CORDIERITE

Cordierite – (Mg,Fe)2Al4Si5O18 – is a rarer cyclosilicate, a mineral typical of contact metamorphic rocks. Synthetic cordierite is used in the manufacture of catalytic converters. Natural cordierite is only economically important when violet and transparent: in this case it is called “iolite” and used as a gemstone.

It forms a series with sekaninaite, an extremely rare orthorhombic cyclosilicate of very specific lithotypes. The high-temperature dimorphic mineral of cordierite is indialite, a very rare hexagonal cyclosilicate generated by melting or polymetamorphism of pelitic sedimentary rocks. Macroscopically cordierite can be confused with members of the Osumilite Group, another hexagonal cyclosilicate.

Cordierite can contain Ca, K, Na, Ti, Mg and Fe and forms pseudohexagonal crystals.

1. Characteristics

Crystal system: Orthorhombic bipiramidal.          

Color: Blue, bluish, bluish violet, greenish, yellowish brown, gray.    

Habit: Usually granular, massive. Pseudo-hexagonal prismatic twins.

Cleavage: {100} distinct, {001} and {010} poor. Striations // to [001].

Tenacity: Brittle.        

Twinning: Common, simple, lamellar, cyclic, by {110} and {130}.

Fracture: Subconchoidal.       

Mohs Hardness: 7 – 7.5

Parting: No.         

Streak: White.         

Lustre: Resinous, vitreous.          

Diaphaneity: Transparent.           

Density (g/cm³): 2.57 – 2.66

          

2. Geology and Deposits

In small amounts it occurs in magmatic rocks such as granites, gabbros, rhyolites and andesites. Cordierite is an index mineral for the anatectic origin of the rock or contamination by Al-rich host rocks (such as clay-mineral-rich sediments).

Cordierite is very common in contact metamorphism rocks. At the outer contact aureloas, cordierite prophyroblasts form schists; cornubianites with cordierite are also formed.

Cordierite also occurs in metapelites that have undergone high-grade regional metamorphism, such as gneisses to cordierite. It is also common in granulites and charnochites, as well as pegmatitic metatexites.

 

3. Mineral Associations

In igneous rocks and metatexites, it occurs with quartz, K-feldspar, plagioclase, andalusite, sillimanite, kyanite, biotite, garnet, rutile, ilmenite, pyrrhotite, chalcopyrite, pyrite and orthopyroxene (enstatite).

In schists it forms as neoformed blasts in a matrix of quartz, biotite, sericite, chlorite, mullite, talc, hematite, anthophyllite and graphite.

In cornubianites it occurs with biotite, muscovite, quartz, plagioclase, andalusite, enstatite and garnet.

In silica undersaturated rocks, cordierite coexists with corundum and spinel.

In rocks formed by contact metamorphism, it typically occurs with sillimanite (which forms fibrous inclusions known as fibrolite), garnet, biotite, ilmenite, and hercynite. May occur detrital.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.527 – 1.560    nβ: 1.532 – 1.574     nγ: 1.538 – 1.578

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless if the thin section is of standard 30 microns thickness.

In thick thin-section it is pleochroic between yellowish and pale blue or pale purple, in volcanic rocks sometimes pleochroic between pale blue and pale purple.  

Relief: Low negative to low positive.           

Cleavage: {100} poor, which is not visible under a microscope. It often has very irregular fractures.           

Habits: Usually as anhedral crystals. Very rarely it occurs as euhedral crystals in volcanic rocks, forming short pseudo-hexagonal prisms elongated according to z.

In metamorphic rocks such as cornubianites, it forms anhedral grains (porphyroblasts) with very irregular contours; Poikiloblastic textures with intergrowths with quartz, biotite and/or graphite are typical. In volcanic rocks, quartz inclusions are common; in paragneisses there are fibrolytic inclusions of sillimanite. May be “dusty”, pinitized, with yellow pleochroic halos around zircon and apatite inclusions.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors:  Birefringence between 0.008 and 0.018: 1st order and upper 1st order colors, from gray to straw yellow. 

Extinction: Tends to be parallel, which is generally not possible because the grains are xenomorphic (anhedral). 

Elongation sign: ES(-) by cleavage, normally cannot be determined.            

Twins: Common by {110} and {310}, can be simple, lamellar or polysynthetic.

Polysynthetic and pseudo-hexagon trillings are common, easily recognizable under NC. Of the 6 sectors of the twin, 2 tend to be extinct or with the same interference colors, as they belong to the same crystal. Interpenetration trillings are common in cornubianite shales.        

Zoning: No.             

CONVERGENT LIGHT

Character: Usually B(-), may be B(+).          

2V angle:  35 – 106º        

Alterations: during alteration, it is progressively replaced, from the limits and along the fractures, by a dense aggregate of sericite and/or chlorite and biotite. Pseudomorphs of cordierite composed of sericite (muscovite) and other phyllosilicates (e.g., chlorite) are called “pinite”, the process is called pinitization. This transformation consumes water and is an indicator of hydrothermal interactions.

May be confused with: quartz and potassic feldspars, when the crystals do not show any typical cordierite habits.

Quartz is very similar, but quartz is never altered and is U(+).

Orthoclase without twins is very similar to cordierite; it is necessary to consider paragenesis.

Plagioclase presents polysynthetic twins, tends to be zoned and is usually altered to sericite and saussurite.

It is much easier to identify cordierite using polished sections under Reflected Light (see section below).  

Cordierite 01: In PPL, idiomorphic crystal of cordierite. If anhedral (xenomorphic) it is easily confused with untwinned potassic feldspar.
Cordierite 01: The same section as of the previous image, now in XPL. The interference colors are the same of quartz and K-feldspar.
Cordierite 02:In PPL, kyanite crystals with higher relief and cleavage besides cordierite crystals with low relief, many inclusions and without cleavage. At the top, chlorite.
Cordierite 02: The same section as of the previous image, now in XPL. Interference colors of the kyanites are a bit higher than the interference colors of the cordierites. In the middle cordierite, it is possible to observe a polysynthetic twin.
In XPL, trilling twinning in cordierite. It is an interpenetration twin formed by three individuals. These twins, when with perfect development, form an aggregate of hexagonal contour, divided into 6 sectors, 2 of which are always extinct. In the image, the twins are less perfect, but it is clear that the crystals are interpenetrated.
In XPL, trilling in cordierite. The black areas are holes in the thin section.
Cordierite 03: In contact metamorphic rock, cordierite porphyroblasts. In the dark level, these are de darkest points in the image. Picture: Gustavo Treméa Kothe.
Cordierite 03: The same section as in the previous image, now in XPL. The crystals are very small.
Cordierite 04: In PPL, detail of the cordierite porphyroblasts of the previous images.
Cordierite 04: The same section as of the previous image, now in XPL. The anhedral habit and the large amount of inclusions of these cordierite porphyroblasts can be seen. Pictures: Gustavo Treméa Kothe.
Cordierite 05: In PPL, high-grade aluminous metamorphic rock, magnetite (black), chloritoid (green), sillimanite (colorless, high relief) and, in the center of the image, cordierite (colorless, low relief)
Cordierite 05: The same section as of the previous image, now in XPL. Cordierite remains similar do quartz and untwinned feldspar. Almost in the center of the image, in the extinction position (black), one of the grains is pseudo-hexagonal.
Cordierite 06: In PPL, chloritoid (green), sillimanite (colorless, high relief), opaque (black = magnetite) and cordierite (colorless, low relief).
Cordierite 06: The same section as of the previous image, now in XPL. Cordierite shows low colors and is very similar to quartz and potassic feldspar without twins. The interference figure is B(-).
Cordierite 07: In PPL, in the center of the image, cordierite with simple twin. It is not possible to recognize cordierite in PPL. The otehr minerals are chloritoid (green), magnetite (black) and sillimanite (colorless, high relief, in needles).
Cordierite 07: The same section as of the previous image, now in XPL. The cordierite twin is very evident, but may be confused with a K-feldspar twin. To assess the distribution of cordierite in the rock, it is recommended to make a polished block, as the color of cordierite allows it to be easily recognized under Reflected Light.
In XPL, on the diagonal of the image, with slightly yellowish interference colors, cordierite porphyroblast with biotite, garnet, spinel and others. Quartz and feldspar around it are gray.

5. Reflected Light Microscopy

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

Sample preparation: The polishing of cordierite is relatively simple and, with a little persistence, it becomes of excellent quality, as the quartz and feldspars that occur associated.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, like quartz and feldspars.       

Pleochroism:  No.     

Reflectivity: Low (<10%)        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Anisotropy was not observed,.        

Internal reflections: Widespread in the same colors of the mineral in hand sample, varying between lighter and darker shades according to the thickness of the mineral at the point considered.

The most frequent color of cordierite is a yellow-brown, caramel color of medium intensity. Therefore internal reflections tend to be this color: yellowish with a brown to brown tinge.

In the iolite variety, one can see, with great attention, some internal reflections in violet and lilac tones, but they tend to be very faint and only localized.      

May be confused with: other transparent minerals of low reflectivity and yellowish colors. But the recognition of cordierite is much simpler in Reflected Light than in Transmitted Light. Considering the paragenesis and looking for caramel-colored minerals, the identification of cordierite is easy, as quartz, feldspar, pyroxenes, amphiboles and micas (biotite and muscovite) have very different colors (and therefore internal reflections under Reflected Light).

Cordierite 01: In PPL, feldspars on the left and cordierite on the right. Both have the same color and the same polishing quality.
Cordierite 01: The same section as of the previous image, now in XPL. Feldspar has white internal reflections and the cordierite has yellowish to caramel internal reflections, which correspond to the color of the hand specimen.
Cordierite 02: In PPL, biotite (top left), quartz (bottom left) and cordierite (right). Quartz and cordierite have the same appearance, but biotite is darker and has more polishing scratches.
Cordierite 02: The same section as of the previous image, now in XPL. Biotite shows brown internal reflections, quartz has white reflections and cordierite shows yellowish to caramel internal reflections.
Cordierite 03: In PPL, cordierite, variety iolite (blue, violet, lilac). There are no diagnostic features in PPL.
Cordierite 03: The same section as of the previous image, now in XPL. Some tenuous internal reflections of very faintly bluish can be seen.