MANGANAXINITE Chapter16. Sorosilicates and cyclosilicates

MANGANAXINITE

Ca₂MnAl₂BSi₄O₁₅(OH)
Triclinic, P1, a = 7.161, b = 9.213, c = 8.974 Å,
a = 91.80, b = 98.11, g = 77.24^o, Z = 2.




	Figure 16-15. Crystal drawings of manganaxinite from the Trotter Mine (top drawing) and the Parker Shaft area (lower drawing). Drawings are from Palache (1935) who provided crystallographic data.

Manganaxinite was described from Franklin by Genth et al. (1891) who provided the first morphological description and chemical analysis. Ford (1903) established the correct water content using Franklin samples similar to those described by Genth. The morphology of the superb yellow crystals was the subject of additional detailed studies by Aminoff (1919) and Palache (1935). Dunn (1979c) and Dunn et al. (1980c) provided chemical data and Lumpkin and Ribbe (1979) provided crystallographic, compositional, and optical data for several specimens; these are given here. Manganaxinite is the species generally referred to as common axinite in the older literature.

Description

Franklin manganaxinite occurs in two substantially different habits: as massive, light-yellow to white material and as rich deep-yellow to light-yellow to colorless euhedral crystals up to 1 cm in size. The yellow crystals were the subject of all the early investigations. Franklin crystals are mostly yellow; brownish crystals have been found at Sterling Hill. Crystals are sharp and wedge-shaped and have typical axinite habits (Figures 16-15, 16-16, and 16-17); excellent morphological descriptions are provided by Palache (1935). The cleavage is good on {100}; the density is 3.328 g/cm³; and the luster is vitreous. Optically, it is biaxial, negative, 2V = 76^o, with a = 1.685, b = 1.692, and g = 1.695; pleochroism is distinct.




	Figure 16-16. Tabular triclinic manganaxinite crystals from Franklin. Field of view is 0.6 mm in maximum dimension.

The rich yellow, well-crystallized manganaxinites and the distinctly yellow massive material generally have no discernible fluorescence in ultraviolet. However, nearly-colorless euhedral crystals and some massive light-yellow to white samples fluoresce with a strong red color in shortwave and a less intense red in longwave ultraviolet. The intensity of the fluorescence varies. There is insufficient data to establish firm correlations between fluorescence and composition, but the samples which fluoresce have minimal FeO and MnO values less than 12 wt. %. Massive manganaxinite can be confused with a number of species, among them andradite, grossular, and fine-grained feldspars.

Composition




	Table 9. Chemical analyses of manganaxinite, uvite, and dravite.

Manganaxinite is a calcium manganese aluminum boron silicate hydroxide mineral of the axinite group. Franklin material varies in composition, but there is only limited solid-solution toward ferroaxinite or magnesioaxinite. Analyses of specimens with maximum and minimum Mn contents (from 36 analyses) are given in Table 9, together with a partial analysis of a Sterling Hill specimen (Dunn, 1979c; Dunn et al., 1980c). Most Franklin specimens have MnO values of 12-16 wt.%. Numerous additional analyses by the writer are on file at the Smithsonian Institution and Harvard University.

Occurrence and paragenesis

Manganaxinite is known from both Franklin and Sterling Hill. However, the overwhelming preponderance of material, and the best, is from Franklin, specifically from the Trotter and Parker Mines; Sterling Hill specimens are rare.




	Figure 16-17. Euhedral triclinic manganaxinite crystals from Franklin. Field of view is 0.8 mm in maximum dimension.

Manganaxinite is known from a number of specific Franklin assemblages. Additionally, it may form as an accessory mineral in many calcium-silicate assemblages. It is the preferred host for boron in the orebody at Franklin; tourmalines are sparse and mostly occur in the Franklin Marble. Among the most important and esthetic Franklin assemblages is one of crusts of bright-yellow manganaxinite crystals (Figures 16-16 and 16-17) which, together with rhodonite, form the vein assemblage which hosts marsturite (Figures 17-15 through 17-18) and ganophyllite (Figure 18-36); this is described under those species. Manganaxinite and rhodonite also form fine crystals in vuggy seams and veins; contacts with ore are common, and datolite is locally found as younger late-stage coatings.

Much of the less spectacular specimen material occurs as light-yellow druses and crusts and as aggregates in veins and vuggy recrystallized assemblages, associated with andradite, rhodonite, johannsenite, mica, barite, hancockite, willemite, datolite, and other species.

Also found in quantity was a massive variety, light yellow to white, with intense to dull to absent red fluorescence as noted above, and associated with andradite, feldspar, and mica. In massive form, manganaxinite was volumetrically abundant at the north end of the Franklin deposit.

Axinite, undifferentiated as to species, is also found as microcrystals under calcite in veins in the camptonite dikes at Franklin; some of this material has been labeled as titanite.

At Sterling Hill, manganaxinite is a rare mineral. It was found with rhodonite on the 800 level and with epidote, calcite, and heulandite in another assemblage. Among the best Sterling Hill occurrences is one of fine euhedral light-brown crystals, associated with fluorite, epidote, amphibole, scheelite, and calcite on the 1300 level. Both Sterling Hill occurrences were confirmed as manganaxinite by chemical analyses. An analysis of a Sterling Hill specimen is given in Table 9.

FOOTER LBI




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