TEPHROITE

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).
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 Bums 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.
Secondary tephroite crystals from vein assemblages, 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.
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.
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.
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 have an "alexandrite-effect" as do glaucochroite, gageite, and some other Mn-bearing species. The luster is vitreous to slightly greasy; cleavage is good, but is seldom observed. There are strong partings 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. 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.
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, 1 cm barite crystals, and tiny pyrobelonite crystals. Of equivalent quality are the bluish-gray crystals reported by Gordon (1922, 1923a), associated with willemite, andradite, calcite, and caryopilite in a vuggy assemblage.
At Franklin, ordinary tephroite occurs as gray masses associated with zincite, franklinite, willemite, and calcite in the primary, high-temperature ores; 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).
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 and symplectites in contact with Mn-silicate minerals.
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. 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.
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. (Dunn, 1995)

Location Found:

Franklin and Ogdensburg (Type Locality)

Year Discovered:

1823

Formula:

Mn₂²⁺SiO₄

Essential Elements:

Manganese, Oxygen, Silicon

All Elements in Formula:

Manganese, Oxygen, Silicon

IMA Status:

Valid - first described prior to 1959 (pre-IMA) - "Grandfathered"

To find out more about this mineral at minDat's website, follow this link Tephroite

References:

Dunn, Pete J. (1995). Franklin and Sterling Hill New Jersey: the world's most magnificent mineral deposits. Franklin, NJ.: The Franklin-Ogdensburg Mineralogical Society. p.336

Frondel, Clifford (1972). The minerals of Franklin and Sterling Hill, a checklist. NY.: John Willey & Sons. p.79

The Picking Table References

	PT Issue and Page	Description / Comment
View Issue	V. 7, No. 2 - August 1966, pg. 14	The Minerals of Sterling Hill 1962-65 by Frank Z. Edwards - Tephroite
View Issue	V. 2, No. 2 - September 1961, pg. 12	Tephroite by Cornelius S. Hurlbut, Jr.

Images


Brown tephroite with exsolution willemite lamellae, franklinite (black) and minor calcite (white) from Sterling Hill Mine, NJ. Photo by JVF.		Brown tephroite with exsolution willemite lamellae, franklinite and minor calcite from Sterling Hill Mine, NJ under shortwave UV light. The willemite fluoresces green the calcite red-orange and the tephroite and franklinite are non-fluorescent. Photo by JVF.

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