THE MINERAL XENOTIME
- Chemistry: YPO4, Yttrium Phosphate
- Class: Phosphates
- Uses: As mineral specimens and as a source of yttrium.
Xenotime is one of the very few minerals, let alone words, that starts with the letter X; a good word to remember in some word games. It is a widely spread mineral throughout the world although good crystals and therefore specimens are somewhat hard to find.
It is one of the few yttrium minerals known to science. Wakefieldite and chernovite-(Y) are other yttrium minerals, although a vanadate and an arsenate respectively. Often uranium or other rare elements such as erbium, thorium, ytterbium, zirconium and the not so rare calcium are found as traces in xenotime replacing the yttrium. Due to the presence of at least some of these radioactive elements, xenotime is frequently slightly radioactive.
Xenotime also will contain traces of silicon dioxide and arsenate replacing the phosphate anion. In fact it forms a solid solution series with the previously mentioned chernovite-(Y) whose formula is YAsO4. It is unusual to have a solid solution series involving the principle anions but it is not as complete a solid solution series as the more famous pyromorphite-vanadinite-mimetite series. That is a more complex phosphate-vanadate-arsenate series.
Also of note is that xenotime is one of the few phosphate minerals that does not contain water molecules, hydroxides or chlorides. It belongs to an informal group of phosphates and called the anhydrous phosphates along with monazite, purpurite and lithiophyllite.
The crystals of xenotime are similar to zircon and can easily be confused with the duller lustered, less transparent samples of zircon. However, the cleavage and the softness of xenotime are sufficient to distinguish them.
- Color is most commonly various shades of brown but also gray, greenish brown, muted red and yellow.
- Luster is vitreous to resinous.
- Transparency: Crystals are translucent to opaque.
- Crystal System is tetragonal; 4/m 2/m 2/m
- Crystal Habits include stubby to elongated prismatic crystals that are terminated by variously slanted dipyramids. Also known for nice rosette and radial aggregates.
- Cleavage is perfect in two prismatic directions.
- Fracture is uneven.
- Hardness is 4 - 5
- Specific Gravity is approximately 4.4 - 5.1 (heavy for translucent minerals)
- Streak is pale brown to yellow or red.
- Other Characteristics: Trace amounts of uranium and other rare earth elements may render crystals slightly radioactive.
- Associated Minerals are quartz, micas especially biotite, monazite and some feldspars.
- Notable Occurrences include Arendal, Hittero and Tvedestrand, Norway; Madagascar; several locations in Brazil and also in the United States in Colorado, California, Georgia and North Carolina.
- Best Field Indicators are color, crystal habit, cleavage, softness and luster.
THE MINERAL XONOTLITE
- Chemistry: Ca6Si6O17(OH)2, Calcium Silicate Hydroxide.
- Class: Silicates
- Subclass: Inosilicates
- Uses: Only as a mineral specimen.
Xonotlite is named for its type locality, Tetela de Xonotla, Puebla, Mexico. It has also been called Eakleite.
It frequently forms tuffs of fine white acicular crystals in veins in serpentine formations and along contact zones.
- Color is Colorless, Gray, Lemon white, Light gray, or Pink.
- Transparency: Specimens are transparent.
- Luster: Vitreous to silky.
- Crystal System is monoclinic.
- Crystal Habits include tuffs of acicular fibers and massive.
- Hardness is 6.5.
- Specific Gravity is approximately 2.7 (average).
- Streak is white.
- Notable Occurrences are Tetela de Xonotla, Puebla, Mexico, Arizona, USA, and South Africa.
- Best Field Indicators are locality, color, crystal habit and hardness.