Piemontite
Monoclinic
Ca2Al2Mn3+[Si2O7][SiO4]O(OH)
Piemontite is the manganese-bearing variety of epidote and shows very distinctive deep red and violet colors and pleochroism. The name derives from the Piedmont region of Italy, as the first described specimens of piemontite were from Prabornaz mine, located in the Aosta Valley (formerly a province of Piedmont, which became an autonomous region after 1946). The name changed several times since the original designation of röd Magnesia (Cronstedt, 1758) before being named Èpidote manganésifere by Haüy (1822) and Piemontischer Braunstein by Werner (1817), which was finally shortened to to piedmontite in 1853 by Kenngott.
Structure and chemistry
Strictly speaking, piemontite is pure Ca2Al2Mn3+[Si2O7][SiO4]O(OH). However, in nature, piemontites are rather Mn3+-bearing epidotes that normally contain less than 40% of the piemontite end-member. Likewise epidote, piemontite consists of isolated [SiO4] tetrahedrons and [Si2O7] double tetrahedral groups that connect chains of edge-sharing octahedral sites (M) containing Fe3+,Al, and Mn3+, which are oriented parallel to the intermediate axis b. The tetrahedral and octahedral sites surround large cation sites (A sites) that accommodate Ca cations. There are three different octahedral sites: M2 octahedral chains, separated from the other octahedral sites, and M1 – M3 chains, which are attached to one another. The M3 site is distorted and non centrosymmetric. Al has a strong preference for the M2 site, while Fe3+ is concentrated in the larger M3 site. Mn also has a preference for the M3 site, where it is present both in trivalent (3+) and divalent (2+) oxidation state. The increase in Mn-content in epidote corresponds to an increase in the b and c cell parameters.
The compositional range of piemontites varies greatly between epidote [Ca2Al2Fe3+Si3O12(OH)] and pure piemontite [Ca2Al2Mn3+Si3O12(OH)], with significant proportions of [Ca2AlMn2Si3O12(OH)]. However, pure piemontites are rare in nature, due to the general scarcity of Mn in natural rocks. Therefore, technically the piemontite grains found in rocks ara rather piemontite-bearing varieties of epidote.
Properties
Habit: prismatic
Hardness: 6
Density: 3.38-3.61 g/cm3
Cleavage: {001} perfect
Twinning: {100} lamellar, uncommon
Color: red, reddish brown, black
Luster: vitreous
Streak: red
Alteration: –
In thin section…
α(α^c = 2-9°): 1.730-1.794
β(//b): 1.740-1.807
γ(γ^a = 27-34°): 1.762-1.829
2Vγ: 64-106°
Color: violet, pink, yellow
Pleochroism: α yellow to lemon or orange yellow, β pale pink, violet pink, amethyst, γ red, crimson, magenta
Birefringence (δ): 0.025-0.073 (high but often masked by the strong pleochroism)
Relief: high
Optic sign: +
[Mindat]
[HoM]
Field features
Piemontite in the field occurs as reddish/violet (often deep red to even reddish black) prismatic grains in low-grade schists of greenschist-, blueschist-, and amphibolite-facies. Even tiny grains, too small to be detected on hand samples, can give their host rock a distinctive purple color. Piemontite can also be found in Mn-rich hydrothermal deposits.
Piemontite in thin section
Piemontite shares many of the optical and physical properties of other members of the epidote group, such as prismatic habit, high relief, presence of a single set of cleavage planes, straight extinction on prismatic sections parallel to b, and oblique extinction on all other sections. However, its most distinctive feature is the red/violet/pink color in thin section, associated with a strong pleochroism varying from yellow to pink/violet and red/crimson. The variability of pleochroism is maximum on sections containing the a- and c-axis (pale yellow to deep red). Piemontite technically shows very high interference colors at CPL, but these are commonly masked by the pleochroism.
Video. The typical colors of pleochroism of piemontite, ranging from yellow/pale yellow to violet, purple, and red. PPL. Width: 2.5 mm. Piemontite schist, Sanbagawa belt. Asemigawa, Shikoku, Japan.
Video. The pleochroism of epidote varies depending on the section considered from α yellow/orange to β pink/purple and even γ red/crimson. PPL. Width: 2.5 mm. Piemontite schist, Sanbagawa belt. Asemigawa, Shikoku, Japan.
Video. Likewise other minerals of the epidote group, piemontite is elongated on the b-axis. This implies that elongated grains can either be length slow (section containing α and β) or length fast (section containing γ and β), as shown by the two neighboring piemontite grains in the video above. CPL, CPL + λ. Width: 1.2 mm. Piemontite schist, Sanbagawa belt. Asemigawa, Shikoku, Japan.
CPL + λ
CPL
PPL
⇔ slider. Piemontite crystals, surrounded by quartz, white mica, chlorite, and hematite in a schist. The colors are highly variable due to differences in orientation and presence of zoning patterns (i.e. the small basal section close to the center). Width: 2.5 mm. Piemontite schist, Sanbagawa belt. Asemigawa, Shikoku, Japan.
CPL
PPL
⇔ slider. Basal section of piemontite, showing an increase in color from core to rim (zoning), corresponding to an increase in the Mn content. Width: 1.2 mm. Piemontite schist, Sanbagawa belt. Asemigawa, Shikoku, Japan.
Examples of piemontite-bearing rocks
Piemontite schist
Piemontite in a low-grade (greenschist-facies) schist, characterized by the presence of Fe3+-bearing oxides (i.e. hematite). Piemontite likely formed due to breakdown of Fe3+ and Mn3+-bearing minerals.
Sample: piemontite schist, Sanbagawa metamorphic belt
Assemblage: albite, piemontite, quartz, white mica, chlorite, hematite, apatite
Locality: Asemigawa (Asemi river), Motoyama, Kochi, Shikoku, Japan
Occurrence
Piemontite is a typical product of low-grade regional and high-pressure metamorphism of Mn-bearing rocks. It is found in greenschists, blueschists, and amphibolites in schists, marbles, and metabasic rocks. It forms from the breakdown of Mn-oxides and Mn-bearing phyllosilicates and, at medium- to high-metamorphic grade, it breaks down to form spessartine-rich garnet. Piemontite is also found in a variety of hydrothermal Mn-deposits in association with other Mn-bearing minerals.
Bonazzi, P., & Menchetti, S. (2004). Manganese in monoclinic members of the epidote group: piemontite and related minerals. Reviews in Mineralogy and Geochemistry, 56(1), 495-552.
Dollase, W. A. (1969). Crystal structure and cation ordering of piemontite. American Mineralogist: Journal of Earth and Planetary Materials, 54(5-6), 710-717.
Kawachi, Y., Grapes, R. H., Coombs, D. S., & Dowse, M. (1983). Mineralogy and petrology of a piemontite‐bearing schist, western Otago, New Zealand. Journal of Metamorphic Geology, 1(4), 353-372.
Keskinen, M., & Liou, J. G. (1987). Stability relations of Mn–Fe–Al piemontite. Journal of Metamorphic Geology, 5(4), 495-507.
Miyashiro, A., & Seki, Y. (1958). Enlargement of the composition field of epidote and piemontite with rising temperature. American Journal of Science, 256(6), 423-430.
Resources
An introduction to the Rock-Forming Minerals. Deer, Howie & Zussmann.
Optical Mineralogy: Principles & Practice. Gribble & Hall.
Transmitted Light Microscopy of Rock-Forming Minerals: An Introduction to Optical Mineralogy (Springer Textbooks in Earth Sciences, Geography and Environment). Schmidt.
Traduzione in corso!
Le pagine in Italiano dovrebbero essere disponibili nuovamente nel giro di qualche mese.
English 
Italiano