RECRYSTALLIZATION Chapter 12. Mineral assemblages

Recrystallization of minerals




	Figure 12-35. Coarsely crystallized zincite (central and left dark areas), willemite (white), and franklinite (black at right and black-gray at top), from Franklin. Specimen is 16 cm in maximum dimension. Mineralogical Museum, Harvard University, #2484. Photo by the author.

The extent and effects of recrystallization of medium-grained assemblages to coarse-grained ones is unpublished. Limited evidence suggests such recrystallization played a major role locally, particularly near the Trotter and Parker Mines in Franklin.

Parts of both orebodies have been subjected to recrystallization of the primary minerals. Some of this is due to contact effects from local pegmatites and solution infusion but, with exceptions, specimens showing recrystallization have not been attributed to any one cause. Recrystallized areas may be composed of ore, silicates, or mixtures of both.




	Figure 12-36. Coarsely crystallized willemite (light gray-white), zincite (black), calcite (white), and tephroite (medium gray), in ore specimen from Franklin. Specimen is 10 cm in maximum dimension. Mineralogical Museum, Harvard University, #113591. Photo by Vic Krantz.

The most abundant of these recrystallized silicate minerals are andradite and willemite.

Recrystallized specimens are characterized in part by greatly increased grain-size of ore, silicates, or both; diminishment or loss of the primary gneissic textures; and, in many cases, a reduction of the elemental contaminants in solid solution, a “cleansing” as it were, relative to the primary ore. For example, the writer’s unpublished investigations show that recrystallized willemite almost invariably contains less Fe, Mg, and Mn than primary willemite. These recrystallized features were discussed by Palache (1935), who attributed large crystals to recrystallization, by Frondel (1972), and by Frondel and Baum (1974).




	Figure 12-37. Zincite (black, elongate, thick-or-thin areas) from Franklin, associated with willemite (light gray-white), calcite (white), franklinite (black equant areas lower left and right) and tephroite (medium gray, left side). Note the concentrations of zincite outlining some grain boundaries, as well as its occurrence as apparently random aggregates. Specimen is 16 cm in maximum dimension. Smithsonian Institution, #C2838. Photo by the author.		Figure 12-38. Coarsely crystallized willemite (gray-white), franklinite (black), leucophoenicite (medium gray contacting franklinite above willemite), zincite (isolated black blebs, top center) in ore specimen from Franklin. Specimen is 15 cm in maximum dimension. Franklin Mineral Museum, unnumbered. Photo by Vic Krantz.

The “Parker-Shaft minerals”

The term “Parker-Shaft minerals” is a misnomer; it erroneously implies that all the minerals which came from the Parker Shaft part of the mine have some common, restricted, source area. However, after the underground linkages of the old mines, and until the shaft’s closing in 1910, much of the material from underground mining throughout the Franklin Mine was taken out through the Parker Shaft. Thus, a great variety of general mineral matter was recovered through this shaft, rendering the specificity of the appellation incorrect in a formal sense. However, the term “Parker-Shaft minerals” is pervasive in the history and literature of Franklin, and has a century of common use. It is known world-wide to collectors and curators, and is a staple term of the local argot and traditions, and it now serves to designate a broad assemblage of minerals from an extensively recrystallized part of the deposit. The term “Parker-Shaft minerals” is one of great utility and intentionally is given currency here.

As noted in the discussion of the Parker Mine, in 1895 two vertical raises (vertical upward openings without surface egress) 20-30 feet apart were extended vertically two hundred feet (Figure 3-17). These raises mined a spectacular zone of recrystallized minerals, many new to science. Many of these minerals were first described by Penfield, Warren, and others in the 1890’s using specimens found on the Parker Dump, but the fact that they had occurred in this restricted area was not recognized at the time. From this occurrence came the discoveries of roeblingite (1897), clinohedrite (1898), nasonite (1899), leucophoenicite (1899), glaucochroite (1899), hancockite (1899), margarosanite (1916), charlesite (1983), minehillite (1984), and possibly also cahnite (1927), marsturite (1978), and franklinfurnaceite (1987).

Fortuitously, this area was to be mined again in the mid-20^th century, providing a fresh infusion of specimen material to laboratories, museums, and collectors. This second period of mining occurred when the Palmer Shaft support-pillar (a solid area of ore left in place over the Palmer Shaft to prevent collapse of the hanging wall) was removed during the final stages of mining in approximately 1944-1954. The original source areas for these rare minerals, the 1895 raises, had been hidden from access within the Palmer Shaft support-pillar during the intervening half-century. The rare and uncommon minerals, when found again, quickly were recognized as being those found at the end of the preceding century, and substantial quantities were preserved by collectors. Thus, some of the more rare and interesting minerals were preserved by persons several generations apart.

The localization of the Parker Mine recrystallization area, near the very abundant pegmatites in the north end of the orebody, coupled with the presence of much water and boron in the minerals, suggests that this assemblage might be related to the intrusion of, or to emanations from, a different, much wetter, and more volatile-containing pegmatite than was common near the orebodies. Hydrothermal activity was intense in this northern part of the orebody. Boron mineralization was common and extensive, and almost all primary silicate minerals were extensively altered and recrystallized. Here the feldspars are altered and chalky, the micas are severely altered and replaced (caswellite), vuggy textures are common, open veins and cavities are abundant, and many species occur in superb euhedral crystals. Both high-temperature and very low-temperature minerals were found together here. Hydrated species include charlesite with 48 wt. % H₂O, ganophyllite, and clinohedrite. In addition to the new species discovered in this area of the mine, many others occurred there. These include most of the lead silicates excepting esperite and larsenite, together with willemite, vesuvianite, datolite, cahnite, thomsonite, xonotlite, hodgkinsonite, manganaxinite, andradite, rhodonite, prehnite, cuspidine, pectolite, and a great many others. Hancockite, andradite/grossular, and manganaxinite were particularly abundant.

The Trotter Shaft minerals

The Trotter Mine recrystallization was due to a very large pegmatitic intrusion, a microcline pegmatite (Double Rock), which cut the orebody and resulted in abundant contact effects. The mineral specimens from the Trotter Shaft were substantial in quality and held in much esteem by mineralogists and collectors in the last century. Garnet, rhodonite, pyroxenes, and amphiboles were abundant, together with allanite-bearing (rare-earth bearing) green microcline, and a very anomalous suite of nickel arsenides.

The “north orebody” at Sterling Hill

The term “north orebody” is almost another misnomer in that it suggests there was an orebody north of the Sterling Hill orebody and geologically distinct from it. Such is not the case; the east limb of the Sterling Hill orebody was truncated by the Zero Fault and, as a result, its northernmost part, located between the 1850 and 2550 levels of the Sterling Mine, is physically separate from the rest of the east limb, but it is not a geologically separate entity. The minerals of the north orebody are characterized by much dolomitization of the carbonates; extensive sussexite, serpentine, and pyrochroite; much hematite derived from in situ alteration of franklinite; and a host of other features. The mineralogy of the north orebody remains wholly unstudied; a superb suite of representative specimens has been deposited at the National Museum of Natural History by Richard Bostwick.




	Figure 12-39. Sussexite vein from the north orebody at Sterling Hill, consisting of sussexite (gray banded mineral), zincite (dark gray at center), and franklinite (black). Banded sussexite is surrounded by impure calcite with zincite impurities. Dark gray at lower left is zincite with calcite impurities. Specimen is 14 cm in maximum dimension. Smithsonian Institution, #164055. Photo by the author.		Figure 12-40. Franklinite (lustrous black) shows direct alteration to hematite (dark gray rimming franklinite), associated with calcite (white), serpentine (black in center), and impure mixtures of serpentine and calcite (top left), from the north orebody at Sterling Hill. Specimen is 12 cm in maximum dimension. Smithsonian Institution, #R19126. Photo by the author.

Giant crystals

In addition to many other distinctions, Franklin and Sterling Hill have produced a great number of giant crystals, commonly in the presence of much calcite. Recrystallization of the calcium silicate units and oxides has generated large crystals, but it is not at all certain or even probable that all giant crystals have formed in such a manner, or even a preponderance of them. There is evidence, as noted previously, for extremely slow cooling of the deposits, a process favoring the development of large crystals.

Although giant crystals of some minerals are known from other localities, many are one-of-a-kind occurrences (Palache, 1932; Rickwood, 1981). The best local sources of giant crystals have been at Sterling Hill, specifically at the Noble Pit; at an area between this and the Passaic Pit (Baum, 1962b) where superb, euhedral, giant crystals were recovered after their weathering from enclosing calcite and other minerals; and at an area along the east side of the east limb near the crossmember. Even larger crystals than those recovered have been reported by responsible and careful miners and geologists alike; most were fractured tectonically or by explosives-blasting and were broken upon attempted removal. Vast numbers of others were assuredly destroyed in the explosive blasting processes of mining and were never witnessed at all. Among those preserved, and reported by Palache (1932), are:

Andradite, Gooseberry Mine, 57 cm in circumference.

Augite, Sterling Hill, 30 cm.

Gahnite, Sterling Hill, octahedron 12.5 cm on edge.

Franklinite, Sterling Hill, octahedron 17.5 cm on edge.

Hornblende, Sterling Hill, 15 x 45 cm.

Microcline, Franklin, 30 cm.

Pyroxene, Sterling Hill, 26 cm.

Rhodonite, Franklin, 6 x 6 x 20 cm.

Willemite, Sterling Hill, 10 x 25 cm.

Additionally, Frondel (1935) reported a formless single-crystal of zincite from Franklin, which was 5 cm perpendicular to the cleavage and 6 cm across the cleavage. Frondel and Baum (1974) reported a rhodonite crystal 18 inches (~45 cm) in length. The writer has not verified that the above are record sizes for preserved local crystals of these species; there might be larger ones. No recent compilation has been made.

Although these dimensions are impressive, it is notable that fine crystals of many local minerals have been found in smaller but still very significant 5-15 cm sizes. Among these are amphibole, augite, rhodonite, andradite, spessartine, franklinite, willemite, gahnite, and others. Most major public collections have such great crystals among their treasures.

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