TEPHROITE Chapter15. Nesosilicates

TEPHROITE

Mn₂SiO₄
Orthorhombic, Pbnm, a = 4.899, b = 10.621, c = 6.251 Å, Z = 4.




	Figure 15-1. Crystal drawings of tephroite from Franklin. Drawings are from Palache (1935) who provided crystallographic data.

Tephroite was originally described from Sterling Hill by Breithaupt (1823) and Rammelsberg (1844). The early history of this mineral and information on morphology, optics, and composition was given by Palache (1935); see also Gordon (1922, 1923a) and Schaller (1933a).

The structure of Franklin tephroite was determined by O’Daniel and Tscheischwili (1944), and tephroite from both deposits was studied in detail by Hurlbut (1961). Tephroite is found at both Franklin and Sterling Hill and is the most abundant Mn-silicate at these deposits. Picrotephroite is an obscure name for magnesian tephroite.

Description

Tephroite occurs in both massive and well-crystallized samples. Franklin has produced superb euhedral crystals which have been described in detail by several investigators and summarized by Palache (1935). The best crystals are 2-3 mm in length and gray to gray-blue; see figures 15-1 through 15-7.

Massive tephroite occurs in large specimens and is much more common than isolated crystals. The color of massive Franklin tephroite varies from dominant gray, to pink, reddish-brown, and brown; most Sterling Hill tephroite is brown to brownish-red. Metsger et al. (1958) attributed the red color to minute inclusions of franklinite. Superb transparent euhedral bluish-gray crystals (Figures 15-1 and 15-2) have an “alexandrite-effect” as do glaucochroite, gageite, and some other Mn-bearing species.




	Figure 15-2. Crystal drawings of tephroite from Sterling Hill. Drawings are from Palache (1935) who provided crystallographic data.

The luster is vitreous to slightly greasy; cleavage is good on {001} but is seldom observed. There are strong partings on {100} and {010} which host willemite exsolutions; these were discussed in superb detail by Hurlbut (1961) and are noted below. These partings have erroneously been considered cleavages. The density is 3.87-4.10 g/cm³. Optically, tephroite is biaxial, negative, 2V = 60^o, with a = 1.770, b = 1.807, and g = 1.825; dispersion is perceptible, r > v. There is no discernible fluorescence in ultraviolet. Massive material is easily confused with the Mn-humites (Dunn, 1985a), and is best differentiated from them by X-ray diffraction methods. The exsolution of willemite is helpful in differentiation inasmuch as the Mn-humites do not have exsolved willemite. Some brownish-red tephroite from Sterling Hill resembles willemite.




	Figure 15-3. Highly-modified prismatic tephroite crystals on andradite from Franklin. Compare with drawing in figure 15-1. Field of view is 0.5 mm in maximum dimension.		Figure 15-4. Stubby prismatic crystals of tephroite in parallel growth, associated with highly-modified equant willemite crystal in foreground, from Franklin. Field of view is 1.2 mm in maximum dimension.

Composition

Tephroite is a manganese silicate mineral of the olivine group. There is much substitution of Fe and Mg and limited substitution of Zn in local tephroite and, as with most olivines, the local samples are dominantly binary solid-solutions, generally with less than 10 mole percent of the third component. Tephroite with near end-member composition is known, and locally there is a solid solution series to 66 mole percent forsterite. Most investigated samples are highly ordered, with Mn in the larger M2 site (Brown, 1970; Francis, 1980, 1985b).




	Table 1. Chemical analyses of minerals in the olivine-, humite-, and manganese-humite groups.

The definitive works on tephroite compositions and exsolutions are those of Hurlbut (1961) and of Francis (1980, 1985b) who studied the Mg/Mn relations and who provided precise data on coexisting pairs of exsolved willemite/tephroite. Francis determined that the original high-temperature tephroite had approximately 20 mole percent Zn₂SiO₄, of which approximately half was retained in tephroite and half exsolved as willemite upon cooling. Using Francis’s data in part, Lumpkin et al. (1983) provided detailed determinative procedures for solid solutions of the known end-members. The relations between tephroite and fayalite were worked out by Burns and Huggins (1972), but there are few such samples at these deposits, due both to the locally limited amount of iron substitution and the pervasive Mn/Mg solid solution.

Selected analyses are given in Table 1; many others are given in the cited studies. Secondary tephroite crystals from vein assemblages, illustrated in figures 15-3 through 15-7, are similar in composition. They contain, on average, only 0.3 wt. % FeO, 0.9 wt. % MgO, 2.5 wt. % ZnO, and 0.2 wt. % CaO; they are close to end-member composition.

Occurrence and paragenesis




	Figure 15-5. Euhedral crystals of Franklin tephroite. Field of view is approximately 1 mm.

Among the best crystals from Franklin are those reported by Palache (1928a), associated with the best willemite from Franklin. These bluish-gray crystals are up to 2 mm in length with black tips and occur on a druse andradite, associated with hodgkinsonite of unusual habit (Figure 15-52), 1 cm barite crystals, and tiny pyrobelonite crystals (Figures 12-43, 25-33, and 25-34). Of equivalent quality are the bluish-gray crystals reported by Gordon (1922, 1923a), associated with willemite, andradite, calcite, and caryopilite in a vuggy assemblage (Figures 15-4 through 15-7).




	Figure 15-6. Prismatic highly-modified tephroite crystals from Franklin. Field of view is 0.2 mm in maximum dimension.

At Franklin, ordinary tephroite occurs as gray masses associated with zincite, franklinite, willemite, and calcite in the primary, high-temperature ores (Figures 12-4 and 12-7); leucophoenicite is sometimes present. Large masses were found on the west limb near the open cut (Palache, 1935). Massive gray tephroite was found sporadically and locally at Franklin, but in substantial quantities, and thus specimens are abundant. Fine gray massive tephroite occurs, associated with zincite and jacobsite in the “zincian vredenburgite” assemblage described by Mason (1946) and McSween (1976).




	Figure 15-7. Euhedral crystal of Franklin tephroite with modified franklinite crystal. Compare with drawing in figure 15-1. Field of view is approximately 1 mm.

Much tephroite was found both in the ore units and in the calcium silicate units. Of interest is an uncommon occurrence in manganosite, with sonolite and leucophoenicite. It is found with many other species at both deposits, and willemite exsolution, as noted above, is very common. Together with hardystonite, it forms symplectite-like coronas around some bright-pink bustamite and is a common component of reaction zones (Figures 12-28 and 12-29) and symplectites in contact with Mn-silicate minerals (Figures 12-30, 12-32, and 17-12).




	Figure 15-8. Part of a vein assemblage from Franklin showing zincite (dark band), below which is a zone of massive tephroite (light gray) with black franklinite. Below the tephroite is willemite with franklinite (whitish zone) and finally, at the bottom, smeared-out willemite-zincite-franklinite ore. Specimen is 11 cm in maximum dimension. Smithsonian Institution, #17637. Photo by the author. See figure 12-7.

Tephroite is common at Sterling Hill, but New Jersey Zinc Company studies and mapping did not distinguish among true tephroite, the black fayalite of the black-willemite zone, or the Mn-humites, and thus should be read with caution. Some tephroite from the east limb at Sterling Hill occurs as crystals isolated in calcite and rimmed by dark brown sonolite. Crystals are up to 15 cm with 1 cm rims, although most such composite crystals are smaller, and 1 cm crystals were common (Figures 12-33, 12-34, 15-9, and 15-20). Much Sterling Hill tephroite occurs in the common granular ore and some, as noted by Palache (1935), resembles chondrodite; manganoan chondrodite is also known from here. Tephroite may have been more common at Sterling Hill than is known, but might have been unrecognized as such prior to the underground use of ultraviolet lamps; the visible-light color of tephroite and willemite can be nearly identical here. Red-brown tephroite occurred in large quantities at Sterling Hill and in early years was sometimes mined by mistake.




	Figure 15-9. Tephroite crystals (gray) observed in cross-section, partly rimmed and/or replaced by sonolite (black in and on tephroite), with franklinite (solid black), and white calcite, from the 700 level, Sterling Hill. The visible surface is polished. Specimen is 10 cm in maximum dimension. Privately owned. Photo by the author. See figures 12-33 and 12-34.

Tephroite forms under both primary and secondary conditions; secondary tephroite can replace willemite. In some cases, secondary material is indistinguishable from primary material, but primary tephroite is generally higher in Zn and may be higher in Fe (Squiller, 1976). Additional study is needed.

Tephroite in some cases alters to a white to pinkish, porous siliceous material, commonly hard and somewhat chalky or opaline in part and associated with nontronite, chalcophanite, and secondary manganese oxide minerals. Bauer found one such specimen to have SiO₂ 83.02, ZnO 11.63, FeO 0.69, MnO 0.96, MgO 0.50, loss on ignition 3.02, total = 99.82 weight percent. Willemite exsolution lamellae in primary tephroite commonly persist unchanged in this white altered mixture.

The paucity of information in this section is a reflection of the very limited studies to date; the paragenesis of tephroite is one of the untold stories, and much work remains to be done.

Name

Tephroite is named for the Greek word for ash-colored, in allusion to the color of much Franklin material.

FOOTER LBI




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

Link to homepage
This page created: January 11, 2001