1911 Encyclopædia Britannica/Monazite

by Leonard James Spencer

MONAZITE, a mineral consisting of anhydrous phosphate of the cerium metals (Ce, La, Di)PO₄, together with small and variable amounts of thorium (ThO₂, 1–10%) and yttrium. It is of considerable commercial importance as a source of thoria for the manufacture of the Welsbach and other mantles for incandescent gas-lighting: the cerium is used to a limited extent in pharmacy.

The following analyses are of monazite from: (I.) Burke county, North Carolina; (II.) Arendal, Norway; (III.) Emmaville, Gough county, New South Wales.

	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)$\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}$	30·88	26·26	30·21
Yttrium oxide (Yt2O3)	—	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

Thoria and silica being often present in the molecular ratio 1 : 1, it has been suggested that they exist as thorite (ThSiO₄) as a mechanical impurity in the monazite.

Crystals of monazite belong to the monoclinic system, and are usually flattened parallel to the ortho-pinacoid (a in the figure). The large (up to 5 in. in length) reddish-brown, dull and opaque crystals from Norway and the Urals are simple in form, whilst the small, translucent, honey-yellow crystals from the Alps are bounded by numerous bright faces. Crystals of the latter habit were described in 1823 from Dauphiné under the name turnerite, and owing to their rarity were not until many years afterwards analysed chemically and proved to be identical with monazite. Monazite from the Urals was described by A. Breithaupt in 1829, and named by him from Gr. μονάζειν to be solitary, because of the rarity of the singly occurring crystals. The hardness is 51/2, and the specific gravity 5·1–5·2. Light which has traversed a crystal or grain of monazite exhibits a characteristic absorption spectrum, and this affords a ready means of detecting the mineral.

As minute idiomorphic crystals monazite is of wide distribution in granites and gneisses, being present in very small amounts as an accessory constituent of these rocks. By powdering the rock and washing away the lighter minerals in a stream of water the heavy minerals (zircon, anatase, rutile, magnetite, garnet, monazite, xenotime, &c.) may be collected. This separation has been effected naturally by the weathering and disintegration of the rocks and the accumulation of the heavier minerals in the beds of streams. Under these conditions monazite has been found as rounded water-worn grains in the alluvial gold-washings of the Urals, Finland, Siberia, the United States, Brazil, Colombia, New South Wales, &c., and in tin-gravels in Swaziland, South Africa. Larger crystals of monazite are found embedded in pegmatite veins in the Ilmen Mountains (southern Urals); at Arendal and other places in southern Norway, where it is collected in the feldspar quarries to the extent of about one ton per annum; and in the mica mines at Villeneuve in Quebec, where masses of monazite weighing 20 ℔ have been found. The small crystals of the “turnerite” habit occur implanted, often with anatase and rutile, on the crystallized quartz and albite, which line crevices in the crystalline schists of the French, Swiss and Tirolese Alps; similar crystals with the same associations occur very exceptionally in the clay-slate at Tintagel in Cornwall. Microscopic crystals of monazite (cryptolite, from κρυπτός, concealed) have been observed embedded in the crystallized apatite of Arendal in Norway.

The deposits worked commercially are the monazite-bearing sands of North Carolina and Brazil, and to a smaller extent those of South Carolina. In North Carolina it occurs over a wide area in the streams rising in the South Mountains, an eastern outlier of the Blue Ridge. The rocks of the district are granitic biotite-gneiss and hornblende-gneiss, and are intersected by veins of auriferous quartz. The percentage of monazite in the river-gravels varies from very small amounts up to 1 or 2%. The heavy minerals contained in the gravels are collected in the same manner as in washing for gold (which is often also present); magnetite is separated with a magnet; but other minerals, such as zircon, rutile, garnet, corundum, &c., cannot be separated by mechanical means. The product is a line-grained yellowish sand containing 65–85% of monazite and 3–9% of thoria. In Brazil it occurs in river-gravels and also in the sand on the sea-beaches; an extensive accumulation of very rich monazite sand occurs on the seashore near Alcobaça in Bahia, and this has been shipped as ballast in the natural state.

See H. B. C. Nitze, “Monazite” (16th Annual Report of the United States Geological Survey, pt. iv. (1895), pp. 667–693).  (L. J. S.)