SAMFOWLERITE Chapter16. Sorosilicates and cyclosilicates

SAMFOWLERITE

Ca₁₄Mn₃Zn₂(Zn,Be)₂Be₆(SiO₄)₆(Si₂O₇)₄(OH,F)₆
Monoclinic, P2₁/c, a = 9.068, b = 17.992, c =14.586 Å,
b = 104.86^o, Z = 2

Samfowlerite was first described from Franklin by Rouse et al. (1994). It has not been found at Sterling Hill.

Crystal structure




	Figure 16-20. Stout prismatic-to-tabular samfowlerite crystals from Franklin. Field of view is 0.3 mm in maximum dimension.

The crystal structure of samfowlerite contains SiO₄ and Si₂O₇ groups and is related to those of the epidote and pumpellyite groups and to that of vesuvianite. The structure is composed of layers of vertex-sharing TO₄ tetrahedra (T = Si, Be, Zn) alternating with layers of vertex- and edge-sharing CaO₈ bicapped trigonal prisms and MnO₆ octahedra, the layers being parallel to (102). The TO₄groups and CaO₈-MnO₆ groups share vertices with one another across the layer boundaries to form the three-dimensional structure. Within the layers of tetrahedra, the TO₄ groups form a network of 4-, 5-, and 8-membered rings.

Description

Samfowlerite occurs as 0.5 mm euhedral vitreous colorless crystals; they may appear white in the aggregate. Only a few specimens have been found, and the crystals have not been studied goniometrically; recently found crystals are consistently twinned, but the twin law has not been determined. Representative photographs are presented in figures 16-20 and 16-21. Cleavage is absent; the density is 3.28 g/cm³.

Optically, samfowlerite is biaxial, negative, with 2V = 29^o, a = 1.674, b = 1.680, and g = 1.681; no dispersion was observed. The orientation is Y = b, X L a = 44^o, Z L c = 29^o. Some samfowlerite is fluorescent in both longwave and shortwave ultraviolet with an exceedingly weak red response color. Samfowlerite is visually similar to some nonfluorescent clinohedrite, but it has a much higher luster on the crystal faces. The apparently common but unstudied twinning (Figure 16-21) may also aid in visual recognition.

Composition

Samfowlerite is a zinc manganese beryllium calcium silicate hydroxide mineral. Microprobe and ion-microprobe analyses, with water by difference, yielded SiO₂ 36.9, MgO 0.2, CaO 34.1, ZnO 9.5, MnO 9.3, BeO 5.6, F 1.0, H₂O 3.8, less O=F 0.4, total = 100.0 wt. %.

Occurrence and paragenesis

Samfowlerite is known from but one rare assemblage from Franklin. Fortunately, however, it is a well-known assemblage for cahnite and was found in 1927, as reported by Palache (1935). The assemblage consists of franklinite-willemite-andradite ore which has vuggy areas. These vugs are lined with sparse 1-cm crystals of willemite and platy barite. Zoned garnet (andradite-grossular) has lined the vugs, grown contemporaneously with the willemite and barite crystals, and persisted in growth after their formation.




	Figure 16-21. Prismatic-to-tabular crystals of samfowlerite from Franklin; some are twinned. Field of view is 0.7 mm in maximum dimension.

The zoned garnet is the most notable megascopic feature of the assemblage, varying in color and texture from yellow-brown granular material to an outer zone of whitish fine-grained zoned material forming dodecahedra with a thin pinkish outer skin.

Of particular interest are very small, submillimeter-sized crystals of various minerals which occur as late-stage crystallites on this outermost zone of the garnet. These minerals are euhedral twinned cahnite crystals, pseudohexagonal clinochlore (called biotite by Palache, 1935), thin druses of very pale-pink leucophoenicite, and euhedral crystals of samfowlerite (Figures 16-20 and 16-21). Although based on the observation of very few specimens, samfowlerite tends to occur alone in vugs, or with clinochlore, and interstitial to platy crystals of barite.

Name

Samfowlerite is named in honor of Dr. Samuel Fowler, a pre-eminent citizen of Sussex County and a pivotal figure in Franklin’s history, as noted elsewhere in this volume.

FOOTER LBI




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