The Chemistry of Allanite – Britholite Single Crystal in Alkaline Volcanic Rocks, from Gabal Umm Shaghir Area, Central Eastern Desert, Egypts

The present study deals with the mineral chemistry and genesis of two rare earth elements bearing silicate minerals, namely; allanite and britholite, co-existing in a single crystal, encountered in quartz trachyte dyke, considered to be an offshoot from Gabal Umm Shaghir trachytic plug, central Eastern Desert of Egypt. The size of present allanite-britholite crystal is about 0.25 mm in length and 0.10 mm in width. The allanite phase occupies the main bulk of the crystal, while the britholite phase, exits in the form of transversal spine-shaped lamellae. The mineral chemistry and the genesis of the present allanite - britholite phases are discussed based on 10 scanning electron microprobe point analyses. The present study also discuses, through elemental mapping, the distribution and concentration of selected 9 elements; Si, Fe, Mg, Al, Na, Ca, P, La and Ce, and their frequencies between different mineral phases. The average structural formula of present allanite based on 12.5 (O2−) and 8 cations is: (Ca2+)1.00 (Ce $^{\mathrm {3+}}_{\mathrm {0.34}}$ , La $^{\mathrm {3+}}_{\mathrm {0.19}}$ , Nd $^{\mathrm {3+}}_{\mathrm {0.14}}$ , Sm $^{\mathrm {3+}}_{\mathrm {0.02}}$ , Y $^{\mathrm {3+}}_{\mathrm {0.11}}$ , Ca $^{\mathrm {2+}}_{\mathrm {0.25}})_{\mathrm {1.05}}$ (Mg $^{\mathrm {2+}}_{\mathrm {0.07}}$ , Fe $^{\mathrm {2+}}_{\mathrm {0.69}}$ , Fe $^{\mathrm {3+}}_{\mathrm {0.19}})_{\mathrm {0.95}}$ (Ti $^{\mathrm {4+}}_{\mathrm {0.02}}$ , Fe $^{\mathrm {3+}}_{\mathrm {0.13}}$ , Al $^{\mathrm {3+}}_{\mathrm {0.85}})_{\mathrm {1.00}}$ (Al3+)1.00 (Si4+)2.99 (O2−)12.5. The average structural formula of present britholite based on 12.5 (O2−) and 8 cations is: (Ca $^{\mathrm {2+}}_{\mathrm {1.73}}$ , K $^{\mathrm {+}}_{\mathrm {0.05,\thinspace }}$ Mg $^{\mathrm {2+}}_{\mathrm {0.10}}$ , Fe $^{\mathrm {2+}}_{\mathrm {0.22}}$ , Ti $^{\mathrm {4+}}_{\mathrm {0.01,\thinspace }}$ Al $^{\mathrm {3+}}_{\mathrm {0.14,}}$ La $^{\mathrm {3+}}_{\mathrm {0.50}}$ , Ce $^{\mathrm {3+}}_{\mathrm {1.19}}$ , Nd $^{\mathrm {3+}}_{\mathrm {0.44}}$ , Sm $^{\mathrm {3+}}_{\mathrm {0.10}}$ , Y $^{\mathrm {3+}}_{\mathrm {0.50,\thinspace }}$ Th $^{\mathrm {4+}}_{\mathrm {0.01}})_{\mathrm {5.00}}$ (Al $^{\mathrm {3+}}_{\mathrm {0.12}}$ , P $^{\mathrm {5+}}_{\mathrm {0.39}}$ , Si $^{\mathrm {4+}}_{\mathrm {2.49}})_{\mathrm {3.00}}$ (O2)12.5. The present study concluded that, the coexistence of britholite - allanite mineral phases were resulted from rare earth elements (REEs) + P enriched hydrothermal solutions, undergone percolation and attacking the allanite crystal along cleavage planes. The reaction processes involved, essentially, substitution of P2O5 and REEs2O3 for Al2O3 and SiO2.