Monazite

Monazite
Monazite-(Ce)
General
Category	Phosphate minerals
Formula (repeating unit)	(Ce,La,Th)PO4
Strunz classification	8.AD.50
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H–M symbol)
Space group	P21/n
Identification
Color	Orange, purple, reddish brown, brown, pale yellow, pink, blue, green, gray,
Crystal habit	Commonly as prismatic or wedge-shaped crystals
Twinning	Contact twins common
Cleavage	Distinct on [100] poor on [010]
Fracture	Conchoidal to uneven
Mohs scale hardness	5.0–5.5
Luster	Resinous, vitreous to adamantine
Streak	White
Diaphaneity	Translucent to opaque
Specific gravity	4.6–5.7 (4.98–5.43 for monazite-Ce)
Optical properties	Biaxial (+)
Refractive index	nα = 1.770–1.793 nβ = 1.778–1.800 nγ = 1.823–1.860
Pleochroism	Weak
2V angle	10–26°
Melting point	1900–2100
Other characteristics	Radioactive if uranium and/or thorium-rich, dull brown cathodoluminescence, paramagnetic
Magnetism	Paramagnetic, moderately strongly
References	[1][2]

Monazite is a primarily reddish-brown phosphate mineral that contains rare-earth elements. Due to variability in composition, monazite is considered a group of minerals.^[3] The most common species of the group is monazite-(Ce), that is, the cerium-dominant member of the group.^[4] It occurs usually in small isolated crystals. It has a hardness of 5.0 to 5.5 on the Mohs scale of mineral hardness and is relatively dense, about 4.6 to 5.7 g/cm³. There are five different most common species of monazite, depending on the relative amounts of the rare earth elements in the mineral:^[5]

• monazite-(Ce), (Ce,La,Nd,Th)PO₄ (the most common member),
• monazite-(La), (La,Ce,Nd)PO₄,
• monazite-(Nd), (Nd,La,Ce)PO₄,
• monazite-(Sm), (Sm,Gd,Ce,Th)PO₄,
• monazite-(Pr), (Pr,Ce,Nd,Th)PO₄.

The elements in parentheses are listed in the order of their relative proportion within the mineral: lanthanum is the most common rare-earth element in monazite-(La), and so forth. Silica (SiO₂) is present in trace amounts, as well as small amounts of uranium and thorium. Due to the alpha decay of thorium and uranium, monazite contains a significant amount of helium, which can be extracted by heating.^[6]

The following analyses are of monazite from: (I.) Burke County, North Carolina, US; (II.) Arendal, Norway; (III.) Emmaville, New South Wales, Australia.^[7]

		I.	II.	III.
Phosphorus pentoxide (P2O5)		29.28	27.55	25.09
Cerium oxide (Ce2O3)		31.38	29.20	36.64
Lanthanum oxide (La2O3) Didymium oxide (Di2O3)	${\displaystyle {\Big \}}}$	30.88	26.26	30.21
Yttrium oxide (Y2O3)		—	3.82	—
Thorium oxide (ThO2)		6.49	9.57	1.23
Silica (SiO2)		1.40	1.86	3.21
Alumina (Al2O3)		—	—	3.11
Iron oxide (Fe2O3)		—	1.13	—
Lime (CaO)		—	0.69	—
Water (H2O)		0.20	0.52	—
		99.63	100.60	99.49
Specific gravity		5.10	5.15	5.001

Monazite is an important ore for thorium,^[8] lanthanum, and cerium.^[9] It is often found in placer deposits. India, Madagascar, and South Africa have large deposits of monazite sands. The deposits in India are particularly rich in monazite.

Monazite is radioactive due to the presence of thorium and, less commonly, uranium. The radiogenic decay of uranium and thorium to lead enables monazite to be dated through monazite geochronology. Monazite crystals often have multiple distinct zones that formed through successive geologic events that lead to monazite crystallization.^[10] These domains can be dated to gain insight into the geologic history of its host rocks.

The name monazite comes from the Ancient Greek: μονάζειν, romanized: monázein (to be solitary), via German Monazit, in allusion to its isolated crystals.^[11]