STAUROLITE

Staurolite – (Fe,Mg)2(Al,Fe)9O6[Si4O22](O,OH)2 – is a more or less common nesosilicate that is characteristic of pelitic metamorphic rocks. It is important as an industrial mineral, but economically viable occurrences (in placers) are so rare that the industrial use of staurolite is restricted by the simple lack of ore.

Twins are common in staurolite. There may be “X” twins, when two intergrown crystals make an angle of 60° to each other, with the twin plane being {231}. More rarely, cross-shaped twins are formed, when two intergrown crystals form a 90° angle to each other, with {031} being the twin plane. Twins according to the {230} plan occur very rarely.

Staurolite can contain Mn, Co, Zn, Ti, Cr, Li and H2O. As varieties there are “lusakite” (with Co) and zinciferous staurolite.

1. Characteristics

Crystal system: Monoclinic prismatic, pseudo-orthorhombic.          

Color: Dark reddish brown to black brown or yellowish brown. Rarely blue.     

Habit: Usually prismatic, can occur granular.       

Cleavage: {010} distinct.       

Tenacity: Brittle.        

Twinning: See above.      

Fracture: Subconchoidal.       

Mohs Hardness: 7 – 7.5

Parting: No.        

Streak: White to gray.         

Lustre: Subvitreous to greasy.          

Diaphaneity: Transparent.           

Density (g/cm³):  3.74 – 3.83

          

2. Geology and Deposits

Staurolite typically occurs in metamorphic rocks of pelitic composition and medium metamorphic grade (low to medium amphibolite facies) formed by regional metamorphism, such as mica-schists, phyllites and gneisses. It is more rarely found in quartzites and metabauxites.

It is also found as a detrital mineral in sediments (placers), when it can then be mined as an industrial mineral for abrasives.

 

3. Mineral Associations

Staurolite is associated with common rock-forming minerals such as quartz, potassic feldspars, chalcosodic feldspars (oligoclase) and micas (muscovite, biotite, paragonite, chlorite).

In addition, it occurs with the typical minerals of metamorphic pelitic rocks, such as garnet (almandine), minerals from the Al2SiO5 Group (andalusite, sillimanite, kyanite), chloritoid and cordierite.

Also occurs with tourmaline, ilmenite and magnetite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.736 – 1.747   nβ: 1.742 – 1.753   nγ: 1.748 – 1.761

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism:  Colorless to yellow, typically with pleochroism.

Pleochroism is very clear in the longitudinal sections:

X = colorless;

Y = colorless, pale yellow, yellowish brown;

Z = light yellow, golden (honey color), reddish brown.

Basal sections show weak pleochroism or none!   

Relief: High.            

Cleavage: {010} distinct, usually not visible under the microscope.

Usually, staurolite has a large number of fractures.           

Habits: Longitudinal sections form short euhedral prisms, elongated along the Z axis.

 Basal sections may appear pseudo-hexagonal (6 sides).

Sieve texture, also called “swiss cheese”, is very common, consisting of staurolite containing many rounded quartz inclusions. Staurolite can form anhedral grain masses and oriented intergrowths (epitaxies) with kyanite.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors:  Relatively low birefringence, between 0.009 and 0.014: higher 1st order interference colors, ranging from light gray to orange.          

Extinction: Parallel in longitudinal sections, symmetric in basal sections.           

Elongation sign:  ES(+).           

Twins: Cyclic twins with an angle of 60° are common. Cross or “X” shaped interpenetration twins also occur. Under a microscope these twins are usually unobservable.         

Zoning: Rarely zoned.             

CONVERGENT LIGHT

Character: B(+)         

2V angle: 80 – 90º           

Alterations: staurolite is relatively resistant to alteration. With the increase in metamorphism, staurolite is replaced by garnet (almandine) + kyanite or by garnet (almandine) + sillimanite. By retrograde alteration, staurolite is replaced by chlorite and sericite.  

May be confused with: few other minerals, as yellow color, yellow/orange pleochroism, high relief and low interference colors are very typical; paragenesis also helps a lot (metamorphic rocks with micas, quartz, etc.).

Tourmaline may look similar (pleochroism sometimes in strong yellow tones) but has an elongated prismatic habit (long prisms), trigonal basal sections, and is U(-).

Chondrodite also show yellow colors and pleochroism, but the colors are weaker, the interference colors are stronger, extinction is oblique and ocurrs in other paragenesis.

Among the common minerals, titanite, barite, epidote and piemontite may occasionally show yellow pleochroism, but generally their characteristics in XPL are different from those of staurolite        

 

Staurolite 01: Longitudinal section of staurolite in PPL showing one of its pleochroism colors. Around it, quartz (colorless) and biotite (brown). The somewhat irregular contours, the virtual absence of cleavage and inclusions in variable numbers are typical.
Staurolite 01: The same section as of the previous image, now rotated 90 degrees to show the second color of pleochroism of staurolite. This yellow/orange colors are very typical and diagnostic for staurolite.
Staurolite 01: The same section as of the two previous images, now on XPL. Staurolite shows low interference colors. Other colors are paler yellows and shades of gray. Extinction tends to be parallel, usually somewhat difficult to determine due to the subhedral or anhedral shape of the grains.
Staurolite 02: Large intergrown staurolite crystals (possibly a twin!) with the “sieve texture”, formed by many rounded quartz inclusions. In PPL, the typical yellow color of staurolite can be seen. Biotite (brown), quartz (colorless and low relief) and garnet (colorless, roundish and high relief) are associated.
Staurolite 02: The same section as of the previous image, now in XPL. One of the staurolite grains, is in the extinction position; the other shows the maximum interference color of staurolite, a deep yellow-orange. Garnet, as usual, is isotropic (black).
In PPL, xenomorphic (anhedral) staurolite grain. Shapeless grains are much more common than defined prismatic shapes.
Staurolite basal section in PPL. The shape is rhombic (pseudo-orthorhombic), pleochroism almost absent and the extinction is approximately symmetrical.
In XPL, twinned staurolite, with an interpenetration twin (gray and yellowish colors).
In XPL, a twinned staurolite with a contact twin, forming a wide-open "V".
Staurolite 03: In PPL, garnet-staurolite-micaschist, with garnet (colorless, high relief), quartz (colorless, low relief), muscovite (colorless, medium relief) biotite (brown) and staurolite (pale yellow). The staurolite developed in the pressure shadow of the garnet grain in the center of the image.
Staurolite 03: The same section as in the previous image, now in XPL. Staurolite to the left of the garnet (isotropic) shows weak interference colors (gray), while the staurolite to the right of the garnet shows red anomalous interference color and lamellar twins.

5. Reflected Light Microscopy

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

Sample preparation: Staurolite easily acquires an excellent polish, despite its high hardness. The quality of the polish is better than that of quartz and much better than that of the generally associated micas. The large amount of inclusions causes problems in polishing, such as holes, flaws and others.       

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, approximately the same color as quartz. Much lighter than the reflection color of micas, pyroxenes and amphiboles.       

Pleochroism: No.     

Reflectivity: Low (<10%).        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Anisotropy is not observed.        

Internal reflections: Generalized in colors between orange yellow, caramel and brown red. They can appear in very dark brown colors, almost black, if the grain thickness in the polished section is high. Along fractures, multicolored internal reflections arise.      

May be confused with: titanite and sphalerite, which can have very similar internal reflections.

General Characteristics: 

Cleavage can be quite visible.  

Symmetrical extinction can be seen in basal sections that are rhombic (idiomorphic).

Staurolite 01: In PPL, staurolite on the right with excellent polishing, contrasting with the bad polish of the micas.
Staurolite 01: The same section as of the previous image, now in XPL. Internal reflections dominate in staurolite, always in yellow, orange and reddish-brown colors.
At right in the image, in XPL, staurolite crystal with contrasting thicknesses showing internal orange reflections in the thin portions and dark red, almost black, reflections in the very thick portions. On the left, muscovite.
Basal section (rhombic) of staurolite in XPL showing the different shades of the internal reflections, with multicolored reflections along the fractures. These sections show symmetrical extinction.