Crystal chemical characteristics have been studied for neptunite from alkaline granites of the Khan Bogdo massif (Southern Mongolia). The mineral occurs to be the practically clear Fe-variety of its species. Refinement of its crystal structure was carried out in two space groups: centric C2/c and acentric Cc. It is indicated that correct space group is the acentric Cc due to ordering of octahedral cations Ti and Fe. The structure was refined to R ₁ = 0.025 (wR ₂ = 0.057) for 4889 reflections with |Fo| ≥ 4σF. Unit cell parameters: a = 16.4542(7), b = 12.5115(4), c = 9.9980(4) Å, β = 115.542(5)°, V = 1857.10(15) Å ³ , Z = 4. Ti octahedra show strong distortions: one bond Ti-O 1.75 Å, another bond 2.20 Å, and four Ti-O bonds around 2.0 Å. Fe octahedra have a more regular shape with an average bond length Fe-O 2.13-2.14 Å. Empirical formula calculated on the basis of chemical analysis is (K _0.86 Cs _0.02 Zn _0.02 Ca _0.01 ) _S0.91 Na _1.90 Li _0.82 (Fe _1.54 ²⁺ Fe _0.22 ³⁺ Mn _0.16 Mn _0.04 ) _S1.96 Ti _2.1 (Si _7.76 Al _0.25 )O ₂₄ . Structural formula is K _0.97 Na _1.90 (Li _0.94 Na _0.06 ) _S1.00 (Fe _1.80 Mn _0.15 Mg _0.05 ) _S2.00 Ti ₂ Si ₈ O ₂₂ (O,OH) ₂ . Idealized formula is KNa ₂ LiFe ₂ Ti ₂ Si ₈ O ₂₄