The chemical formula of mineral Monazite is indicated by (Ce, La, Th) PO4. Monazite is a Phosphate mineral. It is known to crystallize in the monoclinic system. In optical mineralogy, the monoclinic system of crystal formation comprises crystals having three axes of unequal lengths. Two of which are usually found in a position that is oblique or not perpendicular to one another. However, both of which are commonly found perpendicular to the third axis. When viewed under polarized microscope, Monazite usually shows biaxial positive figure. The indices of the mineral range from 1.777 to 1.849. The birefringence of the mineral also ranges from 0.045 to 0.052. The optic angle shown by Monazite when it is viewed under polarized microscope used in optical mineralogy is within the range of about 6 degree to 19 degree. When viewed closely under petrographic polarizing microscope, mineral Monazite is commonly found exhibiting a high positive relief. Monazite is most commonly found occurring as small euhedral crystals that may appear equant, flattened, or elongate.
When Monazite is evaluated in thin section, it is usually found colored pale yellow, pale green, colorless, or sometimes gray. Under petrographic polarizing microscope, mineral Monazite in thin section is commonly found exhibiting a weak pleochroism from lighter to slightly darker yellow or greenish yellow. However, the pleochroism of the fragments in grain mount that may be more strongly colored corresponding to the hand sample colors, may be found better. Monazite may produce pleochroic halos when viewed under polarizing microscope in enclosing biotite or other minerals because it often contains radioactive elements.
When mineral Monazite is evaluated using the Mohs scale method, it is commonly found having a hardness measure of about 5. The specific gravity measure of the mineral is ranging from 4.6 grams per cubic centimeters to 5.4 grams per cubic centimeters. When examined in hand samples, it is commonly found colored yellow to reddish yellow or reddish brown. When rubbed on a white porcelain streak plate, Monazite is usually found leaving a white streak. When viewed in reflected light of polarizing microscope for mineralogists, Monazite is commonly found exhibiting a resinous to waxy luster.
Monazite is known to have a single distinct cleavage in one direction and another fair to poor cleavage at right angle in another direction. There are also some reported poor to very poor cleavages and a basal parting on one direction that can be noticed when the evaluation is made with the aid of the petrographic polarizing light microscope. The cleavages are usually not very distinct in some samples that are not significantly altered. However, in some grains that are relatively strongly altered, both cleavages and the basal partings are sometimes shown relatively well. There are also some simple twins on a twin plane that can be found clearly exhibited under polarized microscope. Lamellar twinning however is very rare.
The maximum extinction angle of Monazite is 0 degree to 10 degree with the slow ray nearly parallel to the trace of the one cleavage. The indices of refraction of Monazite generally increase with the substitution of thallium and silicon and decrease with the alteration. It was found in some tests in optical mineralogy that the interference colors in standard thin section of Monazite sample range up to the upper third or lower fourth order. A maximum birefringence can be found lying on one of the cleavages of those fragments as examined closely under polarizing microscope for mineralogists. Low interference colors are found lying in the basal partings of the fragments.
The basal sections show slightly eccentric acute bisectrix figures and also the fragments lying on the basal partings, and these figures are usually more clearly exhibited under petrographic polarizing microscope. Some fragments may be also found producing flash figures. The high birefringence property of Monazite allows for the numerous isochromes that can be found clearly exhibited under polarized microscope used in optical mineralogy. The optic axis dispersion of Monazite is typically weak, as well as its horizontal dispersion. Mineral Monazite is generally relatively stable in the weathering environment but may also be altered into some mineral, possibly limonite. Monazite is usually formed in metamorphosed dolomites. It may be also found in detrital grains in clastic sediments. Monazite is also an accessory mineral in granitic rocks, syenites, carbonatites, and granitic pegmatites.
