Pyrochroite is a manganese hydroxide mineral and the Mn-analogue of brucite. 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). Local pyrochroite has been but little studied beyond the extensive morphological studies of Palache (1935). It is of no economic significance locally.
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, [and] rhombic in cross-section; rhombic and scalenohedral forms are common. Some crystals are pseudocubic, as described by Palache (1935). Columnar, 6 cm masses of platy crystals up to 1 cm in diameter are known; most crystals are much smaller.
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 is commonly impure, fine-grained, and dull-lustered.
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, masses, and stringers, commonly associated with sussexite of either fibrous or cherry 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. 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. (Dunn, 1995)

 Location Found: Franklin and Ogdensburg
 Year Discovered: 1864
 Formula: Mn(OH)2
 Essential Elements: Hydrogen, Manganese, Oxygen
 All Elements in Formula: Hydrogen, Manganese, Oxygen
 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   Pyrochroite

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.601

Frondel, Clifford (1972). The minerals of Franklin and Sterling Hill, a checklist. NY.: John Willey & Sons. p.72

The Picking Table References
 PT Issue and PageDescription / Comment
View IssueV. 36, No. 2 - Fall 1995, pg. 27Closest-Packed Mineral Structures of Franklin-Ogdensburg: Kepler's Gift of the Snowflake, Part one of two parts, Paul B. Moore - Pyrochroite
View IssueV. 7, No. 2 - August 1966, pg. 12The Minerals of Sterling Hill 1962-65 by Frank Z. Edwards - Pyrochroite

Pyrochroite, willemite, Sterling Hill Mine, Ogdensburg, NJ
Pyrochroite (black) and some willemite (off white) from the Sterling Hill Mine, Ogdensburg, NJ. field of view 1". From the collection of, and photo by, WP.

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