PYROCHROITE

Mn(OH)₂ 
Hexagonal

	Figure 22-69. Crystal drawings of pyrochroite from Franklin. The lower left drawing is of a crystal from the Buckwheat Mine. Drawings are from Palache (1935) who provided crystallographic data.

Pyrochroite was first reported from Franklin by Roepper (1892), probably seen associated with sussexite by Brush (1868), and subsequently reported from both Franklin and Sterling Hill by Palache (1935). A possible association of lead and pyrochroite was briefly mentioned by Boström (1964), but is unknown to this writer. Local pyrochroite has been but little studied beyond the extensive morphological studies of Palache (1935). It is of no economic significance locally.

Description

Pyrochroite occurs as euhedral crystals, aggregates of such, and massive coarse- and fine-grained material. The commonly euhedral crystals are for the most part lustrous, simple, equant in habit, rhombic in cross-section, and rarely elongate on [0001]; rhombic and scalenohedral forms are common (Figures 17-44, 22-64, and 22-69). Some crystals are pseudocubic, as described by Palache (1935) and as shown here in figures 22-69, 22-70, and 22-71. Columnar, 6 cm masses of platy crystals up to 1 cm in diameter are known; most crystals are much smaller.

	Figure 22-70. Pyrochroite crystals from Franklin. Field of view is 0.3 mm in maximum dimension.		Figure 22-71. Pyrochroite crystals from Franklin. This is a close-up view of crystals in figure 22-70. Field of view is 0.1 mm in maximum dimension.

Pyrochroite is colorless, light pink (Franklin), or light blue (Sterling Hill) when found fresh. However, upon removal from the mine, it quickly becomes increasingly opaque upon exposure, and known specimens are almost invariably black or dark brown and nearly opaque. Deeply altered material is decidedly brown. Coarse-grained material occurring in vugs is commonly lustrous with a slightly satiny appearance on the surfaces; this grades, imperceptibly, to dull-lustered material. Surficial oxidation may leave sooty residues or slightly iridescent tarnishes. Pyrochroite found in closed veins (Figure 12-42) is commonly impure, fine-grained, and dull-lustered. Cleavage is imperfect on {0001} and easily produced.

	Figure 22-72. Pyrochroite (black) rimmed by rhodochrosite (light gray). Other minerals are impure calcite (white-gray at right), calcite (white hemispherical area at center), and mooreite (gray at bottom left) from Sterling Hill. Specimen is 7 cm in maximum dimension. Smithsonian Institution, #R6491. Photo by the author.		Figure 22-73. Black pyrochroite in white calcite from Sterling Hill. Rhodochrosite is present as a very thin, indiscernible reaction-rim at the pyrochroite-calcite interface. Specimen is 10 cm in maximum dimension. Privately owned. Photo by the author.

Composition

Pyrochroite is a manganese hydroxide mineral and the Mn-analogue of brucite. Local material has not been analyzed since Palache’s time, and little is known of the limits of substitution of Mg and Zn, which are known to occur in local material, or their variance at Franklin and Sterling Hill.

Occurrence and paragenesis

In general, pyrochroite from Franklin occurs in well crystallized assemblages, occurring as druses and aggregates of fine crystals in vugs and seam openings. The best assemblages are those described in substantial detail by Palache (1935); some of these are:

a)         Pyrochroite in vugs with garnet, calcite, gageite, zincite, and other species. 

b)         Pyrochroite as lustrous, rough, flattened crystals on rhodochrosite which, in turn, coats common willemite/franklinite  ore.

c)         Pyrochroite occurring as crude, coarsely-prismatic aggregates, up to several cm in length and/or diameter, in calcite with sussexite.

d)         Euhedral pyrochroite crystals, several mm in size, occurring sporadically on massive hodgkinsonite, as described under hodgkinsonite by Palache (1935).

Any such listing is incomplete; this one is all the more so because pyrochroite was more abundant at Franklin than indicated above, but commonly occurring in small amounts. It probably is present in a number of minor occurrences in which it is not recognized.

At Sterling Hill, it is likely that pyrochroite is as common or more abundant than at Franklin, but not occurring, as a rule, in free crystals. Sterling Hill pyrochroite is in general more fine-grained and occurs in massive form as veins (Figure 12-42), masses, and stringers, commonly associated with sussexite of either fibrous or cherty texture and with or without calcite. Some fine specimens are found with an unstudied pink carbonate, presumably rhodochrosite, and with lawsonbauerite, torreyite, mooreite, and other manganese-bearing species (Figures 22-72 and 22-73). Secondary zincite is occasionally associated. Some Sterling Hill pyrochroite is commonly unattractive and may have been undercollected relative to other species. Feitknechtite is an alteration product of pyrochroite (Bricker, 1965), but not all altered material contains feitknechtite.

Copyright © 1995 by Pete J. Dunn			Website by Herb Yeates
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This page created: January 11, 2001