Roeblingite

Formula:	Pb2Ca6(SO4)2(OH)2(H2O)4[Mn(Si3O9)2]
Crystal system:	Monoclinic	Space group:	C2/m

Occurence

Roeblingite is a rare mineral found locally in altered secondary assemblages in the northern part of the Franklin mine. It has not been found at Sterling Hill.

	Roeblingite. Nodule embedded in a fine-grained mixture of secondary minerals. Andradite and franklinite form the bulk of this specimen (lower half of photo). Note fractures visible in the andradite-franklinite. These are filled with the fine-grained mixture and secondary willemite. Franklin, NJ. Width 10 cm.

Roeblingite is white, cream or gray in color, and forms compact nodular masses up to 15 cm in size. The appearance of some of these nodular masses, on fracture surfaces, is not unlike that of unglazed porcelain or fresh un-ground coconut. Some roeblingite fluoresces pinkish-red under short-wave ultraviolet light. No euhedral crystals have been found at Franklin.

	Roeblingite. Several nodules embedded in a very fine-grained mixture of secondary minerals. Franklin, NJ. Width 5 cm.

Roeblingite was first discovered in 1897 and was reported to occur in the 1,000 foot level of the Parker mine. It was also reported the following year to have been found on the 800 foot level of the same mine. A number of unusual and rare minerals were first found in the Parker mine, and on the surface waste dumps from workings of the Parker mine, and would later become known as "Parker shaft minerals" among local mineral collectors.

	Roeblingite nodule. Close-up of specimen shown above. Franklin, NJ. Width 2 cm.
	Roeblingite nodule fluorescing. Same nodule above, exhibiting zoned red-violet fluorescence under short-wave ultraviolet light.

The mineral was also found in quantity (many kilograms of specimens were recovered) during exploration and removal of the Palmer shaft pillar, leading up to the closing of the Franklin mine in 1954.

Roeblingite is typically associated with hancockite, andradite and willemite, and smaller amounts of a wide range of associated secondary minerals such as prehnite, xonotlite, datolite, clinohedrite, manganaxinite, barite and ganophyllite.

Typically, roeblingite is found as irregular segregations embedded in an ill-defined mixture of very fine-grained minerals, possibly including some of those listed above. These mixtures are clearly secondary, and adjacent micas, andradite and franklinite often appear little affected.

	Roeblingite nodule. Fragment of a large, pure nodule. Reddish-brown staining on exterior of this nodule fragment (visible upper) is due to associated hancockite. Franklin, NJ. Width 4 cm.
	Roeblingite nodule fluorescing. Same nodule above, fluorescing pinkish-red under short-wave ultraviolet light.

Some specimens clearly show multiple stages of secondary crystallization involving later hydrous or hydroxl-bearing phases, such as prehnite and ganophyllite, and microcrystals of these may rim the irregular nodules of roeblingite. Such specimens are highly prized by local collectors.

For further information on roeblingite, see Dunn (1995).

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