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Monazite is a reddish-brown phosphate mineral containing rare earth metals. 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 at least four different kinds of monazite, depending on relative elemental composition of the mineral:〔(Monazite group on Mindat.org )〕 * 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 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.〔("Helium From Sand", March 1931, Popular Mechanics ) article bottom of page 460〕 Monazite is an important ore for thorium, lanthanum, and cerium. 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. Because of its radioactive nature, monazite is a useful for radiometric dating of geological events, such as crystallization, heating, or deformation of the rocks containing monazite. The name monazite comes from the Greek μονάζειν (to be solitary), via German ''Monazit'', in allusion to its isolated crystals.〔''Oxford English Dictionary'', 3rd edition, 2002〕 ==Mining history== Monazite sand from Brazil was first noticed in sand carried in ship's ballast by Carl Auer von Welsbach in the 1880s. Von Welsbach was looking for thorium for his newly invented incandescent mantles. Monazite sand was quickly adopted as the thorium source and became the foundation of the rare earth industry. Monazite sand was also briefly mined in North Carolina, but, shortly thereafter, extensive deposits in southern India were found. Brazilian and Indian monazite dominated the industry before World War II, after which major mining activity transferred to South Africa and Bolivia. There are also large monazite deposits in Australia. Monazite was the only significant source of commercial lanthanides but concern over the disposal of the radioactive daughter products of thorium, bastnäsite came to displace monazite in the production of lanthanides in the 1960s due to its much lower thorium content. Any future increase in interest in thorium for nuclear energy will bring monazite back into commercial use. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Monazite」の詳細全文を読む スポンサード リンク
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