Silicocarnotite, Ca5 [(SiO4)(PO4)](PO4), a new ,,old'' mineral from the Negev Desert, Israel, and the ternesite–silicocarnotite solid solution: indicators of high-temperature alteration of pyrometamorphic rocks of the Hatrurim Complex, Southern Levant

Abstract

The new mineral silicocarnotite, Ca 5 [(SiO 4 )(PO 4 )](PO) 4 ( Pnma ; a = 6.72230(1), b = 15.4481(2), c = 10.0847(2) A; V = 1047.37(2) A 3 , Z = 4), has been discovered in pyrometamorphic gehlenite-bearing rocks of the Hatrurim Complex, Negev Desert, Israel. The name “silicocarnotite” has been used for the synthetic phase Ca 5 [(SiO 4 )(PO 4 )](PO) 4 for more than 100 years. The holotype specimen is a gehlenite–fluorapatite rock with minor andradite and pseudowollastonite. Silicocarnotite is colourless with a white streak and a vitreous lustre. The microhardness is VHN 50 = 523–552 kg m −2 , mean = 537 kg m −2 . Mohs hardness is about 5. Cleavage and parting are not observed. The calculated density is 3.06 g cm −3 . Silicocarnotite is biaxial (+), with α = 1.618(2), β = 1.621(2), γ = 1.628(2) (589 nm), 2V (meas.) = 75(5)°, 2V Z (calc.) = 67°; dispersion is medium, r > v; the orientation is X || b , Y || a , Z || c ; the mineral is nonpleochroic. The empirical formula of holotype silicocarnotite is Ca 5.011 Sr 0.006 P 1.774 Si 1.106 S 6+ 0.113 O 12 . The strongest diffraction lines are [ d hkl , ( I )]: 2.815 (100), 2.596 (62), 2.575 (50), 3.285 (48), 3.903 (40), 3.007 (39), 3.176 (36), 1.746 (29), 3.082 (29). Minerals of the silicocarnotite–ternesite solid solution, Ca 5 [(SiO) 4 (PO 4 )](PO) 4 –Ca 5 (SiO 4 ) 2 SO 4 , occur in larnite- and gehlenite-bearing pyrometamorphic rocks of the Hatrurim Complex, distributed in the Dead Sea rift area on the territories of Israel, Palestine and Jordan. New data on morphology, composition, structure, mineral associations, mechanisms and conditions of growth of the ternesite–silicocarnotite solid-solution series are presented. Large ternesite porphyroblasts (metacrysts) in fine-grained larnite–ye’elimite matrix are the most striking evidence for high-temperature alterations of early “clinker minerals”. Silicocarnotite–ternesite grains grew as a result of reactions between primary pyrometamorphic minerals (larnite, flamite, fluorellestadite–fluorapatite) with sulphate-bearing melts, which are side-products of the combustion processes during the pyrometamorphism.