Abstract:
The crystal structures of brandtite [Ca2(Mn,Mg)(AsO4)2 (H2O)2, monoclinic, a 5.877(1), b 12.957(2), c 5.675(1) Å, β 108.00(3)°, V 411.0 Å3, space group P21/c, Z = 2], collinsite [Ca2(Mg,Fe) (PO4)2(H2O)2, triclinic, a 5.729(1), b 6.778(1), c 5.444(1) Å, α 97.31(3), β 108. 63(3), γ 107.25(3)°, V 189.8 Å3, space group P1, Z = 1] and fairfieldite [Ca2(Mn,Mg) (PO4)2(H2O)2, triclinic, a 5.795(1), b 6.576(1), c 5.495(1) Å, α 102.39(3), β 108.63(3), γ 90.29(3)°, V 194.0 Å3, space group P1, Z= 1] have been refined to R indices of 2.6, 1.5 and 1.7%, respectively, based on 1200, 1089 and 1137 unique [ Fo > 5σF] reflections measured with an automated four-circle single-crystal X-ray diffractometer equipped with a serial detector and a MoKα X-ray source. The brandtite, collinsite and fairfieldite structures are based on infinite [M(TO4)2(H2O)2] chains parallel to the c axis and with a repeat distance of ~5.55 Å. Interstitial Ca occurs between the chains. Although brandtite, collinsite and fairfieldite are chemically quite similar, differences in their hydrogen-bonding arrangements result in significant structural differences. The three distinct structure-types are compared, with particular emphasis on the hydrogen-bonding arrangement in each structure. Several other structural arrangements are examined, based on different arrangements of hydrogen bonds involving the structural unit [M2+(T5+O4) 2(H2O)2].
