- Chemical Formula: (Ca, Na)2Ta2O6(O, OH, F);
Calcium Sodium Tantalum Oxide Hydroxide Fluoride.
- Class: Oxides
- Group: Pyrochlore
- Uses: A very minor ore of tantalum, as mineral specimens and sometimes cut as a gemstone.
Microlite is one of the tantalum/niobium oxides that are generally difficult to distinguish.
Fortunately there are few of them that form well shaped octahedral crystals.
Microlite crystallizes in the
isometric symmetry class
and forms fine octahedral crystals that are typically and characteristically
modified by other isometric forms.
Other members of the Pyrochlore Group
also form octahedrons, but can sometimes be reliably differentiated by color, streak and
Microlite generally contains impurities of radioactive elements called rare earths
and this produces the slight radioactivity in this mineral.
It is occasionally placed in the informal group of minerals called the
Rare Earth Oxides,
but does not contain a significant amount of these metals.
Microlite is an end member of a solid-solution series between itself and the mineral
pyrochlore from where the
gets its name.
The two minerals have similar structures and properties, but microlite is the tantalum
rich end member and pyrochlore is the niobium rich end member.
Microlite is found mostly in granitic pegmatite dikes and more rarely in calcite rich
rocks called carbonatites.
The rarer pyrochlore on the other hand is more common to the
carbonatites and alkalic pegmatites called nepheline syenites.
The mineral gahnite
also forms octahedrons but is neither a rare earth mineral nor apart of the
pyrochlore Group of minerals.
Gahnite is a member of the Spinel Group
of oxide minerals.
Its green color makes it easy to confuse with the green varieties of microlite,
but microlite is generally denser and much softer.
Remember, this is a slightly radioactive mineral and should be stored away from
other minerals that are subject to damage from radioactivity and of
course human exposure should be limited !
- Color is pale yellow, reddish-brown, red, olive or even emerald green.
- Luster is vitreous to resinous.
- Transparency: Crystals are generally translucent with darker specimens being opaque.
- Crystal System is isometric; 4/m bar 3 2/m
- Crystal Habits typically include octahedral crystals that are modified by other isometric forms; also found granular as disseminated grains and massive.
Although the name microlite was applied to the mineral for the tiny crystals that were first found, larger crystals up to 2 cm have been found.
- Cleavage is in four directions (octahedral), but is indistinct.
- Fracture is subconchoidal to uneven.
- Hardness is 5 - 5.5
- Specific Gravity is approximately 4.3 - 5.7 (heavy for non-metallic).
Variation caused by extent of inclusion of trace metals into the structure.
- Streak is white or pale yellow to brown.
- Other Characteristics: Slightly radioactive.
- Associated Minerals include
- Notable Occurrences include Virgem da Lapa region of Minas Gerais, Brazil;
Iveland, Norway; Verutrask, Sweden; Greenland; Wodinga, Australia; Madagascar and
Amelia Courthouse, Virginia; Black Hills, South Dakota; Dixon, New Mexico; California;
and many of the rare earth pegmatites of New England, USA.
- Best Field Indicators are crystal habit, luster, fracture, color, hardness,
radioactivity, associations, environment and specific gravity.