PIEMONTITE

Piemontite – Ca2(Al,Mn3+,Fe3+)3O(Si2O7)(SiO4)(OH) – is a sorosilicate with a very restricted distribution. It has no economic importance.

Piemontite is one of the members of the Epidote Group. It can contain Na, K, Fe, Ti and H2O and there is a variety with Sr.

In many cases, a deep red variety of epidote, containing Mn3+, is erroneously called “piemontite”. Therefore, care is needed when calling any red-colored sorosilicate from the Epidote Group “piemontite”.

1. Characteristics

Crystal system: Monoclinic prismatic.          

Color: Red, red–violet, brown–red to reddish black.

Habit: Prismatic, tabular, granular, acicular, may form radial aggregates.

Cleavage: {001} perfect, {100} poor.       

Tenacity: Brittle.        

Twinning: Lamellar on {100}, rare.       

Fracture: Irregular.       

Mohs Hardness: 6 – 6.5

Parting: No.         

Streak: Reddish.         

Lustre: Vitreous.          

Diaphaneity: Transparent.           

Density (g/cm³): 3.46 – 3.54

 

2. Geology and Deposits

Piemontite is a relatively rare mineral, forming in Mn-rich metamorphic rocks generated by low to medium grade regional metamorphism (greenschist to amphibolite facies), such as schists and gneisses.

It occurs in hydrothermal veins of felsic rocks such as rhyolites, andesites and diorites that have undergone hydrothermal alteration, as well as in metasomatic deposits (skarns) of Mn minerals.

 

3. Mineral Associations

Piemontite occurs with quartz, epidote, zoisite (thulite), tremolite, glaucophane, orthoclase, calcite, chloritoid, garnet (spesartine) and braunite.

Also to some rare minerals such as yttrocrasite-(Y) and lavoisierite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.725 – 1.756   nβ: 1.730 – 1.789    nγ: 1.750 – 1.832

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism:  Strong colors: pink, yellow, orange, red and violet, with strong pleochroism: X = yellow, Y = violet-red, Z = dark red.

Relief: High to very high.           

Cleavage: {001} perfect and {100} poor.           

Habits: Prismatic crystals, 6-sided sections, acicular, may form radial aggregates such as epidote.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence from 0.025 to 0.073: intense 2nd to 4th order colors. Interference colors are usually masked by the intense colors of the mineral. Colors change little or do not change between PPL and CPL.           

Extinction:  Parallel or oblique, with an angle of 0 – 34º. Large sections usually show parallel extinction.

Elongation sign: ES(+) or ES(-), it is not diagnostic.            

Twins: Rare, polysynthetic, lamellar, according to {100}.

Zoning: No.             

CONVERGENT LIGHT

Character: B(+) or B(-)          

2V angle:  64 – 106º        

Alterations: it hardly alters, but it can alter to chlorite.          

May be confused with:  the colors and pleochroism are very characteristic and there is no other common mineral with these characteristics.

Zoisite has a red variety called “thulite”, which is very similar, but has a pleochroism between pink and yellow (it does not have the violet tones of piemontite).

Clinozoisite has a pink variety called “clinothulite”, typical of granitic pegmatites, which has anomalous interference colors in CPL and not the intense colors of piemontite.

Cuprite (CuO) at first glance may be similar, but it does not have pleochroism and the red tone is different, deeper, less pink and does not have violet tones.

Cinnabar (HgS) may be similar, but is much less transparent (darker colors) and the paragenesis is different.

Realgar (As4S4) also features red colors. 

If the piemontite crystals are large, they are easily visible on the thin section seen with the unaided eye.
Piemontite 01: In PPL, piemontite showing, in this position, one of its pleochroism colors.
Piemontite 01: The same section as of the previous image, also in PPL, now rotated 90 degrees to show the second pleochroism colors.
Piemontite 01: The same section as of the two previous images, now in XPL. The colors, due to their intensity, remain practically the same.
Piemontite in XPL with polysynthetic twins.
In XPL, piemontite crystals showing almost the same intense colors, red and violet, that they showed in PPL. Several are twinned. Around it, plagioclase (gray, twins).
Piemontite in PPL, showing strong pink colors in its variations. Pleochroism is sharp and relief is high.
Piemontite 02: In PPL, piemontite with pink colors and high relief. Habit is typically granular.
Piemontite 02: The same section as of the previous image, now in XPL. Strong pink colors mask interference colors. As can be seen by the yellow interference color of the quartz grains, the thin section is a little too thick.

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical method for the identification of piemontite. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with piemontite.

Sample preparation: the polishing of piemontite offers no problems other than those caused by its excellent cleavage, as is the case with epidote as well. The polishing quality is approximately equal to that of quartz, but with several holes caused by loss of material during grinding.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, a little lighter than quartz, with a slight greenish hue.       

Pleochroism: No.      

Reflectivity: Low (<< 10%)        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Distinctive in shades of gray, visible despite intense internal reflections. 

Internal reflections: Intense, widespread between various shades of strong pink, orange and red. The thicker the grain at the point considered, the darker the shade of pink and vice versa.

May be confused with: few other common minerals. Possibly with cinnabar and realgar.

Piemontite 01: In PPL. in quartzite(?), grains of a mineral of Mn(?)(white, lamellar, fusiform), piemontite (lighter gray, slightly greenish, in the center of the image), quartz (dark gray) and holes (black).
Piemontite 01: The same sectionas of the previous image, now in XPL. Piemontite shows intense internal reflections between orange, pink and red. Quartz shows colorless to white internal reflections.
Piemontite 02: In PPL, a mineral classified as “piemontite”, but the crystals with associated green internal reflections indicate the possibility that it is just an epidote-Mn. At right, quartz.
Piemontite 02: The same section as of the previous image, now in XPL. As said, may be not piemontite.