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This is the structure of the mineral hornblende. It is a 2 x 2 x 2 supercell of an idealized structure, with Mg occupying all 6-fold sites M1-M3, Ca occupying all 6-8-fold sites M4, Na occupying all A sites, no F- substitution for OH-, and no Al-for-Si substitution in the tetrahedra. Ca in light green (no bonds), Na in purple (no bonds), Mg in green (6-fold coordinated), Si in ochre, O, in red, and H in white.