, in a typical calcium-silicate vesuvianite assemblage.

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