Batagayite, CaZn₂(Zn,Cu)₆(PO₄)₄(PO₃OH)₃·12H₂O, is a new secondary phosphate mineral from the Këster deposit, Arga-Ynnykh-Khai massif, NE Yakutia, Russia. It is monoclinic, P2₁, a = 8.4264(4), b = 12.8309(6), c = 14.6928(9) Å, β = 98.514(6)^o, V = 1571.05(15) ų and Z = 2 (from single-crystal X-ray diffraction data). Batagayite crystals are blades up to 2 mm long, flattened on {001} and elongated on [100]; blades often grow in radial aggregates. Associated minerals are arsenolite, native copper, epifanovite, fluorapatite, libethenite, Na-analogue of batagayite, pseudomalachite, quartz, sampleite, tobermorite, and Mg-analogue of hopeite. The streak is white and the luster is vitreous. The mineral is brittle and has a perfect cleavage on {001}, no parting was observed. The Mohs hardness is 3. Density, determined by the float-sink method in Clerici solution, is 2.90(3) g/cm³, and the calculated density is 3.02 g/cm³ (using the empirical formula and single-crystal unit-cell parameters). Batagayite is biaxial, optically negative, α = 1.566 ± 0.002, β = 1.572 ± 0.002, γ = 1.573 ± 0.002 at 589 nm. 2V_meas. = 40(5)°, 2V_calc = 44.3°. Optical orientation: Z is perpendicular to (001), further details unclear. No dispersion or pleochroism were observed. The mean chemical composition determined by electron microprobe is: Na₂O 0.31, MgO 1.39, Al₂O₃ 0.55, SiO₂ 0.48, P₂O₅ 34.37, K₂O 0.17, CaO 2.76, MnO 1.03, CuO 5.80, ZnO 35.62, CdO 0.24 wt%. The H₂O content estimated from the crystal-structure refinement is 16.83 wt%, giving a total of 99.55 wt%. The empirical formula calculated on the basis of P + Si = 7 is (Zn_6.22Cu_1.04Ca_0.70Mg_0.49Mn_0.21Al_0.15Na_0.14K_0.05Cd_0.03)_Σ9.03(P_6.89Si_0.11)_Σ7.00O_24.91(OH)_3.09·12.10H₂O. The mineral easily dissolves in 10% room-temperature HCl. The eight diagnostic lines in the X-ray powder-diffraction pattern are (I-d[Å]-hkl): 100-14.59-001, 25-6.34-012, 11-6.02-111, 37-4.864-003, 13-4.766-112, 20-3.102-1 2¯4¯, 11-2.678-2 3¯3¯, 16-2.411-044. The crystal structure of batagayite was solved by direct methods and refined to R₁ = 0.069 for 3847 independent reflections with F_o > 4σ(F_o). It is based upon complex heteropolyhedral [M₈(PO₄)₄(PO₃OH)₃(H₂O)₉]²⁻ layers parallel to the (001) plane. The layer can be considered as consisting of three sublayers, one A and two B. The central A layer has the composition [M₄(PO₄)₄(H₂O)₄]⁴⁻ and consists of the zigzag chains of edge-sharing (MO₆) octahedra running parallel to the a axis and linked into layers by sharing peripheral O atoms. The (PO₄) tetrahedra are attached above and below the holes created by the linkage of zigzag octahedral chains. The B sublayer consists of chains of (ZnO₄) and (PO₃OH) tetrahedra. The interlayer space is occupied by the Ca²⁺ cations and H₂O molecules. Batagayite is a secondary low-temperature mineral formed as a result of alteration of primary minerals such as native copper and fluorapatite. On the basis of its structural complexity calculated as 1058.257 bits/cell (taking into account contributions from H atoms), batagayite should be considered as a very complex mineral. The high complexity of batagayite is due to its high hydration state and the modular character of its structure, which contains both octahedral-tetrahedral layers and tetrahedral chains.