RHODONITE

Rhodonite – (Mn,Fe,Mg,Ca)SiO3 – is an inosilicate of the Pyroxene Group. It is a rarer mineral, characteristic of specific paragenesis of Mn minerals. It has no economic importance other than its use as a decorative stone.

It is classified in the Rhodonite Group and generally contains traces of Mg, Fe, Al, Zn and Ca. It has four varieties and there is a high pressure, low temperature polymorph, which is pyroxmangite. Rhodonite forms epitaxy with tiragalloite and marturite.

1. Characteristics

Crystal system:  Triclinic pinacoidal.         

Color: Red, pink, reddish-brown to gray. Black crusts of Mn minerals are possible.     

Habit: Coarse crystals, tabular to elongated, with rounded edges. It can be micritic.       

Cleavage: {110} perfect, {001} good.      

Tenacity: Brittle.        

Twinning: . Lamellar on {010}.       

Fracture: Irregular.       

Mohs Hardness: 5.5 – 6.5

Parting: No.         

Streak: White.         

Lustre:  Vitreous, pearly.         

Diaphaneity: Transparent.           

Density (g/cm³): 3.57 – 3.76

          

2. Geology and Deposits

Rhodonite is a rare mineral, always associated with Mn deposits, whose origin can be through hydrothermal processes, contact metamorphism (scarns) or regional metamorphism (schists).

It also occurs in deposits with Mn originated by sedimentary processes. When rhodonite occurs in pelites, it is very fine-grained, difficult to recognize. As in the case of calcite, we have to distinguish rhodonite in microscopic crystals (easier to recognize) and micritic rhodonite in submicroscopic crystals, much more difficult or even impossible to recognize.

 

3. Mineral Associations

Occurs with quartz, garnet (andradite), pyrite, sphalerite, galena, axinite-Mn and cahnite.

In Mn ores of sedimentary origin, it associates with calcite, quartz, goethite, hematite, clay minerals (illite, kaolinite), chlorite, pyrite, anatase, gibbsite and Mn minerals: pyrolusite, cryptomelane, nsutite, todorokite, litiophorite, manganopyrosmaltite and ramsdellite.

There are some unique deposits where rhodonite occurs:

   – in Franklin (New Jersey, USA) it is associated with calcite, franklinite and willemite.

   – in Bald Knob (North Carolina, USA) it is associated with calcite, alleghanyite, tephroite, galaxite, grunerite and magnetite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.711 – 1.738    nβ: 1.714 – 1.741     nγ: 1.724 – 1.751

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless. May have a slight pink color. In this case, there may be weak pleochroism between:

X = yellowish red,

Y = pinkish red and

Z = pale yellowish red.

Relief: High.           

Cleavage: {110} perfect. Longitudinal sections show only one cleavage.

Basal sections show two cleavage directions that intersect at 92.5° angles. Furthermore, rhodonite has good cleavage at {001}.            

Habits: Usually massive or in cleavable masses. The rare crystals are thick tabular.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Maximum birefringence of 0.013, corresponding to colors from gray, white to 1st order yellow, does not reach orange.           

Extinction: Oblique up to 25º.           

Elongation sign: No information available. Being triclinic, it probably doesn’t apply.

Twins: May have lamellar twins.         

Zoning:  No.            

CONVERGENT LIGHT

Character: B(+)          

2V angle:  58 – 73º           

Alterations: alters to Mn oxides which are opaque in thin section.

May be confused with: the two cleavages in the basal sections, which resemble a pyroxene, are very diagnostic. Macroscopically, it has a very diagnostic red-pink color, similar, a bit ligther, to that of rhodochrosite.         

Rhodonite 01: In PPL, in the center of the image, a basal section of rhodonite showing the two cleavages intersecting at nearly right angles.
Rhodonite 01: The same section as of the previous image, now in XPL. Rhodonite shows weak interference colors up to the middle of the first order, like kyanite.
In XPL, longitudinal sections of rhodonite showing interference colors between dark gray and yellow.

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical method for the identification of rhodonite. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with rhodonite, as Mn minerals. But other analytical techniques are usually necessary for the conclusive identification of rhodonite, especially if the ore granulometry is submicroscopic, which is a very common situation in manganese ores.

Sample preparation: rhodonite in large crystals acquires a great polish without much effort. There are clear differences between aggregates of larger crystals (whose polishing is much better) and aggregates of microscopic and submicroscopic crystals (whose polishing is of much lower quality).

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, lighter than quartz and feldspars.       

Pleochroism: No.      

Reflectivity: Low (<<10%).        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Anisotropy was not observed.        

Internal reflections:  Widespread in colors ranging from colorless and white to very intense red, with or without a slight brownish hue.     

May be confused with: rhodochrosite, a Mn carbonate that can occur in association, but which shows the typical cleavage of carbonates if in well developed crystals. 

Rhodonite 01: In PPL, an unidentified Mn mineral (left in image) and rhodonite (right in image). There are no diagnostic aspects for either.
Rhodonite 01: The same section as of the previous image, now in XPL. The Mn mineral is dark and the rhodonite presents widespread internal reflections in various shades of pink, sometimes very light, sometimes more intense.