The crystal structure of ‘Ca-hingganite-(Y)', triclinic P1, a 9.863(4), b 7.602(3), c 4.762(2) Å, α 90.002(15), β 90.073(7), γ 90.020(5)°, V 357.1(5) Å3, Z = 1, has been refined to an R1 index of 2.28% for 4186 observed (|Fo| > 4σF) reflections. Significant observed (|Fo| > 20–50 σF) reflections violate the presence of a 21-screw axis and an a-glide plane, negating the space group P21/a previously found for minerals of the gadolinite supergroup. Averaging of the X-ray data in Laue groups 2/m and forumla gives the agreement indices 2/m (2.90%) and forumla (2.61%). The internal agreement index from averaging of identical reflections collected at multiple positions along the diffraction vector (Rpsi) is 2.12%, where 13,801 reflections were collected, 4204 are unique for P1 symmetry, and Rpsi is based on a mean data-redundancy factor of >3. The |E2 – 1| value of 0.783 is in agreement with P1 as the correct space group. The general formula for the gadolinite supergroup is A2MQ2T2O8φ2 (Z = 2), which we have expanded to 20 anions (Z = 1) to show the A-site cation ordering present in ‘Ca-hingganite-(Y)'. Ca and Y(Ln) are ordered over four A sites: Y(Ln) is dominant at A1 [= 0.90 Y + 0.07 Er + 0.03 Ca], Ca is dominant at A2 [= 0.92 Ca + 0.08 Y], and there is near-equal Ca and Y(Ln) at A3 [= 0.55 Ca + 0.42 Y + 0.03 Er] and A4 [= 0.50 Ca + 0.47 Y + 0.03 Er]. The resulting site-specific empirical formula is A(Ca2.021Y1.811Ln0.133)Σ3.965M(□1.191Fe0.794Mn0.009Mg0.006)Σ2Q(Be2.24B1.58Si0.224)Σ4.044TSi4O16φ[(OH)2.382O1.618]Σ4. The dominant constituent at the M site is vacancy, and Ca does not occur at the M site. Significant B and Si are assigned to the Be-dominant Q sites, and the T sites are occupied by Si. The simplified formula for ‘Ca-hingganite-(Y)' (Z = 1) is A[(Y,Ln)Ca(Y,Ca,Ln)2]M(□,Fe)2Q(Be,B,Si)4TSi4O16[(OH),O]4.

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