Dondoellite, Ca2Fe(PO4)2·2H2O, a New Mineral Species Polymorphic with Messelite, from Rapid Creek, Yukon, Canada

A new mineral species, dondoellite, ideally Ca2Fe(PO4)2·2H2O, was found in the Grizzly Bear Creek, Dawson mining district, Yukon, Canada. It is polymorphic with messelite, a member of the fairfieldite group. Dondoellite occurs as spherical aggregates (diameters up to 2 cm) of radiating bladed crystals. Associated minerals include hydroxylapatite, siderite, and quartz. No twinning or parting is observed. The mineral is colorless to pale yellow in transmitted light, is transparent with white streak, and has vitreous luster. It is brittle and has a Mohs hardness of 3½–4, with perfect cleavage on {001}. The measured and calculated densities are 3.14(5) and 3.15 g/cm3, respectively. Optically, dondoellite is biaxial (+), with α = 1.649(5), β = 1.654(5), γ = 1.672(5) (white light), 2V (meas.) = 55(2)°, 2V (calc.) = 58°. An electron probe microanalysis yields an empirical formula (based on 10 O apfu) Ca1.99(Fe0.89Mg0.13Mn0.01)Σ1.03(P1.00O4)2·2H2O, which can be simplified to Ca2(Fe2+,Mg,Mn2+)(PO4)2·2H2O. Dondoellite is triclinic, space group P, a = 5.4830(2), b = 5.7431(2), c = 13.0107(5) Å, α = 98.772(2), β = 96.209(2), γ = 108.452(2)°, V = 378.71(2) Å3, and Z = 2. The crystal structure of dondoellite is characterized by isolated FeO4(H2O)2 octahedra that are linked by corner-sharing with PO4 tetrahedra to form so-called kröhnkite-type [Fe(PO4)2(H2O)2]2– chains along [100], similar to that observed in messelite. These chains are connected to one another by large Ca2+ cations and H bonds to form layers parallel to (001). The layers are further linked together by Ca–O and H bonds. However, unlike messelite, the crystal structure of dondoellite contains two symmetrically independent PO4 tetrahedra (P1O4 and P2O4) and two distinct CaO7(H2O) polyhedra (Ca1 and Ca2). The kröhnkite-type chains in dondoellite are constructed with P1O4 tetrahedra on one side and P2O4 tetrahedra on the other. Topologically, the dondoellite structure can be considered a combination of the collinsite and messelite structures alternating along [001], thus representing a new structure type for minerals with kröhnkite-type chains. The discovery of dondoellite raises the question as to whether polymorphs of fairfieldite, Ca2Mn2+(PO4)2·2H2O, or collinsite, Ca2Mg(PO4)2·2H2O, might also be found in nature.