Pilanesbergite: a new rock-forming mineral occurring in nepheline syenite from the Pilanesberg Alkaline Complex, South Africa

The new mineral pilanesbergite, with the ideal formula Na 2 Ca 2 Fe 2 Ti 2 (Si 2 O 7 ) 2 O 2 F 2 , was found in a nepheline syenite, locally known as green foyaite, from the Pilanesberg Complex located in the North West Province of South Africa. Pilanesbergite occurs in green foyaite in association, and partly intergrown, with aegirine. The two minerals share an assemblage of inclusions, comprising euhedral nepheline, titanite and minor sodalite. Pilanesbergite belongs to the wohlerite group and is isomorphic with lavenite, normandite and madeiraite. It is related to these species through the homovalent chemical substitutions Mn 2 + <-> Fe 2 + and Zr 4 + <-> Ti 4 + . The empirical formula calculated on the basis of 18 anions is Na 2.00 (Ca 1.74 Na 0.26 ) sigma 2.00 (Fe 1.00 Mn 0.52 Ca 0.49 Zr 0.05 ) sigma 2.06 (Ti 1.69 Zr 0.14 Mg 0.09 Nb 0.08 ) sigma 2.00 (Si 2 O 7 ) 2.00 O 1.84 F 2.16 ( Z = 2 ). The new mineral is translucent with a brown orange colour and a brownish streak. The Mohs hardness is estimated between 5 and 6 by comparison with lavenite, and no cleavage is observed. Measured and calculated densities are D meas = 3.47 g cm - 3 and D calc = 3.40 g cm - 3 . In the thin section the pleochroism is strong, between straw yellow and orange red, while in immersion the strong pleochroism is observed between light yellow ( alpha ) and yellowish orange ( gamma ). The crystals are optically biaxial ( + ) with alpha = 1.743 ( 3 ) , beta = 1.768 ( 3 ) , gamma = 1.795 ( 5 ) and a 2 V angle close to 90 circle . The crystal structure is monoclinic ( P 2 1 / a ), with the unit-cell parameters a = 10.7811 ( 2 ) , b = 9.7836 ( 1 ) , c = 7.0348 ( 1 ) angstrom, beta = 108.072 ( 2 ) circle and V = 705.41 ( 2 ) angstrom 3 , and has been refined to R 1 = 2.06 %. The strongest lines of the powder X-ray diffraction pattern [ d , angstrom ( I , %) (h k l)] are 3.219 (60) (310), 2.851 (100) (12-2), 2.802 (51) (320), 2.743 (27) (22-2), 2.423 (19) (40-2) and 1.723 (19) (44-2). Pilanesbergite formed under relatively reducing conditions from an agpaitic nepheline syenite magma that had evolved by fractional crystallization mainly of aegirine. Further crystallization of arfvedsonite caused an increase in oxygen fugacity and a change towards higher Mn / Mn + Fe of the magma, causing a change of mineral composition from pilanesbergite towards normandite.