TAPIOLITE

Tapiolite – (Fe,Mn)(Ta,Nb)2O6 – is a very rare oxide, characteristic of some granitic pegmatites. It is not considered an ore due to its rarity.

The term “tapiolite” is used for unanalyzed minerals of the Tapiolite-(Fe) – Tapiolite-(Mn) Series.

Tapiolite has the same composition as columbite and tantalite, but is tetragonal instead of orthorhombic. Tapiolite-(Fe) is quite rare and tapiolite-(Mn) is extremely rare. Only with detailed chemical analyses is it possible to classify the mineral correctly.

1. Characteristics

Crystal system: Tetragonal ditetragonal bipyramidal.

Color: Black.     

Habit: Short prismatic or equigranular. Crystals without twinning appear monoclinic or orthorhombic due to distortion.

Cleavage: No. 

Tenacity: Brittle. 

Twinning: On {013}, very common, simulating orthorhombic symmetry. On {101}, rare.

Fracture: Irregular. 

Mohs Hardness: 6 – 6.5

Parting: No.

Streak: Brownish black. 

Lustre: Sub-adamantine, metallic to sub-metallic. 

Diaphaneity: Transparent.

Density (g/cm³): Tapiolite-(Fe): 7.9    Tapiolite-(Mn): 7.72

 

2. Geology and Deposits

Tapiolite is characteristic of late portions of very acidic granitic pegmatites (high silica content). It occurs in three forms: (i) independently, (ii) as an alteration product of columbite, and (iii) as exsolved particles in cassiterite.

It can be found in placers, so much so that it occurs relatively commonly in heavy sand concentrates, but always in minimal quantities.

 

3. Mineral Associations

It is associated with typical minerals of granitic pegmatites, such as quartz, feldspars (albite, microcline), micas (biotite, muscovite), tourmaline, pyrochlore, microlite, and sulfides (pyrite, arsenopyrite, bismuthinite).

In the paragenesis, several very similar black minerals occur, such as cassiterite, columbite, tantalite, stibiotantalite, and wolframite.

Also with rare minerals such as simpsonite, brazilianite and wodginite.

 

4. Transmitted Light Microscopy

Refraction indices:  nω: 2,270      nε: 2,420  

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Strong pleochroism ranging from pale reddish-brown to almost opaque (black).    

Relief: Very high.           

Cleavage: No.           

Habits: Short prismatic, equigranular, massive.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Maximum birefringence of 0.150, very high, corresponding to pearly colors that are difficult to define, up to the 7th order, like calcite.            

Extinction: Paralell.

Elongation sign: No information available. 

Twins: Always present, lamellar, they can be in two groups resembling the cleavage of carbonates.

Zoning: No information available.              

CONVERGENT LIGHT

Character: U(+)          

2V angle: No.         

Alterations: No information available.           

May be confused with: No information available.          

Caption.

5. Reflected Light Microscopy

Sample preparation: Tapiolite acquires a good polish only very slowly, but with superior quality to the polishing of cassiterite. Its hardness upon polishing is similar to that of columbite. During the preparation of the polished section, in the sanding stage, the abrasive typically acquires an intense and deep brown color, called “Zimtbraun” (= cinnamon brown) in German literature.       

PLANE POLARIZED LIGHT – PPL

Reflection color: Colors range from medium gray to grayish-white to bluish-gray.

Compared to the color of sphalerite, the color of tapiolite is very similar.

Compared to the color of cassiterite, the color of tapiolite is much lighter. 

Pleochroism: No.      

Reflectivity: 15.95 – 17%        

Bireflectance: Weak to strong; one color is similar to columbite, the other is much lighter.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Strong anisotropy, which can range from slate gray to reddish-brown or from dark blue to greenish-gray.
Sometimes the anisotropy is masked by internal reflections.        

Internal reflections: They are red and reddish-brown, but not abundant.      

May be confused with: No information available.

General Characteristics: 

Exsolutions of tapiolite, lamellar to acicular, can occur in high-temperature cassiterites.

Lamellar twins, parallel to (011), always occur, resembling those of cassiterite. They are generally quite coarse, but in some cases very fine. The twins can occur as two sets, intersecting like the cleavages of carbonates.

Cataclasis is quite common; in this case, it may be without the occurrence of associated pressure twins.

Reaction rims of varying thicknesses of tapiolite can form around Nb-rich columbite.

Oriented intergrowths occur with cassiterite and wodginite.

Tapiolite 01: In PPL, tapiolite (lighter) and solicates (darker). The cataclastic texture is evident, but the reflection color is not diagnostic.
Tapiolite 01: The same section as of the previous image, now in XPL showing its first anisotropy color. This one has a reddish-brown hue.
Tapiolite 01: The same section as of the two previous images, also in XPL but rotated 90 degress, now showing its second anisotropy color. This color is very dark, subtly bluish, referred to in the literature as "slate gray." Despite the difficulties in defining the color, the anisotropy is quite obvious, easy to verify, and an important diagnostic feature in relation to other minerals in the paragenesis.
In XPL, twinned tapiolite. Twins are an important diagnostic feature.
In XPL, tapiolite with two sets of intersecting twins, closely resembling the cleavage of carbonates. The twins occur in large quantities, covering the entire grain.