The chemical formula for mineral Xenotime is indicated by YPO4. Xenotime is a Phosphate mineral. It is known to crystallize in the tetragonal system. In optical mineralogy, the tetragonal system of crystallization comprises crystals having three axes, which are all in a position perpendicular to one another. Two axes are usually found having the same or equal length. Xenotime is a uniaxial positive mineral. The indices of Xenotime range from 1.690 to 1.827. The birefringence of Xenotime is also usually found within the range of 0.070 to 0.107. Xenotime is commonly found exhibiting a high positive relief when viewed under several adjustments on the aperture diaphragm of the polarizing light microscope.
Xenotime is usually found colorless, pale brown, or yellow in thin section. This mineral is most commonly found with weak pleochroism. Xenotime is commonly found as elongate prismatic tetragonal grains that resemble zircon. Xenotime is also occasionally found as radial aggregates or rosettes that are more splendidly exhibited under petrographic polarizing light microscope. Detrital grains of Xenotime are commonly elongate in form as found when the samples are examined under polarized microscopes.
When evaluated using the Mohs scale method, mineral Xenotime is commonly found having hardness measure ranging from 4 to 5. The specific gravity measure of the mineral is usually found ranging from 4.3 grams per cubic centimeters to 5.1 grams per cubic centimeters. For pure YPO4, the calculated density is usually 4.25 grams per cubic centimeters. If examined in hand samples, Xenotime is commonly found yellowish or reddish brown. It is less commonly yellow, gray, greenish, or salmon. When rubbed on a white porcelain streak plate, Xenotime is most commonly found leaving a white streak. Xenotime is usually found exhibiting a vitreous luster in reflected light of petrographic polarizing light microscope.
Xenotime has good prismatic cleavage on one direction that is more clearly visible under polarizing microscope used in optical mineralogy. The two cleavage directions are usually found at right angles. It has been found that the cleavages do not tend to have strong control on fragment orientation. Xenotime crystals are sometimes twinned as seen when examined with the aid of the petrographic microscopes. Simple contact twins are rare and are usually found producing elbow-shaped twins that are clearly exhibited under polarizing microscopes used in optical mineralogy.
The extinction of mineral Xenotime in longitudinal section is parallel to both the trace of cleavage and the length of the crystals. The elongation is usually length slow. The indices of refraction will probably increase with the substitution of thallium for yttrium. The birefringence of mineral Xenotime is very strong and it is commonly found yielding a high-order white interference color. This interference color of Xenotime is often masked by the color of the mineral. Due to the somewhat converging nature of the light coming from the condenser of the polarizing microscope, the basal sections of Xenotime usually produce high-order interference colors.
Due to the small grain size of Xenotime, interference figures are often difficult to obtain. Uniaxial positive optic axis figures with numerous isochromes and isogyres that broaden substantially toward the edge of the field of view of the petrographic polarizing microscope are usually produced by the basal sections on sufficiently large grains of Xenotime. Some fragments lying on one of the cleavage planes yield a flash figure when viewed under polarized microscope used in optical mineralogy. Xenotime is not readily altered. It can be found in metamorphic rocks, detrital grains in placer deposits, as well as in sand stone and related clastic sedimentary rocks.
