Delindeite and lourenswalsite, two new titanosilicates from the Magnet Cove region, Arkansas

These minerals occur as microscopic crystals in miarolitic cavities in nepheline syenite in the Diamond Jo quarry, Hot Spring County, Arkansas. Delindeite forms light pinkish grey flake-like crystallites with a resinous, pearly lustre; biaxial (+) with average refr. ind. approx 1.813; D 3.3 g/cm³; a 21.617, b 6.816, c 5.383 A, beta 94.03o, space group C2/m; strongest X-ray lines 10.80(100), 2.888(31), 3.083(28), 3.54(24), 2.806(20) A. The crystals are submicroscopically twinned on (100) and also give additional continuous diffraction streaks parallel a*, which double the b and c axes. The approximate formula derived from electron and ion probe analyses is (Na,K)_2.7(Ba,Ca)₄(Ti,Fe,Al)₆Si₈O₂₆(OH)₁₄, with Na>K, Ba>>Ca, Ti>>Fe,Al; Z = 1. Lourenswalsite occurs as very thin hexagonal plates in silver grey to light brownish grey rosettes; biaxial (-) with very low 2V; alpha 1.815, beta approx gamma 1.840; D 3.17 g/cm³; hexagonal with a 5.244, c 20.49 A; strongest X-ray powder lines are 2.608(100), 1.5145(80), 1.3111(25), 10.22(20), 3.93(20) A. Microprobe analyses give an idealized formula (K,Ba)₂(Ti,Mg,Ca,Fe)₄(Si,Al,Fe)₆O₁₄(OH)₁₂, with K>Ba, Ti>>(Mg,Ca,Fe), Si>Al>Fe; Z = 1. Both minerals formed under oxidizing weathering conditions and the iron is assumed to be in the Fe³⁺ state. The names are in honour of Henry de Linde, amateur mineralogist and owner of the quarry, and Lourena Wals one of the foremost mineral collectors in Belgium.-R.A.H.