GIBBSITE

Gibbsite – Al(OH)3 -is a very common hydroxide, an important component of soils and one of the minerals that make up laterites and bauxites, which are aluminum ores. Gibbsitic clays are also interesting for the manufacture of refractory materials.

Gibbsite, also called hydrargillite, has the polymorphs bayerite, doyleite and nordstrandite and a basic mica structure; the neutral plates of aluminum hydroxide (“gibbsite layers”) constitute part of the structure of important clay groups, such as the illite, kaolinite and montmorillonite/smectite groups. It may contain Fe and Ga.

The identification of gibbsite and the other minerals that accompany it cannot be carried out by microscopy due to the extremely small size of the crystals. Other analytical techniques such as X-ray Diffraction must be used.

An important diagnostic aspect of gibbsite is its fluorescence, which is easy to observe. Gibbsite can fluoresce in a yellowish-white color (long or short wavelength UV light) or green to greenish-white color (short wavelength UV light).

1. Characteristics

Crystal system: Monoclinic prismatic, pseudohexagonal.

Color: White, light gray, light green. When impure, reddish-white, reddish-yellow.

Habit: Almost always earthy to nodular. Tabular crystals are rare.

Cleavage: {001} perfect, rarely observed due to the submicroscopic size of the crystals.

Tenacity: No information available.

Twinning: Common in {001}, several other types.

Fracture: Irregular. 

Mohs Hardness: 2.5 – 3

Parting: No,

Streak: No information available.         

Lustre: Vitreous, sub-vitreous, pearly, dull, earthy. 

Diaphaneity: Transparent.         

Density (g/cm³):  2.38 – 2.42 

        

2. Geology and Deposits

Gibbsite is a typical alteration mineral of aluminous minerals such as feldspars, being common in lateritic soils, in bauxite, and in stagnant water deposits such as swamps, where it occurs in “bog iron,” a historically important iron ore that forms concretions composed of iron hydroxides, clays, gibbsite, and other minerals.

It also forms in hydrothermal and low-temperature metamorphic environments, replacing aluminous minerals.

 

3. Mineral Associations

In laterites and bauxites, gibbsite occurs with diaspore, boehmite, corundum, kaolinite, and goethite.

It can occur with crocoite, natrolite, dundasite, and aluminoadamite.

It can also occur with secondary copper minerals such as malachite, azurite, cuprite, and chalcoalumite.

 

4. Transmitted Light Microscopy

Identifying gibbsite with the microscope is difficult to impossible due to the extremely small size of the crystals, which are also intergrown with clay minerals.

Refraction indices:  nα:  1.568 – 1.570    nβ: 1.568 – 1.570     nγ: 1.586 – 1.587

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless to pale brown, without pleochroism.

Relief: Low.           

Cleavage: Possible, in one direction.           

Habits: Nodular, massive, spherulites.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence of 0.018: first-order colors from gray to white, yellow, orange to purple.

Extinction: Oblique extinction, of approximately 26º.           

Elongation sign: ES(+) in tabular crystals. 

Twins: They cannot be observed because the crystals are too small.

Zoning: No.             

CONVERGENT LIGHT

Character: B(+), difficult or impossible to ascertain.          

2V angle: 0 to 40º         

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

May be confused with: When it forms spherulites, it is very similar to chalcedony, but chalcedony has lower birefringence.   

Gibbsite 01: In PPL, gibbsite spherulite, whose structure is similar to the structure of chalcedony spherulites.
Gibbsite o1: The same section as of the previous image, now in XPL. The thin section is a little too thick; in XPL the colors should be a little less vibrant. The size of these spherulites varies between 1-2 mm and more than 1 cm.
In XPL, section of the same spherulite of the previous two images, showing details of its structure.
In PPL, details of the structure of the gibbsite spherulite.

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical technique for identifying gibbsite because it is a transparent mineral that always shows submicroscopic crystals. However, the structures and colors that can be seen under reflected light are quite diagnostic.

Sample preparation: Despite its characteristics, polishing gibbsite, provided it is not earthy, is relatively simple. The surface does not become polished due to the submicroscopic size of the particles, but the aggregate structure is easy to observe.       

PLANE POLARIZED LIGHT – PPL

Reflection color: Light gray, much lighter than quartz and feldspars.

Pleochroism: No, but due to the low hardness, the polishing quality is very low and can simulate pleochroism.      

Reflectivity: Very low (<<10%)        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Anisotropy was not observed. 

Internal reflections: Common, widespread, in porcelain-white, in light colors and colorless. 

May be confused with: Many other transparent minerals of light colors.

Gibbsite 01> In PPL, gibbsite spherulite. The irregular, non-polished surface is easy to observe. It is very similar to clay mineral aggregates in vesicles of volcanic rocks. The center of the spherulite is formed by iron (hydr)oxides.
Gibbsite 01: The same section as of the previous image, now in XPL.