Monazite
| Monazite | |
|---|---|
 |
|
| General and classification | |
| chemical formula | see single minerals |
|
Mineral class (and possibly department) |
Phosphates, arsenates, vanadates |
|
System no. to Strunz and to Dana |
8.AD.50 ( 8th edition : VII / A.15) 04/38/03 (Monazit Group) |
| Similar minerals | Xenotime , Brabantite , Gasparite |
| Crystallographic Data | |
| Crystal system | monoclinic |
| Crystal class ; symbol | monoclinic prismatic, 2 / m |
| Space group | P 2 1 / n (No. 14, position 2) |
| Lattice parameters | see crystal structure |
| Formula units | Z = 4 Please complete the source as an individual reference |
| Twinning | frequently |
| Physical Properties | |
| Mohs hardness | 5 to 5.5 |
| Density (g / cm 3 ) | 4.6 to 5.4 |
| Cleavage | {001} clearly; {100} indistinct |
| Break ; Tenacity | uneven |
| colour | brown, yellow, reddish, greenish, colorless |
| Line color | gray-white |
| transparency | translucent to opaque |
| shine | Resin gloss |
| radioactivity | radioactive in the presence of Th |
| Crystal optics | |
| Refractive indices |
n α = 1.785 to 1.800 n β = 1.787 to 1.801 n γ = 1.837 to 1.849 |
| Birefringence | δ = 0.049 to 0.052 |
| Optical character | biaxial positive |
| Axis angle | 2V = 6 to 19 ° |
| Pleochroism | colorless to yellow |
| Other properties | |
| Chemical behavior | insoluble in water |
| Special features | sometimes fluorescent |
Monazite is a collective term for the minerals recognized by the IMA and end members of a seamless mixed series, consisting of monazite (La) , monazite (Ce) , monazite (Nd) and monazite (Sm) as well as their mixed crystals . These rarely occurring minerals belong to the mineral class of "phosphates, arsenates and vanadates" and crystallize in the monoclinic crystal system with the following chemical composition :
- Monazite (La) - (La, Ce, Nd) [PO 4 ] or LaPO 4
- Monazite (Ce) - (Ce, La, Nd, Th) [PO 4 ] or CePO 4
- Monazite (Nd) - (Nd, Ce, Sm) [PO 4 ] or NdPO 4
- Monazite- (Sm) - (Sm, Gd, Ce, Th) [PO 4 ] or SmPO 4
The metals lanthanum (La), cerium (Ce), neodymium (Nd) and samarium (Sm) appearing in the formula are also referred to as rare earth metals . Furthermore, due to their complete solubility in one another, there are always admixtures of the other end members of the mixed series as well as gadolinium (Gd), praseodymium (Pr), yttrium (Y) and up to 20% thorium (Th). Monazite- (Sm) - (Sm, Gd, Ce, Th) [PO 4 ] or SmPO 4 .
Since the lanthanides in nature are never pure, but always occur as mixtures of the corresponding elements, there is the alternative chemical formula Ln [PO 4 ] for monazite . Here the symbol Ln does not stand for the individual element lanthanum (La), but for the sum of all lanthanides.
Along with bastnäsite, monazite is the most important ore for extracting these rare earth metals.
Etymology and history
The word monazite is derived from the Greek μονάζειν [monázein] "live alone" (composed of μόνος [mónos] "alone" and ζήω [zēō] "live", the etymologies of which are unclear), which refers to the individually occurring crystals . Hermit - a synonym for Monazite - was described by Shepard in 1837 and so named, alluding to the distant individuals of this mineral.
Monazite was first observed by Carl Auer von Welsbach in ballast sand in ships from Brazil. He was looking for thorium for the incandescent mantles he invented in the 1880s . Monazite sand soon established itself as a source of thorium for the manufacture of mantles, which was also the cornerstone of the rare earth industry. For a short time, monazite was imported from North Carolina, but abundant sources were soon discovered in India and Brazil. Until the Second World War, Monazite was the main source of rare earth metals, until about 1965 they began to process bastnasite .
classification
In the old (8th edition) and new systematics of minerals (9th edition) according to Strunz , the minerals of the Monazite series are assigned to the department of "anhydrous phosphates without foreign anions ". This department has been further subdivided since the new system, so that the Monazites can now be found in the subdivision of compounds with exclusively large cations and with the other minerals Cheralith , Gasparit- (Ce) and Rooseveltit a system group with the abbreviation 8.AD .50 form (8th edition: Monazite- (La) - VII / A.15-10, Monazite- (Ce) - VII / A.15-20, Monazite- (Nd) - VII / A.15-30 , Monazite- (Sm) - VII / A.15-35).
In the systematics of minerals according to Dana , the minerals of the monazite series also belong to the division of anhydrous phosphates ( Anhydrous Phosphates ) and, according to their general chemical composition, form A + XO 4 (A = metal, X = phosphorus, arsenic, vanadium) and each same crystal structure together with the other minerals Cheralith, Brabantit and Gasparite- (Ce) the Monazite group with the abbreviation 38.4.3.
Crystal structure
| mineral | Lattice parameters |
| Monazite- (La) | a = 6.8313 Å ; b = 7.0705 Å; c = 6.5034 Å β = 103.27 ° |
| Monazite (Ce) | a = 6.7902 Å ; b = 7.0203 Å; c = 6.4674 Å β = 103.38 ° |
| Monazite (Nd) | a = 6.7352 Å ; b = 6.9500 Å; c = 6.4049 Å β = 103.68 ° |
| Monazite- (Sm) | a = 6.6818 Å ; b = 6.8877 Å; c = 6.3653 Å β = 103.86 ° |
All monazites crystallize in the monoclinic crystal system in the space group P 2 1 / n (space group no. 14, position 2) with the lattice parameters indicated opposite, which differ only slightly, as well as four formula units per unit cell . Monazite is closely related to the tetragonally crystallizing xenotime (Y, Yb) [PO 4 ]. Monazite contains the lighter but larger rare earth metals , the coordination number on the rare earth metal is CN = 9. The smaller and heavier rare earth metals in xenotime only have a coordination number of CN = 8, which also enables the higher symmetry of its structure.
properties
The color varies between brown-red and yellow, it crystallizes in a monoclinic manner and has a density of approx. 5 g / cm 3 . It consists of approx. 50% cerium , approx. 20% lanthanum and neodymium , approx. 5% praseodymium and traces of other rare earth compounds. The composition of cerium mixed metal corresponds to the monazite.
Monazite can contain up to 20% by weight of radioactive thorium dioxide (ThO 2 ) and up to 1% by weight of uranium oxide (UO 2 ), as well as their decay products lead (Pb) and very small amounts of helium (He). Its specific activity is up to 250 kBq per kg and is therefore around 100 times stronger than that of other mineral sands. Therefore monazite is an important mineral for the absolute age dating of rocks ( geochronology ).
Education and Locations
Monazite is found in numerous geological environments. It occurs in acid igneous rocks as well as metamorphic rocks . Thanks to its chemical resistance, it is also present in rubble and in soaps . As Monazite sand, it forms deposits in river and coastal sands (soap deposits). They were created by the enrichment of the heavy minerals after the weathering of the bedrock.
Important primary deposits of monazite are found in the Bayan Obo ore mines in China, Mountain Pass in California, Van Rhynsdorp and Naboomspruit in South Africa. The beaches of Trivandrum and Travancore in South India consist of large amounts of sand with a high content of monazite.
Other locations for Monazite have been various regions in Australia ; the coastal strips off Rio de Janeiro , Bahia , Espírito Santo and Paraíba (Parahyba) in Brazil; Antsirabé in Madagascar; Iveland and Narestö in Norway; Miask in the Urals (Russia); Sri Lanka ;
use
Monazites are needed almost exclusively as a raw material for the extraction of rare earth metals . In addition, the thorium contained in the compounds is also extracted, which was needed until the 1970s to manufacture the incandescent mantles that are important for gas lamps . Due to the fact that fissile uranium-233 can be incubated from thorium, mixed with uranium-235 it was used in high-temperature reactors until 1989 .
The rare earth metals can be digested by either acidic or alkaline treatment.
Acid exposure
In the acidic digestion, a mixture of the sand with 98% sulfuric acid is heated in containers made of cast steel to 120 to 150 ° C for several hours. The digestion reaction is strongly exothermic. If the temperature is kept below 250 ° C and an excess of acid is used, the thorium contained becomes water-soluble. If equal weights of acid and sand are used and a higher temperature is set, the thorium becomes insoluble in water. The latter is unusual. The resulting sludge is quenched with water, whereby the sulfates go into solution. The thorium is then precipitated as pyrophosphate or as a basic salt by neutralization. The lanthanoids remaining in solution are then either completely precipitated with oxalic acid or partially obtained as sodium sulfate double salt by precipitation of the cerium group. The remaining yttrium group is processed separately. The phosphates can be separated by precipitating the hydroxides with sodium hydroxide. When the water-containing precipitate is dried in air, the cerium (III) oxidizes to cerium (IV) oxide .
Basic digestion
In the case of basic digestion, the monazite sand is treated with caustic soda at approx. 140 ° C. This produces insoluble oxides and hydroxides as well as sodium phosphate , which dissolves after adding water. The insoluble sludge is treated with dilute hydrochloric acid, the lanthanoid oxides going into solution, while the thorium dioxide remains as a residue, contaminated with other acid-insoluble components. Cerium (IV) oxide can be almost completely precipitated from the solution by oxidation with sodium hypochlorite at the same pH value.
See also
literature
- Paul Ramdohr , Hugo Strunz : Klockmann's textbook of mineralogy . 16th edition. Ferdinand Enke Verlag, 1978, ISBN 3-432-82986-8 , pp. 625 to 626 .
Web links
- Mineral Atlas: Monazite (Wiki)
Individual evidence
- ↑ Stefan Weiß: The large Lapis mineral directory . 5th edition. Christian Weise Verlag, Munich 2008, ISBN 978-3-921656-70-9 , p. 176 .
- ↑ IMA / CNMNC List of Mineral Names (English, PDF 1.79 MB)
- ↑ Etymology of mineral and rock names
- ^ Wolfgang Brendler: Mineral Collections. Part 2. Wilhelm Engelmann, Leipzig 1912, p. 187 archive.org
- ↑ "The name bestowed upon the mineral is derived from ερημία, solitude, in allusion to the isolated manner of its occurrence, with respect to other individuals of the same species." - "The name given to the mineral is derived from [gr.] Ερημία, loneliness, alluding to the way it appears in relation to other individuals of the same species." - Charles Upham Shepard: Notice of Eremite, a new Mineral Species. In: The American Journal of Science and Arts. Volume 32 (1837), p. 341 f. Full text in Google Book Search
- ↑ American Mineralogist Crystal Structure Database - Monazite- (La) (English, 1995)
- ↑ American Mineralogist Crystal Structure Database - Monazite- (Ce) (English, 1995)
- ↑ American Mineralogist Crystal Structure Database - Monazite- (Nd) (English, 1995)
- ↑ American Mineralogist Crystal Structure Database - Monazite- (Sm) (English, 1995)
- ^ Naturally-Occurring Radioactive Materials
- ↑ a b c C.K. Gupta, N. Krishnamurthy, Extactive Metallurgy of Rare Earths. CRC Press, 2005, ISBN 0-415-33340-7 .
- ↑ MinDat - Localities for Monazite (English)