Monazite: Gemstone Information
The latest authorities (Anthony et al., 2000) postulate three different species in place of the single undifferentiated monazite. The three minerals are monazite-(Ce), monazite-(La) and monazite-(Nd). All form crystals of the monoclinic system, the Ce and La form usually tabular and the Nd forms prismatic. Hardness is about 5.5 and SG 5.4; the Ce and La minerals may be radioactive from thorium content. Colours range from rose-red to a more common reddish brown. Only monazite-(Ce) is transparent, the other two translucent; RI for the alpha, beta and gamma rays is 1.774–1.800, 1.777–1.801, 1.828–1.849, biaxial positive with birefringence 0.049–0.055. Very complex absorption spectra provide the best means of separating the three minerals from one another and spectral data handbooks need to be consulted. For convenience, a port manteau formula can be written as (Ce,La,Nd,Th)PO4. The minerals are found in igneous rocks including granite pegmatites. Most examples reaching the gemmologist will have come from Madagascar or Sri Lanka,which produces orange pebbles. Colour change (reddish orange in day light to pinkish brown in tungsten light) is reported but is common enough when RE are involved in the composition. Some specimens have shown a greenish fluorescence under LWUV.