FRIEDELITE

Mn₈Si₆O₁₅(OH)₉(OH) and Mn₈Si₆O₁₅(OH)₉Cl
Monoclinic, I2/m, a = 22.569, b = 13.396, c = 7.447 Å, b = 93.50^o

	Figure 18-9. Pseudo-trigonal stacked aggregates of platy friedelite crystals on willemite from Franklin. The field of view in maximum dimension is 0.75 mm (left), 1.2 mm (center), and 0.3 mm (right).

Friedelite from Franklin has a lengthy history. It was first reported from here by Palache (1910). Comparisons with schallerite were made by Gage et al. (1925) and Bauer and Berman (1928), and Hey (1956) provided new data and discussed the group relations. The group was further expanded, and more insights into friedelite provided, by Frondel and Bauer (1953) in their description of manganpyrosmalite. Additional X-ray powder data were provided by Heubner (1967) and Peacor and Essene (1980).

Crystal-chemical data for friedelite, schallerite, and caryopilite were provided by Dunn et al. (1981c), and they showed the existence of solid solutions between possible Cl- and (OH) end-members of friedelite from both Franklin and Sterling Hill. Additionally, they indicated that the c-axis repeat of local material is approximately 86 Å. Additional unit-cell data is given by the ICDD (PDF #35-572). Ozawa et al. (1983) studied the layer structure of friedelite relative to that of mcGillite. Fine specimens have been found at both localities (Foshag, 1920) and have been available for over 75 years.

Description

	Figure 18-10. Crystal drawings of friedelite. The top and left drawings are of Franklin crystals; the other is undesignated as its locality being Franklin or Sterling Hill. Drawings are from Palache (1935) who provided crystallographic data.

Friedelite occurs as crystals to 4 mm, as partially described by Palache (1935)(Figure 18-10); those found in subsequent years are of markedly similar morphology, and many are hemimorphic. Fine crystals are known from both deposits and commonly occur in subparallel growth with [001] normal to vein surfaces. Aside from these common single-crystal habits, friedelite also occurs as platy aggregates on Franklin willemite crystals (Figures 15-76, 15-77, 15-82, and 18-9). The preponderance of local material is massive, occurring in veins and seams in the ore. It is fine-grained; some is similar to some serpentines in texture and habit. Friedelite is pink, red, reddish- orange, or dark brown; the bulk of the material is reddish-brown; and yellow material is rare. Cleavage is perfect on {001} and is observed in free-growing crystals and sometimes in coarsely-crystallized aggregates. The luster is generally vitreous, but some cleavages are pearly. The surface of some broken fine-grained vein material is similar to that of chert or quartzite. The density is 3.05 g/cm³.

Optically, friedelite is sensibly uniaxial in many specimens; biaxial negative specimens have a very small 2V. The indices of refraction are a = 1.625, b = 1.654, and g = 1.656. There is no discernible fluorescence in ultraviolet.

Massive friedelite is easily confused with a number of species which may form similar aggregates in vein assemblages. It is distinguished from reddish willemite by its lack of fluorescence and from sphalerite by sphalerite’s inferior hardness, resinous luster, and odor of sulfur when broken or intensely scratched. Distinguishing friedelite from the closely related manganpyrosmalite, nelenite, and schallerite is difficult and should be done employing both X-ray and optical methods.

Composition

	Table 15. Chemical analyses of some layer silicate minerals.

Friedelite is a manganese silicate hydroxide chloride mineral of the friedelite group. Chlorine may be absent, however, as noted by Palache (1935). There is extensive solid solution between Mn₈Si₆O₁₅(OH)₉Cl and Mn₈Si₆O₁₅(OH)₉(OH), as demonstrated by Dunn et al. (1981), but there is no nomenclatural distinction. In general, solid solution of Mg, Fe, and Zn, for Mn is very limited (Dunn and Peacor, 1981c), but the late Dr. Jun Ito analyzed one massive dark brown friedelite, associated with radiating willemite, and found 5.2 wt. % MgO and 2.7 wt. % ZnO (HU# 113902). The relations between Cl- and (OH)-bearing material have not been studied in detail. Several microprobe analyses of friedelite are presented in Table 15.

Occurrence and paragenesis

Friedelite is a moderately common secondary mineral at both Franklin and Sterling Hill and is the most abundant OH-bearing secondary Mn-silicate here. Superb crystals are commonly associated with calcite or barite, less commonly with secondary willemite; friedelite has been reported once with oxidized sulfides (Jenkins and Misiur, 1994). Palache (1910, 1935) noted a number of assemblages; there are many more.

The preponderance of local material is present as veins, some of which are centimeters in thickness, in franklinite-willemite  ore which is commonly calcite-bearing. Although such veins may be vuggy and yield crystals or crystal-druse surfaces, the majority of veins are composed of tight, dense, fine- grained, microcrystalline aggregates. These may be layered, or may be uniform and homogeneous in color and texture. The best massive friedelite, from homogeneous veins up to 5 cm thick and occurring in varying shades of red, has provided some gem material for the cutting of cabochons (Webster, 1975).

Friedelite veins are commonly, but not exclusively, monomineralic. Unlike most local vein minerals, friedelite commonly occurs without willemite. Friedelite veins exhibit many of the same features shown by willemite or serpentine veins. Indeed, friedelite is responsive to Mn-silicate concentrations in much the same way that secondary serpentine is to Mg-silicate concentrations, and friedelite occupies the mineralogic niche commonly held by serpentine in Mg-rich assemblages, such as at Sterling Hill.

Friedelite is not always pure; physical mixtures with other minerals, most notably with schallerite and manganpyrosmalite, are not uncommon. These occur in late-stage fracture fillings and breccia cements, such as one hosting franklinite, andradite, and microcline. Arsenian friedelites have been reported (Bauer and Berman, 1928), but have not been subjected to detailed modern studies. The writer considers it probable that these are physical mixtures of friedelite with other arsenic-bearing minerals, possibly nelenite; the detection of such a mixture by X-ray powder methods is almost impossible; the powder patterns are nearly identical. Another possibility for arsenic contamination is a mixture of friedelite with the unnamed arsenite analogue of friedelite (Peacor et al., 1986), not yet known to occur locally. Such mixtures, at least on a small scale, might be more common than recognized and may provide a ready host for minor arsenic in some silicate assemblages.

Copyright © 1995 by Pete J. Dunn			Website by Herb Yeates
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This page created: January 14, 2001