Fluorapophyllite-(Cs) (IMA 2018-108a), ideally CsCa₄(Si₈O₂₀)F(H₂O)₈, is an apophyllite-group mineral from the moraine of the Darai-Pioz glacier, Tien-Shan, Northern Tajikistan. Associated minerals are quartz, pectolite, baratovite, aegirine, leucosphenite, pyrochlore, neptunite, fluorapophyllite-(K), and reedmergnerite. Fluorapophyllite-(Cs) is a hydrothermal mineral. It is colorless and has a vitreous luster and a white streak. Cleavage is perfect; it is brittle and has a stepped fracture. Mohs hardness is 4.5-5. D_meas. = 2.54(2) g/cm³, D_calc. = 2.513 g/cm³. Fluorapophyllite-(Cs) is unixial (+) with refractive indices (λ = 589 nm) ω = 1.540(2), ε = 1.544(2). It is non-pleochroic. Chemical analysis by electron microprobe gave SiO₂ 48.78, Al₂O₃ 0.05, CaO 22.69, Cs₂O 10.71, K₂O 1.13, Na₂O 0.04, F 1.86, H₂O_calc. 14.61, -O=F₂ -0.78, sum 99.09 wt.%; H₂O was calculated from crystal-structure analysis. The empirical formula based on 29 (O + F) apfu, H₂O = 8 pfu, is (Cs_0.75K_0.24)_Σ0.99(Ca_3.99Na_0.01)_Σ4(Si_8.01Al_0.01)_Σ8.02O_20.03F_0.97(H₂O)₈, Z = 2. The simplified formula is (Cs,K)(Ca,Na)₄(Si,Al)₈ O₂₀F(H₂O)₈. Fluorapophyllite-(Cs) is tetragonal, space group P4/mnc, a 9.060(6), c 15.741(11) Å, V 1292.10(19) ų. The crystal structure has been refined to R₁ = 4.31% based on 498 unique (F_o > 4σF) reflections. In the crystal structure of fluorapophyllite-(Cs), there is one ^[4]T site occupied solely by Si,,T-O. = 1.615 Å. SiO₄ tetrahedra link to form a (Si₈O₂₀)^8-sheet perpendicular to [001]. Between the Si-O sheets, there are two cation sites: A and B. The A site is coordinated by eight H₂O groups [O(4) site], A-O(4) = 3.152(4) Å; the A site contains Cs_0.75K_{0.24 0.01}, ideally Cs apfu. The Cs-O bond length of 3.152 Å is definitely larger than the K-O bond length of 2.966-2.971 Å in fluorapophyllite-(K), KCa₄(Si₈O₂₀)F(H₂O)₈. The ^[7]B site contains Ca_3.99Na_0.01, ideally Ca₄ apfu;<B-φ> = 2.417 Å (φ = O, F, H₂O). The Si-O sheets connect via A and B polyhedra and hydrogen bonding; two H atoms have been included in the refinement. Fluorapophyllite-(Cs) is isostructural with fluorapophyllite-(K). Fluorapophyllite-(Cs) is a Cs-analogue of fluorapophyllite-(K).