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Scandium is important in modern technology and is regarded as a strategic metal in many countries. It is highly dispersed in the crust and hardly forms independent minerals. Clinopyroxene is the most important Sc-bearing mineral in some world-class deposits. Nonetheless, factors controlling the Sc distribution in clinopyroxene is underexplored, impeding the understanding of the geochemical behavior of Sc and the formation of Sc-rich clinopyroxene. The newly discovered Mouding Sc deposit in SW China is hosted in a zoned mafic-ultramafic intrusion. Clinopyroxene in the intrusion is diopsidic in composition with high Sc contents (80–105 ppm). In-situ trace element mappings on diopside reveal homogeneous, zoned, swallow-tailed and hourglass internal Sc distribution patterns. These textural patterns can be produced through kinetically controlled incorporation of Sc on different crystal faces. The preferential substitution of Sc can take place at the {100}, {110} and {010} prism faces because of the high flexibility of the octahedral M1 protosites. The fast growth of diopside, which facilitates the kinetically-controlled crystallization, is dominated by the textural coarsening process and promoted by the hydrous parental magma due to its low viscosity and active convection. The active flow and efficient interstitial communication can direct compatible elements from the magma chamber into clinopyroxene, in favor of the formation of Sc-rich clinopyroxene. Our study emphasizes that kinetic effects should be carefully examined in relation to the Sc distribution and enrichment processes of hydrous magmatic Sc deposits.

Additional information: Mr. Zhen-Chao WANG, zhenchao@connect.hku.hk