DUMORTIERITE

Dumortierite – (Al,Fe3+)7(SiO4)3(BO3)O3 – is a fairly rare nesosilicate, characteristic of some types of metamorphic rocks. It is used in the manufacture of porcelain. Macroscopically, it can be confused with sodalite and has been used as an imitation of lapis lazuli.

Dumortierite is classified in the Dumortierite Supergroup, is isostructural with magnesiodumortierite, and is very similar to holtite, which is another orthorhombic nesosilicate containing B. Dumortierite may contain Fe, Ti, Ca, and Mg as impurities. The “blue quartz” sold in rock stores may be quartzite with dumortierite (or quartzite with kyanite).

1. Characteristics

Crystal system: Orthorhombic bipyramidal. 

Color: Blue, can be greenish or pale violet. Brown, pink, red. 

Habit: Long, columnar, fibrous, granular, massive prismatic structure. 

Cleavage: {100} distinct, {110} bad. 

Tenacity: Brittle.        

Twinning: On {110}, “trillings” are possible.       

Fracture: No information available.       

Mohs Hardness: 7 – 8

Parting: On {001}         

Streak: No information available.          

Lustre: Vitreous.          

Diaphaneity: Transparent.           

Density (g/cm³): 3.21 – 3.41

 

2. Geology and Deposits

Dumortierite is a rare mineral, restricted to aluminum-rich rocks that have undergone regional metamorphism.

In this context, it occurs both disseminated in rocks (for example, quartzite with dumortierite) and in pegmatitic veins of granitic composition that cross schists or gneisses.

It can occur in B-rich pegmatites and in some contact metamorphic rocks (skarns).

 

3. Mineral Associations

It occurs associated with the normal minerals of pelitic metamorphic rocks, such as quartz, calcic feldspars (albite), potassium feldspars (orthoclase), micas (biotite, muscovite), epidote, and apatite.

At the metamorphic grade in which it occurs, it is associated with minerals of the Al2SiO5 Group (kyanite, sillimanite, andalusite), cordierite, tourmaline, topaz, pyrophyllite, hematite, and rutile.

It can be closely associated with tourmaline; it also replaces sillimanite and biotite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα:  1,659 – 1,678   nβ: 1,684 – 1,691    nγ: 1,686 – 1,692

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Strong pleochroism: the darkest color occurs when the prismatic crystals are oriented E-W in the field of view.

X = dark blue to violet; green or brown are rarer.

Y = colorless to yellow, pink, green to reddish-violet.

Z = colorless to pale colors: blue or yellow or green.

Pleochroism can range from colorless to pink.

Relief: High.           

Cleavage: {100} distinct, {110} bad, parting at {001}. Cleavage is not very sharp; grains without visible cleavage are common. Fractures are common.           

Habits: Columnar, fibrous to acicular crystals, can form aggregates of parallel fibers, also xenomorphic (anhedral) and massive.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Maximum birefringence of 0.027, corresponding to colors from the 1st order to the beginning of the 2nd order (gray, orange, red, blue, green). Generally, the colors are of the higher 1st order and increase with increasing Fe content.           

Extinction: Parallel (to length and cleavage), as expected of an orthorhombic mineral.          

Elongation sign: ES(-)            

Twins: Common in {110}, it can form cyclic twins of 3 individuals (“trillings”). Twinned crystals may show pseudo-hexagonal basal sections.         

Zoning: No.             

CONVERGENT LIGHT

Character: B(-). The figure is difficult to obtain in small grains and fibers.          

2V angle: 20 – 52º         

Alterations: May alter to sericite (white mica). 

May be confused with: Some minerals that exhibit blue colors and pleochroism.

Glaucophane shows stronger blue colors. In addition, it has oblique extinction and SE(+).

Riebeckite also exhibits blue pleochroism, but it shows the typical cleavage of amphibole minerals, and the extinction can be oblique.

Tourmaline can exhibit blue colors, but these colors are usually zoned (in patches). In addition, it does not have cleavage, and the basal sections are triangular or hexagonal.

Sapphrine also shows pleochroism between blue and colorless, but the interference colors are much lower; at most, it goes to light gray, not reaching yellow.

When dumortierite exhibits a pink color with pleochroism, it is very similar to fibrous andalusite and fibrous sillimanite (= fibrolite). But sillimanite has SE(+). Fibrous andalusite, on the other hand, is difficult to differentiate from dumortierite with pink pleochroism.

Dumortierite 01: Dumortierite in quartzite with muscovite in PPL, showing its pleochroism between colorless and blue and its relief much higher than that of the quartz and muscovite around it. The crystal shapes tend to be prismatic, but granular forms are also common.
Dumortierite 01: The same section as of the previous image, now rotated 90 degrees to see the pleochroism in blue colors.
Dumortierite 02: In PPL, long prismatic crystals of dumortierite, more or less aligned in quartzite.
Dumortierite 02: The same section as of the previous image, now rotated 90 degrees to see the pleochroism in blue. The high relief against quartz is very clear.
In XPL, dumortierite in quartzite. Quartz is gray and dumortierite exhibits colors of upper first order.
Dumortierite can present itself with a very fine, almost fibrous habit, as in the image to the side, obtained in PPL. It can also present itself with pleochroism ranging from colorless to pink, which is the case here. If the fibers are very fine, it is necessary to identify some larger crystals to detect the pink pleochroism.
Dumortierite 03: In PPL, fibrous dumortierite with pink pleochroism, which can be seen in the larger crystals.
Dumortierite 03: The same section as of the previous image, also in PPL, rotated 90 degrees to see the pleochroism of the crystals.
In XPL, fibrous dumortierite showing grayish interference colors.

5. Reflected Light Microscopy

Reflected Light Microscopy is evidently not the recommended analytical method for identifying dumortierite. However, it is important to create a polished slide or section to identify the opaque minerals that occur associated with dumortierite, such as hematite.

Sample preparation: Dumortierite acquires a great polish without difficulty.       

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray like quartz and feldspars, but with a distinct pleochroism: the lighter color corresponds to the color of quartz, the other color is darker.       

Pleochroism: Distinct between light gray and dark gray. The darker color is seen when the prisms are oriented vertically in the field of view.

Reflectivity: Low (<<10%).        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Strong anisotropy between colorless and blue, very diagnostic.        

Internal reflections: Colorless and milky, often multicolored when the cleavage is arranged at a favorable angle relative to the plane of the polished section.      

May be confused with: No other common minerals.

Secondary blue copper minerals show other colors and anisotropies.

Riebeckite is generally very dark, without anisotropy.

Glaucophane exhibits very faint blue reflections, without anisotropy.       

Dumortierite 01: In PPL, prismatic crystals of dumortierite in quartzite with muscovite. The color of quartz and dumortierite grains is practically the same, but their shapes are very different: quartz is granular and xenomorphic, while dumortierite is lamellar and prismatic, like muscovite.
Dumortierite 01: The same section as of the previous image, now in XPL. Quartzo shows colorless, milky and multicolored internal reflections. Dumortierite exhibits blue internal reflections. Rotating the platinum slab 90º, most dumortierite crystals are oriented vertically and appear colorless.
Dumortierite 02: In the center of the image, in XPL, three parallel dumortierite crystals show, in this position, the first of their anisotropic colors, a very faint blue, which can be almost colorless. Next to it, a crystal with multicolored internal reflections, which sometimes occurs.
Dumortierite 02: The same section as of the previous image, also in XPL and now rotated 90 degrees. In this position, the dumortierite crystals show their second anisotropy color, as very dark blue.