MICROLITE

Microlites – (Na,Ca)2Ta2O6(O,OH,F)  or A2-mTa2X6-wZ-n – constitute a group of relatively rare oxides that are ore of Ta.

The Microlite Group is formed by the Ta-rich (and Ti- and Nb-poor) members of the Pyrochlore Supergroup. There are 13 different mineral species, plus 6 varieties. They are minerals that generally occur in the form of octahedrons. They occur as tiny grains (the name comes from this fact) disseminated in pegmatite. Color, shape and paragenesis are the same as pyrochlores.

The classification of microlites was revised by Atencio et al. 2010 and by Christy & Atencio, 2013. More information can be obtained from these articles.

The exact determination of individual mineral species is not possible with microscopy; it is necessary to use a combination of several analytical methods, as is also the case with pyrochlores.

1. Characteristics

Crystal system: Cubic hexaeoctahedral.          

Color: Pale yellow to reddish-brown, sometimes emerald green.     

Habit: Octahedrons or combined cubic shapes.       

Cleavage: {111} bad, sometimes distinct, but usually not recognizable.       

Tenacity: Brittle.        

Twinning: Rare by {111} (Spinel Law).

Fracture: Irregular, splintered, sub-conchoidal. 

Mohs Hardness: 5 – 5.5

Parting: Cleavage can be a partition.

Streak: Light yellow, brownish.

Lustre: Vitreous, resinous          

Diaphaneity: Transparent.           

Density (g/cm³): 5.9 – 6.4 

 

2. Geology and Deposits

Microlites occur in the albitized portions of granitic pegmatites and in miarolitic cavities of granites.

 

3. Mineral Associations

It associates with a large number of minerals typical of pegmatites, such as quartz, feldspars (microcline, albite, including the clevelandite variety), micas (muscovite, lepidolite), spodumene (including the kunzite variety), beryl (including the aquamarine variety), tourmaline (including the rubellite variety), vanadinite, fluorapatite, topaz, garnet (spessartine), fluorite and tantalite, among many others.

 

4. Transmitted Light Microscopy

In PPL they appear colored, yellow to brown, with high relief. In XPL they are isotropic.

Very similar to pyrochlore, it is not possible to distinguish microlites from pyrochlore under a microscope.

5. Reflected Light Microscopy

Microliths cannot be identified by Reflected Light. They are very similar to pyrochlore and some brightly colored silicates.