Yangzhumingite, ideally KMg2.5Si4O10F2, occurs as subhedral to euhedral platy crystals up to ca. 0.5 mm across in metamorphosed carbonate rock from Bayan Obo, Inner Mongolia, China. It is associated with dolomite, calcite, tremolite, norbergite and huanghoite-(Ce) with minor phlogopite, barite, bastnäsite-(Ce), parisite-(Ce), and fluorite. Yangzhumingite is transparent and colorless with white streak and pearly luster. It is optically biaxial (−), n = 1.532–1.537 (589 nm), and 2V = 5–10°. The hardness is 3 on Mohs’ scale (measured on a synthetic equivalent). It has a monoclinic unit cell with a = 5.249(4), b = 9.095(5), c = 10.142(5) Å, β = 99.96(6)°, V = 476.9(5) Å3, Z = 2, space group C2/m, and Dcalc. = 2.807 g/cm3. The five strongest lines in the powder XRD pattern are [d(Å), I/I0, hkl]: (10.03, 95, 001); (3.37, 48, 022), (2.90, 49, 113̅), (2.59, 67, 131̅200) and (2.41, 100, 132̅201). The mean of 15 electron microprobe analyses and SIMS analysis for Li lead to the empirical formula (K0.70Li0.30Na0.01)(Mg2.48Fe0.06) (Si3.96Al0.03)O10[F1.92(OH)0.08] on the basis of 12 anions with (F + OH) = 2. Yangzhumingite is a member of the mica group (Nickel-Strunz grouping 9.EC.10), and it represents the Mg-dominant analogue of montdorite.