THE TANCO PEGMATITE AT BERNIC LAKE, SOUTHEASTERN MANITOBA. XV. ERCITITE, Na Mn3+ PO4 (OH) (H2O)2, A NEW PHOSPHATE MINERAL SPECIES

Abstract

Ercitite, ideally Na Mn 3+ (PO 4 ) (OH) (H 2 O) 2 , is a new mineral species from the Tanco pegmatite, southeastern Manitoba. Ercitite forms 200–400 μm irregular aggregates of lath-like crystals in fan-like divergent sprays. The mineral occurs in a 5-cm nodule embedded in a quartz + spodumene pseudomorph after petalite in the upper intermediate zone (5) of the pegmatite. The outer rind of the nodule contains pink rhodochrosite, green and blue apatite, and minor yellow sphalerite, and the inner region is comprised of orange-brown lithiophilite. The core of the nodule consists of an assemblage of altered lithiophilite, locally stained red (hematite?), colorless lithiophosphate and dark brown-black clots of ercitite on collinsite, fairfieldite, whitlockite and two unidentified phosphates; the new species and associated minerals are alteration products of lithiophilite. Individual laths of ercitite are light brown with a beige streak and a vitreous luster. Laths are generally ≤20 μm across and ≤200 μm long, elongate along [101] and flattened on {101}. Ercitite is brittle, with an irregular fracture on ends of laths, and has two very good orthogonal cleavages, parallel to {101} and {010}. Mohs hardness is 3–4, and the calculated density is 2.75 g/cm 3 . Ercitite is biaxial positive, with α 1.699(2), β 1.715(5), γ 1.737(5), 2 V (obs.) = 86°, 2 V (calc.) = 82°, strongly pleochroic with X yellowish green, Y yellowish brown, Z very dark brown, with absorption Z ≫ Y > X and X = b , Y ^ c = 34° (in β acute), Z ^ a = 53° (in β obtuse). Ercitite is monoclinic, space group P 2 1 / n , a 5.362(5), b 19.89(1), c 5.362(5) A, β 108.97(8)°, V 540.8(6) A 3 , Z = 4. The seven strongest lines in the X-ray powder-diffraction pattern [ d (in A)( I) ( h k l )] are: 9.90(10)(0 2 0), 2.644(8)(1 4 1), 3.273(6)(1 4 1), 3.126(6)(1 5 0, 0 5 1), 4.92(5)(0 1 1, 1 1 0), 2.542(4)(2 0 0, 0 0 2), 2.376(4)(1 7 1). Chemical analysis by electron microprobe gave P 2 O 5 32.37, Al 2 O 3 0.34, Mn 2 O 3 18.81, Fe 2 O 3 16.51, ZnO 0.08, MgO 0.12, CaO 1.09, Na 2 O 12.44, H 2 O (calc.) 20.44, sum 102.20 wt.%, where the valence states of Mn and Fe, and the amount of H 2 O, were determined by crystal-structure analysis. The resulting empirical formula on the basis of seven anions (including OH and 2 H 2 O) is (Na 0.89 Ca 0.04 ) ∑0.93 (Mn 3+ 0.53 Fe 3+ 0.46 Al 0.01 ) ∑1.00 (PO 4 ) 1.01 (OH) (H 2 O) 2 . Ercitite is structurally similar to bermanite, Mn 2+ Mn 3+ 2 (PO 4 ) 2 (OH) 2 (H 2 O) 4 , and is related by the substitution 2Na → Mn 2+ + □ (vacancy).