VERMICULITE

Vermiculite – Mg0.7(Mg,Fe,Al)6(Si,Al)8O20(OH)4·8H2O – is a phyllosilicate (mica), a relatively common alteration product. It has a number of very important industrial applications, which are based on the property of vermiculite to expand (“exfoliate”) when heated, which is due to the water contained between the leaves of the mineral. Famous deposits of vermiculite are the mines of Palabora (Australia) and Libby (USA).

“Vermiculite” is actually not a mineral, but just a generic term applied to a group of phyllosilicates from the Smectite Group formed by alteration of biotite or phlogopite. The individualization of the mineral species of the group is not possible under the petrographic microscope. If it occurs in very fine grains (clay size), vermiculite cannot be recognized under the petrographic microscope.

There are five varieties of vermiculite (Fe, Cu, Cr, white and from the Vaal River in South Africa). Impure vermiculite may contain fibrous amphiboles (“asbestos”) such as tremolite, winchite and richterite, which are considered contaminants and prevent commercial use of vermiculite due to the health problems they can cause. In the USA, asbestos-contaminated vermiculite was sold in large volumes between 1920 and 1990 under the trade name “zonolite”, originating from the Zonolite Mountain mine in Libby (Montana). The site is considered the worst case of community exposure to a toxic substance in US history.

1. Characteristics

Crystal system: Monoclinic prismatic.          

Color: Colorless, white, yellow, green, bronze to yellowish-bronze, may be golden.     

Habit: “Micaceous” habits: reeds, lamellae, leaves. It forms pseudohexagonal plates.       

Cleavage: {001} perfect, (relict from the original mica).       

Tenacity: Flexible.        

Twinning: No.       

Fracture: Irregular.       

Mohs Hardness: 1.5 – 2

Parting: No.         

Streak: Colorless to yellowish.         

Lustre: Dull, resinous, silky.          

Diaphaneity:  Transparent.          

Density (g/cm³):  2.2 – 2.6

 

2. Geology and Deposits

Vermiculite forms as a product of alteration of biotite and phlogopite, by weathering or hydrothermalism. Vermiculite in commercial volumes occurs in basic and ultrabasic rocks, where they are formed by alteration of phlogopite/biotite, chlorite or pyroxenes. The alteration can occur by weathering, hydrothermal processes or by the percolation of groundwater. When formed by the alteration of micas, it can form perfect pseudomorphoses.

It can occur in carbonatites and marbles, and in some cases naturally expanded vermiculite occurs in volcanic rocks. It occurs in soils as one of the constituents of the clayey fraction.

 

3. Mineral Associations

In basic rocks, vermiculite it is associated with corundum (including the ruby variety), apatite, serpentine, chlorite and talc.

Also pargasite, chromiferous diopside, anthophyllite, anorthite and zeolites (stellerite, stilbite).

It forms interstratified minerals with chlorite and biotite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.525 – 1.564     nβ: 1.530 – 1.581    nγ: 1.530 – 1.581

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism:  If the transformation of biotite/phlogopite to vermiculite was complete, the vermiculite is colorless.

If the transformation was not complete, it is pleochroic in various pale shades of brown. The color is paler in X than in Y and Z.

Vermiculite can form pseudomorphs on biotite and phlogopite.

Relief: Low to moderate. 

Cleavage: {001} perfect, typically relict (inherited from biotite or phlogopite). 

Habits: Occurs in sheets that may present pseudo-hexagonal shapes. It also occurs in clay-sized particles. 

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors:  Maximum birefringence of 0.020, which corresponds to interference colors up to the end of 1st order: gray, white, yellow/orange to bluish.

Extinction:  Tends to parallel as in biotite and phlogopite, but is not mottled, which is important!          

Elongation sign: ES(+), like all micas. 

Twins:  No.        

Zoning: No.             

CONVERGENT LIGHT

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

2V angle: 0 – 15º    

Alterations: does not alters, it is a product of alteration.          

May be confused with: biotite and phlogopite if the alteration was not complete.

But biotite and phlogopite, when observed with a stereomicroscope or even on the thin section with the unaided eye, present black colors, while vermiculite presents a very strong, characteristic golden color. In addition, biotite and phlogopite exhibit mottled extinction, while vermiculite no longer exhibits mottled extinction. Vermiculite interference colors are fainter (weaker) than biotite and phlogopite colors. 

Vermiculite 01: In PPL, vermiculite in schist, showing the first of its pleochroism color. Pleochroism is weak but still present. The appearance of the grains is different (“dirty”, “altered”) compared to unaltered biotites or phlogopites. Pleochroism may be much weaker if the alteration to vermiculite proceeds further.
Vermiculite 01: The same section as of the previous image, now rotated 90 degrees to show the second pleochroism color of vermiculite.
In XPL, vermiculite in extinction position: extinction is now no longer mottled, but has a uniform black appearance, which is uncharacteristic for unaltered micas.
Vermiculite in PPL in maximum illumination position: the interference colors are weaker and less colorful. There are no longer those green/red/blue luminosities that are typical of unaltered mica biotite or phlogopite.
Vermiculite 02: In PPL, vermiculite showing its first pleochroism color.
Vermiculite 02: The same crystal of the previous image, now rotated 90 degrees to show the second pleochroism color.
Vermiculite 02: The same crystal as of the two previous images, now in XPL to show its appearance with interference colors. In the maximum dark position, the extinction is no longer mottled. As this vermiculite is not colorless, the alteration of the original phlogopite or biotite was probably not complete.

5. Reflected Light Microscopy

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

Sample preparation: the polishing of vermiculite, as with all micas, is hampered by the excellent cleavage and low hardness of the mineral. That’s why vermiculite acquires a very bad polish, with associated holes and grooves, which is still diagnostic.

PLANE POLARIZED LIGHT – PPL

Reflection color: Very dark gray, almost black, much darker than quartz and feldspars and darker than pyroxenes and amphiboles.

Pleochroism: No.      

Reflectivity: Very low (<<10%)        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Anisotropy is not perceived. Extinction is parallel if the transformation of phlogopite or biotite was not complete. 

Internal reflections:  Very dark, mixed with white reflections from the alteration.

May be confused with: other micas, when only partial transformation of phlogopite or biotite into vermiculite occurred.

Vermiculite 01: In PPL, chalcopyrite (golden), quartz and feldspars (light gray, indistinguishable) and biotite partially altered to vermiculite (very dark gray to black, lamellar shapes).
Vermiculite 01: The same section as of the previous image, now in XPL. Chalcopyrite is very dark, quartz and feldspars show colorless to milky internal reflections and vermiculite shows dark reflections mixed with lighter reflections.