BUSTAMITE Chapter17. Inosilicates

BUSTAMITE

CaMnSi₂O₆Triclinic, P1, a = 7.736, b = 7.157, c = 13.824 Å,
a = 90.52, b = 94.58, g = 103.87^o, Z = 12




	Figure 17-8. Large, crude crystals of bustamite (light gray) from Franklin, associated with franklinite crystals (black), calcite (white), and mica (black at bottom). Specimen is 10 cm in maximum dimension. Smithsonian Institution, #164048. Photo by the author.

Bustamite was reported from Franklin by Larsen and Shannon (1922a); it was separately reported and analyzed by Lewis and Bauer (1922), but they assumed it to be a variety of rhodonite. The keatingine of Shepard (1876) was discredited as bustamite by Palache (1935), but specimens so-labeled in Shepard’s collection at the Smithsonian Institution were examined by this author and found to be rhodonite, as noted by Kemp (1893a). Some bustamite was known informally as manganese wollastonite. A second report of bustamite by Larsen and Shannon (1922b) was subsequently shown by Dunn and Leavens (1986) to be marsturite epitactic on rhodonite.




	Figure 17-9. Franklin bustamite (light gray on right) with rhodonite (dark gray in center) and calcite (white on left), adjacent to franklinite-calcite ore (lower left). Specimen is 9 cm in maximum dimension. Smithsonian Institution, #R3115-1. Photo by the author.

A detailed and superb study of Franklin bustamite and its relations to rhodonite and wollastonite was given by Hey (1929). Sundius (1931), and Mason (1975) have also studied local material and, together with Ohashi and Finger (1978), have studied the relations among these species. The crystal structure of bustamite was determined by Peacor and Buerger (1962), and the unit-cell and relationship to wollastonite was reinterpreted by Peacor and Prewitt (1963). Viswanathan and Harneit (1986) included Franklin bustamites in their study of lattice expansion and ionic substitutions. Bustamite is known from both Franklin and Sterling Hill.

Description

Bustamite occurs as subhedral crystals, up to at least 8 cm (Figures 17-8 and 17-10); they are not mentioned in Palache’s monograph (1935) and have not been measured. Bustamite varies in color; most Franklin material is light pink to pinkish orange. The color may fade on exposure to light, and weathered surfaces may darken, presumably by oxidation of Mn. Sterling Hill bustamite is sparse and brownish orange. Bustamite cleavages are pinacoidal; three are good, and one is fair. The numerous cleavages give an apparent fibrosity to some cleavage fragments. The luster is vitreous, and the density is 3.30 g/cm³. Optically, bustamite is biaxial, negative, 2V = 44^o, with a = 1.664, b = 1.675, and g = 1.679; dispersion is strong and crossed and is enhanced in the most Ca-rich specimens. Some bright pink Franklin bustamite has a moderate-to-dull red fluorescence in longwave ultraviolet, and some specimens are cathodoluminescent, but the intensity of these effects is not proportional to Mn:Ca ratios. Bustamite may be confused with light-colored rhodonite or pyroxmangite; it is differentiated from both by its negative optic sign and from rhodonite by its indices of refraction.

Composition

Bustamite is a calcium manganese silicate of the pyroxenoid group. Calcium and manganese are ordered, and there is a partial solid solution toward both wollastonite and rhodonite. The compositions of Franklin bustamites lie within the known compositional ranges for the species (Deer et al., 1978). The coexistence of bustamite and wollastonite was noted by Mason (1975) who studied a critical Franklin sample, found by John Baum, which contained the minimum MnSiO₃ component required for stability of the bustamite structure. Bustamite is the high-temperature form of CaMnSi₂O₆; the inversion to johannsenite takes place at 830^o C, but may be sluggish (Deer et al., 1978).




	Figure 17-10. Stout crystals of bustamite with minor calcite (white) and andradite (gray) from Franklin. Specimen is approximately 12 cm in maximum dimension. Mineralogical Museum, Harvard University, #99266. Photo by Chip Clark.

Analyses by the writer are presented in Table 10. Those given here were chosen to show the maximum content of Mn, Ca, and Zn; magnesium and iron are very limited in all known local specimens. The relations to wollastonite and rhodonite, well documented in the literature, are supported by these data. Sterling Hill bustamites have higher Mg contents than Franklin material, in keeping with the generally higher magnesium content of Sterling Hill silicates.

Occurrence and paragenesis

Bustamite is moderately common at Franklin and relatively rare at Sterling Hill. Franklin bustamite occurs in a variety of assemblages, associated with calcite, vesuvianite, rhodonite, wollastonite, garnet, diopside, willemite, tephroite, glaucochroite, johannsenite, margarosanite, clinohedrite, and numerous other species (Figures 12-20 and 12-24). Only a few of the most notable occurrences are mentioned here.

The initial description of Franklin material by Larsen and Shannon (1922a) was of pink prismatic crystals, up to 1 cm, associated with blue vesuvianite, garnet, mica, and white hyalophane (Figures 12-22 and 16-25). The intergrowth of these species is very colorful and esthetic. Perhaps selective retention of this material by miners and collectors was practiced; much material has been preserved.




	Figure 17-11. Franklin bustamite (whitish-gray), in contact with franklinite (black). Franklinite at right is rimmed by hardystonite, that at the left is rimmed by willemite. Specimen is 10 cm in maximum dimension. Privately owned. Photo by the author.		Figure 17-12. Bustamite vein cutting massive black franklinite ore from Franklin. Willemite is abundant, but not obvious. The dark gray mineral at the franklinite-bustamite interface is tephroite. The visible surface is polished. Specimen is 11 cm in maximum dimension. Privately owned. Photo by the author.

A bright light-pink bustamite in massive platy aggregates to 10 cm occurs associated with hardystonite and willemite at Franklin; some specimens have coronal rims of tephroite and/or hardystonite with a symplectite-like texture. This is the assemblage studied by Hey (1929). This bustamite is also notable for its unique fluoresence in ultraviolet as noted above (Figure 12-30).

Subhedral to euhedral crystals, light pinkish orange and up to 5 cm in length, occur with calcite, andradite, mica, and traces of willemite at Franklin. Some crystals are slightly resorbed. Little is known of the assemblage.




	Figure 17-13. Massive whitish bustamite with willemite (dark) from Franklin. Field of view is 14 cm in maximum dimension. Smithsonian Institution, #R19230. Photo by the author.		Figure 17-14. Coarse-grained bustamite (white-gray on right half of specimen) with calcite (white on left half) and diopside (black) from Sterling Hill. Specimen is 10 cm in maximum dimension. Smithsonian Institution, #148373. Photo by the author.

Of significance are two assemblages with substantial ZnO (#R19170 and #144636): the first, intimately associated with green microcline, has the maximum ZnO reported in any bustamite and has no exsolution. The second assemblage, with less ZnO, consists of orange-pink Franklin bustamite occurring in large masses, some weighing kilograms. Willemite is commonly associated; in specimens where willemite is clearly exsolved, as plates and/or as rod-like crystals, lesser amounts of a dark red sodic johannsenite is also exsolved as platy crystals. The material was locally abundant.

Aside from these assemblages for which many specimens exist, there are other, rarer assemblages which deserve mention. These are:

a)    Franklin bustamite noted by Leavens et al. (1987) associated with glaucochroite, andradite, and willemite. (HU-114190).

b)    Franklin bustamite with willemite, in a reactive contact with spessartine; the reaction products are gahnite and tephroite.

c)    Franklin bustamite in a coarse-grained, slightly layered assemblage of hardystonite, calcite, and andradite. Minor apatite and clinohedrite are present.

At Sterling Hill, bustamite occurs with tephroite, andradite, calcite, and a dark brown unanalyzed clinopyroxene. It is also found associated with diopside and calcite (Figure 17-14).

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