DIASPORE

Diaspore -AlO(OH) – is a very common hydroxide and—along with gibbsite and boehmite—one of the main components of bauxite, the most important aluminum ore.

It is classified within the Diaspore Group and is isomorphous with boehmite. It typically contains Fe, Mn, Cr, and Si and is cryptocrystalline; consequently, microscopy is not the appropriate analytical technique for its identification.

A famous color-changing gem variety from Turkey is marketed under trade names such as Csarite, Zultanite, Ottomanite, and Turkish diaspore. There is also a Cr-bearing variety and a third variety containing Mn.

1. Characteristics

Crystal system: Orthorhombic bipyramidal.

Color: It can appear colorless, grayish, white, yellowish, greenish, pinkish, lilac, or violet.

The color may vary within a single crystal depending on the viewing direction. It can exhibit a brownish-green color in sunlight and a pink hue in incandescent or candlelight. 

Habit: Massive, stalactitic, lamellar or scaly masses, earthy; crystals flattened or acicular, up to 40 cm. 

Cleavage: {010} perfect, {110} distinct, {100} in traces. Striations in various directions.       

Tenacity: Very brittle.        

Twinning: Rare; heart-shaped twins occur on {021}, and it can form pseudo-hexagonal aggregates. 

Fracture: Conchoidal to irregular.       

Mohs Hardness: 6.5 – 7

Parting: No.         

Streak: White.         

Lustre: Adamantine, vitreous, pearly on cleavage surfaces.          

Diaphaneity: Transparent.           

Density (g/cm³): 3.2 – 3.5

           

2. Geology and Deposits

Diaspore is the end product of the diagenesis of bauxite deposits formed by tropical alteration of aluminosilicate rocks. There the diaspore is cryptocrystalline, its presence must be assumed based on the type of deposit and confirmed by X-ray diffraction. In metalaterites (metabauxites) diasporite can form, when it is in an intermediate stage in the corundum formation process.

It is present in aluminous clays (“fireclays”).

It also occurs through hydrothermal alteration of aluminous minerals. It can occur due to hydrothermal alteration in alkaline pegmatites (nepheline-syenites), in greisen and in felsic volcanic rocks.

It is an alteration product of corundum and emery and is found in limestone and kaolin.

 

3. Mineral Associations

It is associated with other Al minerals such as silicates (sillimanite, andalusite, kyanite) and Al (hydr)oxides (corundum, boehmite, gibbsite, zunyite, spinel).

Also with other (hydr)oxides such as magnetite, hematite, rutile, and lepidocrocite.

With phyllosilicates such as muscovite, pyrophyllite, margarite, chlorite (clinochlore), kaolinite, halloysite, and amesite.

Other minerals that may occur in association include chloritoid, natrolite, garnet (uvarovite), and adachiite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.682 – 1.706    nβ: 1.705 – 1.725      nγ: 1.730 – 1.752

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless.

Colored varieties, or when in thick sections, may exhibit pleochroism between pale colors: reddish-brown, reddish-violet, and greenish-gray to green.  

Relief: Very high.           

Cleavage: {010} perfect, {110} distinct, and {100} in traces.           

Habits: tabular, acicular, or fibrous crystals; it may be stalactitic, massive, or earthy, or form very fine-grained lamellar or scaly masses. 

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors:  maximum birefringence of 0.048, corresponding to strong, intense colors—red, blue, green, cream—up to the middle of the third order.          

Extinction: parallel to the main cleavage.          

Elongation sign: ES(+) or ES(-), it is not diagnostic.            

Twins: Rare; they may occur as contact twins (heart-shaped, with a 60° angle between them) or form pseudo-hexagonal aggregates.

Zoning: May show zoning.             

CONVERGENT LIGHT

Character: B(+), difficult to obtain in very small grains.          

2V angle: 84 – 86º         

Alterations: Can transform into corundum upon heating or dehydration. With the addition of silica, it transforms into kaolinite and other clay minerals.          

May be confused with: other long, prismatic minerals with strong colors in PPL.

Sillimanite has different habits, lower birefringence, and lower refractive indices.

Gibbsite shows lower relief and oblique extinction.

Corundum has lower birefringence.         

Caption.

5. Reflected Light Microscopy

It is not possible to identify diaspore by Reflected Light.

Sample preparation:        

PLANE POLARIZED LIGHT – PPL

Reflection color:        

Pleochroism:       

Reflectivity:         

Bireflectance:        

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:         

Internal reflections:       

May be confused with:        

General Characteristics: 

Caption