RIEBECKITE

Riebeckite – Na2Fe2+3 (Al,Fe3+)2Si8O22(OH)2 – is an inosilicate of the Amphibole Supergroup. It is a comparatively rare mineral. When fibrous, is one of six minerals marketed as “asbestos”, a material that is used less and less nowadays due to the health problems it causes. The variety of riebeckite known as “crocidolite” is the “blue asbestos” which is, of all types of asbestos, the most dangerous to health.

Riebeckite is part of the Riebeckite Group, chemically it is very close to arfvedsonite and may contain, as impurities, Mn, Fe, Ti, Al and Mg.

1. Characteristics

Crystal system: Monoclinic prismatic.          

Color: Black, can be light blue to dark blue, gray blue, gray, brown.

Habit: Long prismatic (typical of amphiboles), up to 20 cm. Acicular, columnar, radiated, tabular, massive, may be earthy.       

Cleavage: {110} perfect, as all amphiboles.      

Tenacity: Brittle.        

Twinning: Simple or multiple, paralell to {100}.       

Fracture: Splintery.       

Mohs Hardness: 5 – 5.5

Parting: No.         

Streak: Pale gray to bluish gray.         

Lustre: Vitreous.          

Diaphaneity: Transparent.           

Density (g/cm³): 3.28 – 3.44

          

2. Geology and Deposits

Riebeckite is not a common mineral. It is found primarily in alkaline igneous rocks, rich in Na, such as syenites (with aegirine), trachytes and alkaline granites. “Ailsite” is a riebeckite-granite.

It can occur in pantellerites and volcanic comendites (alkaline rhyolites to alkaline trachytes).

In metamorphic rocks, riebeckite can occur in schists, gneisses and granulites and is common, with an asbestiform habit, in banded iron formations (BIFs), in the variety known as crocidolite.

It can occur in alkaline metasomatic rocks related to faults.

“Crossite” is a term now discredited, which designated intermediate compositions between glaucophane and riebeckite.

 

3. Mineral Associations

Usually riebeckite is associated with quartz (rock crystal, smoky quartz), feldspars (albite, microcline, “anorthoclase”), feldspathoids (nepheline), arfvedsonite, aegirine, zircon, bastnäsite-(Ce), danalite, astrophyllite, hydroastrophyllite and TiO2 polymorphs (anatase, rutile).

In igneous rocks it occurs with feldspars (sanidine), feldspathoids (nepheline), pyroxenes (aegirine) and amphiboles (arfevdsonite).

In metamorphic rocks it is associated with amphiboles such as iron-actinolite and tremolite.

In banded iron formations, it occurs with Fe oxides (magnetite, hematite), goethite, carbonates (calcite, siderite, ankerite), silica (quartz, chalcedony), grunerite and stilpnomelane.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.680 – 1.698     nβ: 1.683 – 1.700    nγ: 1.685 – 1.706

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Strong pleochroism between:

X = Y = dark blue to blue-green to black and

Z = dark yellow-green to pale yellow-green to brown.

The smaller the crystals and the denser the fiber aggregate, when applicable, the more difficult it is to perceive these pleochroism colors.  

Relief: Moderate to high.           

Cleavage: Perfect {110} cleavage.

In the longitudinal sections, there is only one cleavage. In the basal, rhombic sections, there are two cleavages that intersect at 56º and 124º, as in all amphiboles.

But when riebeckite shows a fibrous or acicular habit, this cleavage is no longer visible.           

Habits: Prismatic wide, short or long. Acicular or fibrous.

Basal sections, when it is possible to observe, have a rhombic shape.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors:

Birefringence from 0.006 to 0.008, corresponding to first-order interference colors, ranging from dark to light gray and white, such as quartz.

But the strong own colors of the mineral mask these interference colors in such a way that the color of riebeckite practically does not change between PPL and XPL.           

Extinction: Tends to oblique with an angle between 0 and 30º.

Therefore, it can simulate being parallel!           

Elongation sign: ES(-), sometimes ES(+), difficult to verify due to the intense own color of the mineral.            

Twins: Simple and lamellar on (100), only visible in larger crystals.

Zoning: Often zoned, usually zonation can be seen only in wider/larger crystals; in fibers is impossible.             

CONVERGENT LIGHT

Character: B(+) or B(-), can simulate being uniaxial.          

2V angle: 40 – 100º, very variable.         

Alterations: to iron oxides and carbonates.          

May be confused with: few other minerals. Its cleavage, colors and pleochroism are very diagnostic. When fibrous, the habit is diagnostic, as other blue minerals do not have it.

Glaucophane occurs in other paragenesis and has different pleochroism colors (violet, etc.) and much more intense interference colors.

Dumortierite usually has a much lighter blue color, pleochroism varies between colorless and blue.

Tourmaline is more rarely blue, it usually has another habit and its colors usually have spots (zoning).

Arfvedsonite is much darker, some of its colors have greenish hues and it has the maximum absorption colors along the z axis.         

Riebeckite 01: In PPL, riebeckite, variety crocidolite (dark) with stilpnomelan (yellow) and quartz (colorless) in a banded iron formation (BIF, also called taconite). Observing with the lower magnification objective (2.5x), as in the images, the deep blue color of riebeckite makes the mineral look like an opaque mineral in both PPL and XPL, but there is no opaque mineral with this habit.
Riebeckite 01: The same section as of the previous image, now in XPL. Quartz in gray/white, stilpnomelane brownish and riebeckite very dark, but the fibrous habit is very diagnostic.
Riebeckite 02: In PPL, in detail, the very fine, fibrous habit of riebeckite. Around it, quartz.
Riebeckite 02: The same section as of the previous image, now in XPL. Riebeckite is so dark that it resembles an opaque mineral.
When the crocidolite fiber bundles are less thick and/or dense, it is possible to perceive the pleochroism colors of the mineral, between dark blue almost black and a pale green yellow. Image in PPL.
Riebeckite 03: In PPL, riebeckite fibers showing its first pleochroism color. The next image shows the second color.
Riebeckite 03: Also in PPL, the same crystals of the former image, now rotated 90 degrees clockwise to show the second color of pleochroism.
Riebeckite 04: In PPL, two aggregates of riebeckite fibers in banded iron formation. The aggregate on the left is less dense and therefore it is possible to perceive pleochroism and blue colors. The aggregate on the right is denser and therefore it is not possible to perceive the pleochroism or the deep blue color, it looks black, almost opaque. Interference colors are masked by the own blue color.
Riebeckite 04: The same section as of the previous image, now in XPL. The interference colors of riebeckite are masked by the own dark blue colors of the mineral.
In PPL and observed with the objective of maximum magnification (63x), crocidolite fibers showing their very fine pointed ends. It is clear that observation of basal sections is almost impossible with riebeckite in this habit.
Riebeckite 05: In PPL, riebeckite in trachyte In this position, riebeckite shows its first pleochroism color. The next image is of the second pleochroism color.
Riebeckite 05: The same section as of the previous image, now rotated 90 degrees and showing the second pleochroism color. The crystals are very small and the bluish colors are difficult to see.
In PPL, riebeckite in trachyte. As the riebeckite crystals are extremely small, their blue color in PPL is so dark that it looks like an opaque mineral.
In XPL, riebeckite in trachyte. Care is needed to detect the blue interference color.
Riebeckite 06: In PPL, riebeckite in granulite. In PPL, the deep blue color and pleochroism are only visible with close attention and high magnification. The color at the maximum illumination position is so dark that the blue is barely noticeable.
Riebeckite 06: The same section as of the previous image, now in XPL. Riebeckite grains have such a deep blue color that they simulate a position of extinction or being opaque. It takes a lot of attention to recognize them. Pyroxenes show their colored interference colors.

5. Reflected Light Microscopy

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

Sample preparation: the degree of polish of riebeckite depends on the size of the crystals. Large grains get very good polish, but aggregates of small crystals don’t polish well and end up with lots of holes. In a way, this can be diagnostic.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray like common silicates (quartz, feldspars). 

Pleochroism: No.      

Reflectivity:  Low (<<10%).       

Bireflectance:  No.      

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Strong anisotropy in the same colors as pleochroism, between deep blue and yellowish. Large grains can be so dark that this anisotropy is not noticeable.       

Internal reflections: Occasional, depending on the orientation of the cleavages in relation to the plane of the polished section. Strong, deep blue colors, very luminous are typical.      

May be confused with: few other minerals, due to the characteristic habit and strong blue colors.  

Riebeckite 01: In PPL, riebeckite (gray), bornite (pink) and chalcocite (white) in copper ore. The habit of riebeckite is already quite diagnostic.
Riebeckite 01: The same section as of the previous image, now in XPL. Bornite and chalcocite are very dark, even with XPL+2°. The riebeckite presents internal reflections in very luminous deep blue.
Not all riebeckite crystals show the reflections in blue, but there are always several where the reflections are well defined, as in this example here. XPL.
Riebeckite 02: In PPL, in a trachyte, riebeckite (left) and sanidine (right). The poor polishing of aggregates of small grains of riebeckite contrast with the good polishing of the feldspars.
Riebeckite 02: The same section as of the previous image, now in XPL. The blue colors of riebeckite are easily visible.
Riebeckite 03: Riebeckite and quartz in a granulite, in PPL, showing its first anisotropy color. The next image shows the second anisotropy color.
Riebeckite 03: The same grains as of the previous image, now rotated 90 degrees to show the second anisotropy color. . Riebeckite shows marked anisotropy with virtually the same colors as its pleochroism under Transmitted Light.
Riebeckite 04: In PPL, granulite with riebeckite and quartz. Quartz and riebeckite have the same reflection colors, pyroxenes are lighter, holes in black.
Riebeckite 04: The same section as of the previous image, now in XPL. Riebeckite shows reflections in deep blue, pyroxene shows reflections in green colors and quartz has milky reflections.