Abstract: Retzian-(La), ideally, Mn₂La(AsO₄)(OH)₄, is a new rare-earth analogue of retzian and retzian-(Nd). It was found associated with willemite, calcite, and franklinite, in the Stealing Hill Mine, Ogdensburg, Sussex County, New Jersey, USA. Retzian-(La) is orthorhombic, space group Pban, with a = 5.670(7), b = 12.01(1), and c = 4.869(8) Å. The strongest lines in the X-ray powder diffraction pattern are (d,I/I₀,hkl): 3.51 80 111, 2.715 100 131, 1.848 50 061, 1.456 40. Microprobe analysis yielded MnO 25.2, MgO 3.7, ZnO 1.2, As₂O₅ 26.5, La₂O₃ 10.6, Ce₂O₃ 8.8, Nd₂O₃ 8.2, Pr₂O₃ 5.1, Sm₂O₃ 2.2, Y₂O₃ 2.5, H₂O [7.7], sum = 101.7 wt. %, corresponding to an La-dominant member of the retzian series.

Retzian-(La) forms 0.5 mm euhedral crystals composed of the forms {001}, {010}, {110}, and {150}; the crystals have pseudohexagonal habit. It is reddish-brown, with vitreous luster, no cleavage, density > 4.2 g/cm³. Optically, it is biaxial, positive, 2V = 82° with α = 1.766, β = 1.773, and γ = 1.788. Dispersion is strong, r > v; pleochroism is very weak; orientation is X = c, Y= b, Z = a; absorption is Z > Y > X.