Ammoniolasalite, [(NH4)2Mg2(H2O)20][V10O28], is a new decavanadate species (IMA2017-094) from the Burro mine, Slick Rock district, San Miguel County, Colorado, USA (38°2′42″N 108°53′23″W). The mineral occurs as crusts on montroseite- and corvusite-bearing sandstone in an NH4-rich secondary assemblage also containing ammoniozippeite and the NH4-bearing decavanadates schindlerite and wernerbaurite. The crusts, up to 2 mm or more across, exhibit consistently oriented faces. Ammoniolasalite crystals are bright orange to orange-yellow with a vitreous luster. The mineral has brittle tenacity, Mohs hardness of 1, no cleavage, and conchoidal fracture. The density (measured) is 2.28(2) g/cm3. Ammoniolasalite is biaxial (–), α = 1.740(3), β = 1.769(3), γ = 1.771(3) as measured in white light. The measured 2V is 31(1)°. Dispersion is very strong, r > v, and the orientation is Y = b, Z ∧ a = 38° in obtuse β. Pleochroism is X yellow, Y yellow orange, Z orange; X < Y < Z. Electron probe microanalysis gave the empirical formula (based on 48 O apfu) [(NH4)1.76K0.24]Σ2.00Mg1.94[V5+10O28]·20H2O. Ammoniolasalite is monoclinic, C2/c, with a 24.478(3), b 10.9413(4), c 17.5508(12) Å, β 119.257(7)°, and V = 4100.9(6) Å3. The strongest four lines in the diffraction pattern are [d in Å(I)(hkl)]: 10.64(24)(200), , 7.62(26)(002,111), and . The atomic arrangement of ammoniolasalite was solved and refined to R1 = 0.0357 for 3628 independent reflections with I > 2σ(I). Ammoniolasalite is isostructural with lasalite, [Na2Mg2(H2O)20][V10O28], from which it derives its name. The structural unit is the [V10O28]6– decavanadate group; charge balance in the structure is maintained by the [(NH4,K)2Mg2(H2O)20]6+ interstitial complex. Linkage between the structural unit and the components of the interstitial complex is attained by bonding between the (NH4,K) site and three oxygen atoms of the decavanadate structural group.