GAGEITE Chapter17. Inosilicates

GAGEITE-2M and GAGEITE-1Tc

(Mn,Mg)₄₂(Si₄O₁₂)₄O₆(OH)₄₀
Monoclinic, a = 19.42, b = 19.42, c = 9.84 Å, g = 89.5^o, Z = 1
Triclinic, a = 14.17, b = 14.07, c = 9.84 Å,
a = 76.5, b = 76.6, g = 86.9^o




	Figure 17-44. Acicular gageite crystals from Franklin, liberally sprinkled with equant multifaceted franklinite crystals and rhombic pyrochroite crystals. Field of view is 0.6 mm in maximum dimension. See figures 22-23, 22-24, and 23-10.

Gageite has been the subject of a number of investigations. It was first described by Phillips (1910); Levison (1918), Gordon (1927), and Palache (1928a, 1935) added additional information. Moore (1968a, 1968b) published a discussion of the crystal chemistry and a crystal structure study (1969). Dunn (1979) published analytical data at variance with the structure and suggested it was incorrect in part. Additional discussion was given by Campagnoni et al. (1983) in their description of the Mg-analogue, balangeroite. Early proposed relationships to leucophoenicite are invalid. Gageite is not known from Sterling Hill, but has been found at other localities (Dunn et al., 1986).

Crystal structure

Ferraris et al. (1987) published a TEM study of gageite, describing the structures and noting the two polytypes, gageite-2M and gageite-1Tc, both found at Franklin. The structure consists of four-repeat silicate tetrahedral chains running in channels built of chains of edge-sharing octahedra; these form two types of octahedral modules, “walls” and “bundles,” composed of 3 x 1 chain units and 2 x 2 chain units, respectively.

Description

Gageite occurs as elongate crystals, commonly forming bundles and 1-3 mm subparallel groupings, which are commonly platy in part (Figures 17-44 through 17-47).




	Figure 17-45. Termination of a typical gageite crystal from Franklin, showing the irregular and ragged ends of such crystals. Crystal is 0.07 mm in length.

Gageite also occurs as very fibrous colorless inclusions in other minerals. Crystals are common in some vuggy assemblages and are elongate and commonly square or rectangular in cross-section; they may be randomly arranged and may occur in imperfect radial aggregates. Gageite crystals with terminal forms are unknown to the writer; those studied in detail have cavernous and highly irregular terminations (Figure 17-45). Morphological studies have not been done; crystals are commonly warped. Small crystals commonly form matted aggregates; once seen, they are easily recognized and are distinguished from willemite by their lack of fluorescence.

Gageite is commonly light pinkish-red and is tinged with red or brown hues in varying specimens; it is rarely orange-pink. Some material, apparently oxidized, is nearly opaque brown, bronzy, or black; such crystals are internally fresh when broken.

Like glaucochroite, chlorophoenicite, tephroite, and other species, gageite commonly exhibits an “alexandrite effect” and has a different apparent color in artificial light than in daylight. The luster is vitreous. Gageite is a brittle mineral; crystals are easily bruised and disintegrate into acicular shards. Cleavage is likely present, but has not been measured. The density is 3.584 g/cm³.

Optically, gageite is biaxial, negative, with a = 1.723, b = 1.734, and g = 1.736. There is no discernible fluorescence in ultraviolet. Gageite is easily distinguished from chlorophoenicite by the latter’s terminal forms and gageite’s commonly curved crystal faces.

Composition

Gageite is a manganese silicate hydroxide. Small amounts of Mg and Zn may substitute for Mn, but are apparently nonessential (Dunn et al., 1986). Franklin material is remarkably uniform in composition, and the octahedral substituents are perhaps to some degree ordered. The average of 7 closely agreeing analyses by Dunn (1979d) is given in Table 3.

Occurrence and paragenesis




	Figure 17-46. Subparallel, slightly curving crystals of gageite from Franklin. Field of view is 1.0 mm in maximum dimension.

The initial description of gageite by Phillips (1910) was of colorless acicular crystals, associated with zincite, willemite, calcite, and leucophoenicite. Palache (1935) described gageite occurring with pyrochroite, willemite, chlorophoenicite, calcite, leucophoenicite, and zincite. He also mentioned the occurrence of gageite in cracks in the ore.

In general, gageite is found in a matrix of willemite and franklinite; secondary zincite is very commonly associated, but not consistently. Most specimens are found in secondary vein assemblages and vug-fillings in ore. The bulk of specimens have a weathered or slightly etched appearance, and some are generally unattractive. In addition to the minerals noted above, gageite is also associated with hetaerolite, calcite, hodgkinsonite, pyrochroite, and, most commonly, chlorophoenicite.




	Figure 17-47. Broken subparallel array of acicular gageite crystals from Franklin. Field of view is 0.5 mm in maximum dimension.

Rare assemblages include that described for the purportedly Be-bearing vesuvianite, in which delicate, radial, fibrous gageite coats leucophoenicite and vesuvianite and is covered by white barite; gageite is commonly seen as inclusions at the silicate/barite interfaces. Gageite remains a rare mineral in collections; occurrences as inclusions in other minerals are not easily recognized. It has also been found as brown fibrous crystals associated with white fibrous sussexite, barite, clinohedrite, and willemite, and in the holotype sclarite assemblage. Numerous other anomalous assemblages likely exist, and gageite may have been more common at Franklin than is presently recognized.

Name

Gageite was named after Robert B. Gage of Trenton, New Jersey, who was pivotal in the description of a number of Franklin minerals.

FOOTER LBI




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