Charleshatchettite, CaNb4O10(OH)2·8H2O, is a new mineral related to franconite and hochelagaite, discovered on a fracture surface of a nepheline syenite at Mont Saint-Hilaire, Québec, Canada. The mineral occurs in white globules (~0.15–0.20 mm in diameter) composed of radiating crystals with individual crystals having average dimensions of ~0.002 × 0.010 × 0.040 mm. Crystals are euhedral, bladed (flattened on ), and are transparent to translucent. The mineral is associated with albite, quartz, muscovite, pyrrhotite, pyrite, ancylite-(Ce), and siderite. Charleshatchettite is inferred to be biaxial (–) with α′ = ~1.72(2) and γ′ = ~1.82(2). Data from chemical analyses (SEM-EDS, n = 8): CaO 7.96 (7.04–8.63), MgO 0.24 (0.08–0.78), Al2O3 0.13 (b.d.–0.49), SiO2 1.04 (0.49–1.88), TiO2 3.64 (2.45–5.05), Nb2O5 68.07 (64.83–71.01), and H2O (calc) 22.96, total 104.04 wt% gives the average empirical formula: (Ca1.00Mg0.04)∑1.04(Nb3.62Ti0.32Si0.12Al0.02)∑4.08O10(OH)2·8H2O (based on 20 anions). This is similar to that of hochelagaite (CaNb4O11·nH2O), although the two are readily distinguished by their powder X-ray diffraction patterns. Results from single-crystal X-ray diffraction analysis give a = 21.151(4), b = 6.496(2), c = 12.714(3) Å, and β = 103.958(3)°, space group C2/c (no. 15). The crystal structure, refined to R = 5.64%, contains 1 Ca site, 2 distorted octahedral Nb sites, and 10 O sites. It consists of clusters of four edge-sharing Nb(O,OH)6 octahedra, linked through shared corners to adjacent clusters, forming layers of Nb(O,OH)6 octahedra. These alternate along  with layers composed of Ca(H2O)8 polyhedra, the two being linked together by H-bonding. Charleshatchettite is a late-stage mineral, interpreted to have developed through the interaction of low-T (<150 °C) aqueous fluids with an alkali-, Nb-rich precursor under slightly reducing conditions and a highly alkaline pH. The precursor mineral(s) is unknown but is considered to have been Nb-dominant, relatively unstable under slightly reducing as well as alkaline conditions, and likely itself would have been a product of near-complete Nb/Ta fractionation due to the paucity of Ta in charleshatchettite. Charleshatchettite is crystallochemically related to Sandia Octahedral Molecular Sieves [SOMS; Na2Nb2–xMxO6–x(OH)x·H2O with M = Ti, Zr, Hf], a group of synthetic compounds with strong ion exchange capabilities.