MANGANOSITE

MnO 
Cubic, Fm3m, a = 4.445 Å, Z = 4

Manganosite was first described from Franklin by Palache (1910) and has not been found at Sterling Hill. A second occurrence was reported from the northern end of the mine by Palache (1935), who also gave the optical data of Ford (1914). Franklin material was studied using X-ray methods by Levi (1924) and Berry and Thompson (1962) and was included in the compendia of Hewitt and Fleischer (1960). The definitive papers on the assemblage are those of Frondel (1940) and McSween (1976).

Description

Franklin manganosite occurs as massive material, intimately associated with zincite. Euhedral crystals are unknown, but octahedral fragments, separated along (111) parting planes by zincite, are commonly 1 cm in size and occur up to 10 cm. Manganosite is a rich dark green, easily oxidized, and most specimens appear black. Cleavage is cubic, the luster is vitreous to adamantine (dull after exposure), and the density is 5.36 g/cm³. Optically, manganosite is isotropic with n = 2.16 (Larsen (1921).

The predominant descriptive characteristic of Franklin manganosite is the very evident mutual exsolution of manganosite in zincite and zincite in manganosite. The minerals break among parting planes, and thus many extant specimens have planar features resembling crystal faces. Ramdohr (1938) had noted the presence of a hausmannite-hetaerolite rim on some manganosite grains. Studies by the writer confirm Ramdohr’s and McSween’s (1976) findings that this is hausmannite, although McSween found Mn²⁺:Zn to be 1:1. The zincite-manganosite exsolution was studied in detail by Frondel (1940), who found zincite {0001} [100] parallel to manganosite {111} [110]. Additional unnamed minerals occurring as yet unstudied inclusions have been noted by Ramdohr, McSween, and Frondel.

Composition

Manganosite is a manganese oxide mineral. Local material is quite near end-member composition. Palache (1935) reported Bauer’s analysis of material with zincite impurities as having 4.89 wt. % ZnO, but McSween (1976) reported only 2.22 wt. % by use of an electron microprobe. Only these few analyses have been published.

Occurrence and paragenesis

Most manganosite is associated with jacobsite, zincite, and calcite. Olivine-group minerals and related minerals (tephroite, leucophoenicite, and sonolite) are also associated. Of these, leucophoenicite and sonolite are the most common. The known specimens are almost entirely from the above-described assemblage. In addition, manganosite is rarely known as thin films on black hetaerolite which occurs as common exsolutions from zincite; one such specimen has manganosite associated with abundant manganhumite.

 

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