Triangle-cut microlite, 0.90 cts, 5.3 x 5.1 mm, Brazil. © The Gem Trader. Used with permission.
Ranging in color from pale yellow to brown, reddish, and green, microlite cabochons are prized by collectors. Faceted gems are very beautiful but extremely rare.
| Data | Value |
|---|---|
| Name | Microlite |
| Colors | Pale yellow to brown, reddish, green. |
| Crystallography | Isometric. Crystals octahedral; also grains and masses. |
| Refractive Index | 1.93-2.02 |
| Luster | Vitreous to resinous |
| Hardness | 5-5.5 |
| Fracture | Subconchoidal to uneven |
| Specific Gravity | 4.3-5.7; usually 5.5. |
| Birefringence | None. |
| Cleavage | Indistinct, Octahedral (not always evident) |
| Luminescence | None. |
| Luminescence Present | No |
| Transparency | Translucent to opaque. |
| Absorption Spectrum | Not diagnostic. |
| Formula |
(Na,Ca)2Ta2O6(O,OH,F) |
| Pleochroism |
None. |
| Optics |
N=1.93-1.94 if slightly metamict; also 1.98-2.02. May show anomalous birefringence. |
| Etymology |
From the Greek mikros for “small,” due to the small size of the crystals found at the original locality. |
| Occurrence |
Primary mineral in granite pegmatites. |
Microlite: Amelia Court House, Virginia (0.3). Photo © Joel E. Arem, PhD, FGA. Used with permission.
As the name suggests, microlite crystals are small. Ranging from translucent to opaque, faceted gems over 4 carats in size would be extremely rare. However, gem carvers can cut massive brownish or reddish material into cabochons up to several inches long.
Microlite belongs to the pyrochlore mineral supergroup and, mineralogically speaking, constitutes a group itself of pyroclores with the element tantalum (Ta) predominant.
This crystal specimen measures 4 x 3 x 2.5 cm and features muscovites and tiny, golden orange microlites on albite. Ipe mine, Governador Valadares, Doce Valley, Minas Gerais, Southeast Region, Brazil. © Rob Lavinsky, www.iRocks.com. Used with permission.
Microlite has a streak color ranging from pale yellow to brown. However, don’t conduct streak testing on finished gems. Test material in inconspicuous spots as a last resort only.
Due to radiation exposure during formation, microlites may become metamict, losing their crystalline structure. This metamictization may also cause anomalous double refraction (ADR).
Brazil produces fine, sometimes gemmy, green crystals.
The Rutherford Mines in Amelia, Virginia also yield green and brown crystals, and gemmy specimens may reach a few inches in length.
Other notable gem-quality sources include the following locations:
Green Brazilian gems weighing less than 1 carat have appeared on the market, and the potential exists there for larger stones.
Generally, faceted gems weigh under 3 to 4 carats. A garnet-red microlite crystal found in 1885 weighed 4.4 carats in the rough and, when faceted, looked like a red zircon.
Some microlites may be slightly radioactive due to impurities of rare-earth elements (REE), therefore, faceters should take precautions to avoid inhaling or accidentally ingesting dust from cutting.
Store microlite jewelry and carvings separately from more commonly encountered gemstones, such as garnet, quartz, topaz, etc. With a hardness ranging from 5 to 5.5, microlites can suffer scratches from these harder materials as well as common objects like a steel file, so use protective settings for ring use. To clean, use a soft brush, mild detergent, and warm water. Consult our gemstone jewelry cleaning guide for more recommendations.
Microlite: Brazil (0.14). Photo © Joel E. Arem, PhD, FGA. Used with permission.
IGS Newsletter