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The occurrence and mineralogical characteristics of ishikawaite in unzoned pegmatite hosted in the monzogranite of Gabal Samma area, Southeastern Sinai, Egypt are investigated. Ishikawaite occurs in highly radioactive pegmatitic bodies in which the averages of uranium and thorium contents are (211&1480 ppm respectively). The mineral presents in association with uranophane, thorite, uranothorite, columbite, betafite, metamict zircon and fluorite. The pegmatites occur as pockets (5x10m) and dykes (1x20m) hosted in the monzogranites. The investigated mineral is generally black to dark brown in color, forming anhedral grains with a black to brown streak and opaque with vitreous luster. The mineralogy and geochemistry of the studied ishikawaite were determined using microscopic examination as well as quantitative analysis by ICP-MS. The ICP-MS shows the average oxides composition: 47.61% Nb 2 O 5 , 22.83% UO 2 , 6.06% Fe 2 O 3 , 5.44% Ta 2 O 5 , 4.69% REE 2 O 3 , 4.34% ThO 2 , 3.23% CaO and 2.99% Y 2 O 3 . The minor to trace elements include Mn, Ti, Al and Zr. The calculated empirical formula is (U 0.231 Th 0.045 Fe 0.207 Ca 0.157 Y 0.071 REE 0.06 Mn 0.048 Al 0.021 Na 0.015 K 0.007 Zr 0.007 Mg 0.002 Hf 0.001 ) ∑0.883 (Nb 0.976 Ta 0.067 Ti 0.028 ) ∑ 1.071 O 4 . The simplified formula is (U, Th, Fe, Ca, Y, REE) (Nb, Ta, Ti) O 4 . The formula of the study ishikawaite is non stoichiometric and show some deficiency in the A-site relative to the B-site. Metamictization, a common feature of ishikawaite is suspected to be the main reason for this deviation from ideal stoichiometry. The deviations of Y/Ho and Zr/Hf ratios of the investigated ishikawaite from chondiritic trace-element ratios are characteristic for strong magmatic with hydrothermal alteration.

OCCURRENCE AND MINERAL CHEMISTRY OF ISHIKAWAITE IN THE MINERALIZED PEGMATITES OF GABAL SAMMA, SOUTHEASTERN SINAI, EGYPT