LEUCOPHOENICITE Chapter15. Nesosilicates

LEUCOPHOENICITE

(Mn,Ca)₇(SiO₄)₃(OH)₂
Monoclinic, P2₁/b, a = 4.828, b = 10.85, c = 11.380 Å,
a = 103.75^o, Z = 2.




	Figure 15-21. Crystal drawing of leucophoenicite from Franklin. Drawing is from Palache (1935) who provided crystallographic data.

Leucophoenicite was originally described from Franklin, New Jersey, by Penfield and Warren (1899). Subsequent studies of its morphology were published by Palache (1928, 1935) and Moore (1967), and optical absorption data were given by Keester and White (1966). The discovery of jerrygibbsite, a polymorph of Mn₉(SiO₄)₄(OH)₂, by Dunn et al. (1984d) led those authors to speculate that there might be additional members of the leucophoenicite family and that jerrygibbsite might be one of these. This hypothesis was verified by the discovery of ribbeite from the Kombat Mine in Namibia.

Leucophoenicite originally was known only from the zinc deposit at Franklin, New Jersey; it has never been reported from Sterling Hill. White and Hyde (1982b) have shown that leucophoenicite occurs at Pajsberg, Sweden. Winter et al. (1983) have noted that a calcian alleghanyite from Italy, originally described by Dal Piaz et al. (1979), is leucophoenicite, based on X-ray powder diffraction data; this was confirmed by Dunn et al. (1988), who also described leucophoenicite from the Kombat Mine in Namibia.




	Figure 15-22. Crystal drawings of leucophoenicite from Franklin; these are two projections of the same crystal. Drawings are from Palache (1935) who provided crystallographic data.		Figure 15-23. Crystal drawings of leucophoenicite from Franklin. These are two projections of the same crystal. Drawings are from Palache (1935) who provided crystallographic data.

Crystal structure

The crystal structure of leucophoenicite was solved by Moore (1970b) who noted that it has fully- occupied silicate tetrahedra and also has half-occupied, edge-sharing silicate tetrahedra, and is based on hexagonal close-packed anions stacked parallel to {010}. The octahedral cations form a kinked serrated chain. Leucophoenicite is structurally distinct from the humite-group minerals, with which it has strong compositional similarity and is sometimes associated. Brovkin and Nikishova (1975) showed the relation of the structures of Mg₅(BO₃)₃F and leucophoenicite. White and Hyde (1983a, 1983b), using TEM techniques, showed that leucophoenicite is a member of a family of crystal structures (including some borates and germanates) and supported Moore’s (1970b) proposal of edge-sharing, half-occupied silicate tetrahedra. Yau and Peacor (1986) studied leucophoenicite-jerrygibbsite intergrowths using TEM methods and showed that the differences between the Mn-humites and leucophoenicites are due to unit-cell twinning.




	Figure 15-24. Crystal drawings of three twinned leucophoenicite crystals from Franklin. Drawings are from Palache (1935) who provided crystallographic data.

They also suggested that leucophoenicite- group minerals might form in the absence of fluorine. Kato (1988) suggested that Ca is not essential to the leucophoenicite structure, and he provided crystal-structure refinements of Ca-rich and Ca-poor leucophoenicites. The description of the crystal structure of ribbeite (Freed et al., 1993) provided additional descriptions of the features common to leucophoenicite-related minerals.

Description




	Figure 15-25. Vein of leucophoenicite (gray) and willemite (white) in franklinite-willemite ore from Franklin. The visible surface is polished. Specimen is 13 cm in maximum dimension. Smithsonian Institution, #R3878-1. Photo by the author.

Leucophoenicite is commonly some shade of pink, but violet-red and brownish-red hues are known. It occurs as both massive material and as euhedral crystals (Figures 15-21 through 15-24); most crystals resemble those in figure 15-21.

No morphological studies have been done since those of Palache (1935) and Moore (1967). The luster is vitreous; cleavage was reported by Penfield and Warren (1899) but was not observed by this writer; and the density is 3.85 g/cm³. Optically, leucophoenicite is biaxial, negative, 2V = 74^o, with a = 1.751, b = 1.771, and g = 1.782; it is faintly pleochroic. There is no discernible fluorescence in ultraviolet. Leucophoenicite is distinguished from hodgkinsonite by the perfect cleavage and lower indices of refraction of the latter. Leucophoenicite is best distinguished using X-ray methods.




	Figure 15-26. Prismatic leucophoenicite crystals with unknown mineral in rosettes, from Franklin. The “curved” termination of the largest leucophoenicite crystal results from contact with another crystal and is not a freely-grown crystal face. Field of view is 1.2 mm in maximum dimension.

Composition

Leucophoenicite is a manganese silicate hydroxide mineral of the leucophoenicite group. Several analyses are presented in Table 1. Dunn (1985a) provided 27 modern analyses and made the following compositional observations:

1. Most leucophoenicites are highly calcic. Of the 27 analyses, 22 have Ca values in excess of 0.48 Ca per 3 Si. It should be emphasized, however, that this ratio is misleading; calcic material is much more common, as evidenced by unpublished data. No samples contained the very high (up to 14 wt. %) CaO values reported by Cook (1969) using XRF analysis. The abundance of samples with values of 0.5-0.7 Ca per 7 octahedral cations suggests that this is a somewhat “stable” calcium content for samples which form in calcic assemblages.

2. Zn is a constant constituent of Franklin material. It is present in all analyses of local leucophoenicite, including many not published here. It is relatively invariant, amounting to approximately 0.3 Zn per 3 Si. No Franklin leucophoenicite was found which contained the very high zinc content (up to 8 wt. % ZnO) reported by Cook (1969) using XRF analysis. Using material from other localities, Dunn et al. (1988) showed Zn to be non-essential to leucophoenicite.

3. Fluorine is essentially absent in leucophoenicite. Some samples may have traces of F, which are well within the error of the microprobe determinations.

Because most samples are highly calcic, the implication is that Ca might be a preferential guest cation in leucophoenicite. One might further speculate that it should be ordered, inasmuch as Ca is ordered in all olivine-related structures (Lumpkin et al., 1983; Ribbe, 1982). However, Kato (1988) found no proof of this in leucophoenicite using Ca-rich and Ca-poor crystals supplied by and carefully analyzed by this writer. The affinity of Ca for leucophoenicite among the humite-related minerals is enigmatic.




	Figure 15-27. Rough-surfaced crystals of leucophoenicite from Franklin; the sharp crystals in the lower section of the photograph are vesuvianite. Field of view is 1.6 mm in maximum dimension. See figure 16-24.		Figure 15-28. Superb crystal of leucophoenicite from Franklin. Field of view is 1.7 mm in maximum dimension.

Occurrence and paragenesis

Leucophoenicite is a moderately common mineral at Franklin, occurring in secondary veins (Figure 15-25) and adjacent ore. Recrystallized material was common, especially in the Parker Mine, but leucophoenicite is not known from Sterling Hill. It occurs in veins and vuggy assemblages in association with a number of associated minerals, and it is found in calcium-rich primary ore, hydroxyl-bearing ore assemblages, and recrystallized aggregates. In vein assemblages, leucophoenicite crystallizes late, commonly contemporaneously with willemite, the most commonly associated mineral. Other associated species are calcite, andradite, sonolite, franklinite, zincite (Figure 22-13), and vesuvianite (Figure 15-27).




	Figure 15-29. Coarse, prismatic crystals of leucophoenicite, on druse of friedelite, from Franklin. Field of view is 4.6 mm in maximum dimension.

Several hundred leucophoenicite samples were examined. The results of this comparison supported the preliminary findings of Dunn et al. (1984d) that leucophoenicite occurs in a great variety of assemblages, most of which contain calcite or Ca- bearing associated minerals such as andradite. Most of the preserved pink-colored material from Franklin is leucophoenicite and not jerrygibbsite. However, it is not clear whether this is due to selective retention of the more attractive bright-pink leucophoenicite by miners’ casual collecting, or if this apparent predominance of leucophoenicite is due to geochemical conditions, a viewpoint tentatively held by the writer.

A very small number of calcium-poor leucophoenicite specimens occur without calcium- bearing species. These leucophoenicites are very uncommon; they occur in two types of assemblages:

1. With franklinite, willemite, and zincite in assemblages commonly devoid of other associated phases. If other silicate phases are present, they are tephroite, sonolite, or jerrygibbsite, the first two commonly in minor amounts.

2. With manganosite, zincite, jacobsite and hetaerolite, in coarse-textured specimens which are in large part manganosite (MnO) by bulk volume. This assemblage was examined in detail in search of the Mn-analogue of norbergite (MAN), which remains unknown in nature. The silicate phases in this assemblage are tephroite, sonolite, and Ca-poor leucophoenicite, which is the dominant silicate.

Name

Leucophoenicite was named for the Greek words for white or pale and purple-red because of its color.

FOOTER LBI




Copyright © 1995 by Pete J. Dunn			Website by Herb Yeates

Link to homepage
This page created: January 11, 2001