NORBERGITE Chapter15. Nesosilicates

NORBERGITE

Mg₃(SiO₄)(F,OH)₂
Orthorhombic, Pbnm, a = 4.7104, b = 10.2718, c = 8.7476 Å, Z = 4.




	Figure 15-12. Crystal drawing of norbergite from the Nicoll Quarry in Franklin. Drawing is from Palache (1935) who provided crystallographic data.

The best euhedral crystals of Franklin norbergite were reported from the Nicoll Quarry at the turn of the century and subsequently described by Larsen et al. (1928); the early history is given by Palache (1935). Nicoll Quarry norbergite was also used by Taylor and West (1929) and Gibbs and Ribbe (1969) for the determination and refinement, respectively, of the crystal structure. Abbott et al. (1989) used the data of Gibbs and Ribbe in a structure-energy calculation. Norbergite is a mineral of the Franklin Marble.

Description

Although fine crystals may attain 2-3 cm (Figure 15-12 and 15-13), most local norbergite occurs as anhedral to subhedral 1-4 mm grains in the host marble. The crystals from the Nicoll Quarry are composed of the forms {001}, {011}, {021}, {110}, {111}, and {221} (Palache, 1910, 1935). Local crystals can be of very high quality; White and Hyde (1982a), using TEM techniques, examined two specimens from Franklin and found them to be perfect crystals. Generally, at least for the well-studied specimens, norbergite is light yellow or light greenish-yellow; however, well-formed 1 cm brown crystals are known. Some samples, carefully verified as norbergite, are visually indistinguishable from common orange-brown chondrodite; the distinction is best made with X-ray diffraction methods. The luster is vitreous; the cleavage is imperfect; and the density is 3.20 g/cm³.

Optically, norbergite is biaxial, positive, 2V = 54^o (calc.), with a = 1.559, b = 1.564, and g = 1.584; pleochroism and dispersion are weak. The fluorescence in ultraviolet for Nicoll Quarry samples is yellow in shortwave and weak yellow to absent in longwave. Anhedral microcrystals of apatite and/or uvite may resemble norbergite.

Composition

Norbergite is a magnesium silicate fluorine hydroxide mineral of the humite group and the most fluorine-rich member of the group. As noted by Jones (1969), there is very little solid-solution of cations substituting for Mg; the known analyses agree well with the theoretical composition. Analyses of norbergite from the Franklin Marble at Franklin and Sterling Hill are presented in Table 1. Norbergite, alone among the humites, has no known analogue in either the manganese-humite or the leucophoenicite groups. Such an analogue may be precluded crystallochemically. The verified brown and orange norbergites have not been chemically analyzed; their color may be due to iron or manganese substitution.

Occurrence and paragenesis




	Figure 15-13. Prismatic norbergite crystals in calcite, from the Nicoll Quarry in Franklin. Smithsonian Institution, #R3877. Specimen is 6 cm in diameter. Photo by Vic Krantz.

Norbergite specimens from the Nicoll Quarry in the Franklin Marble in Franklin (Figure 15-13) have been used for all the major investigations, and it is recommended that they be used for future studies as well. In 1905 another occurrence of norbergite was found by Palache (1935) in the Nicoll Quarry associated with black spinel, pyrrhotite, and fluorite. Fine, yellow, euhedral norbergite crystals, up to 25 mm in clusters to 3 x 2 x 2 cm, were found in the Franklin Quarry in 1982 and have the tabular habit figured by Palache (Figure 15-12). In this occurrence, norbergite is associated with calcite, hercynite, and phlogopite. Norbergite has also been reported from the Fowler Quarry and likely occurs sporadically in many of the other local marble quarries. It also occurs in marble on the 800 level of the Sterling Mine as brownish rims surrounding gray material; both have been verified as norbergite. Norbergite crystals were reported by Larsen et al. (1928) to have cores of chondrodite.

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