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Chemistry and Origin of Enigmatic Monazite and Chevkinite/Perrierite in the Egyptian Black Beach Sand
Author(s) -
Moustafa Mohamed I.
Publication year - 2010
Publication title -
resource geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.597
H-Index - 43
eISSN - 1751-3928
pISSN - 1344-1698
DOI - 10.1111/j.1751-3928.2010.00132.x
Subject(s) - monazite , geology , geochemistry , mineral , mineralogy , zircon , chemistry , organic chemistry
The content of monazite in the Egyptian black beach sand and coastal sand dunes is normally equal to or below 0.01 wt.%. The obtained high grade monazite concentrate includes three minor monazite groups in addition to major canary and lemon yellow coloured monazite: (i) the colourless to pinkish white coloured monazite; (ii) the opaque light to dark resinous, reddish brown and dark brown coloured monazite; and (iii) opaque yellowish red to brownish red coloured monazite grains group. These groups represent 3%, 4% and 2%, respectively, in the high grade monazite concentrate. A negligible amount of euhedral to subhedral black to brownish black chevkinite/perrierite mineral crystals was detected in the obtained monazite concentrate. The presence of these minor mineral groups affects the chemical composition of the obtained high grade monazite concentrate. The Ce 2 O 3 is the main REE in the studied monazite. In the colourless‐pinkish monazite grains, the analyzed REE are the following, in order of abundance; Ce > La > Nd > Pr > Sm > Gd > Dy. UO 2 ranges between 0.11 and 1.74 wt.%. The contents of Eu 2 O 3 is under the limit of detection while ThO 2 ranges between 3.99 and 8.58 wt.% with an average value of 5.57 wt.%. These grains are most probably igneous monazite from a highly differentiated granite. The resinous, brown monazite grains have lower Ce 2 O 3 content (24.63 wt.%) and much lower La 2 O 3 content (6.00 wt.%) but greater content of Eu 2 O 3 (0.41 wt.%) than those of the colourless‐pinkish monazite. These monazites have the lowest contents of Th, U and Ca among the three groups. The resinous, brown monazites are most probably formed by metamorphism or alteration leading to leaching or replacement of pre‐existing minerals. The red monazite group has a lower average Ce 2 O 3 content (25.28 wt.%) than the colourless‐pinkish variety (28.02 wt.%) but slightly greater than that of the resinous, brown ones. The red monazite group has the highest ThO 2 and UO 2 contents; 5.84 wt.% and 1.24 wt.%, respectively. It has the lowest monazite component mole fraction (0.75). The red monazite seems to have been formed by hydrothermal alteration of pre‐existing monazite and other mineral species bearing for Y, REE, Ca, Th and U. The two coupled substitution mechanisms: (Th, U) 4+ + Ca 2+ 2REE 3+ , and (Th, U) 4+ + Si 4+ REE 3+ + P 5+ , are obvious in the studied colourless‐pink monazite.