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Ferro-ferri-katophorite, a new clinoamphibole from the silicocarbonatite dykes in Sierra de Maz, La Rioja, Argentina

2023; Cambridge University Press; Volume: 87; Issue: 2 Linguagem: Inglês

10.1180/mgm.2023.2

1471-8022

Fernando Colombo, Jordi Rius, Elı́es Molins, Héctor Biglia, Miguel Ángel Galliski, María Florencia Márquez‐Zavalía, Edgardo G. Baldo, Agustín Kriscautzky,

Clay minerals and soil interactions

Abstract Ferro-ferri-katophorite (IMA2016–008), ideally Na(NaCa)(Fe 2+ 4 Fe 3+ )(Si 7 Al)O 22 (OH) 2 , was found as xenocrysts up to 3 cm long and replacement rims around aegirine–augite in silicocarbonatite dykes cropping out in the Sierra de Maz, La Rioja province, NW Argentina. Ferro-ferri-katophorite is black and has vitreous lustre and a pale green streak. The new mineral is brittle, with perfect {110} cleavage and has a Mohs hardness of 6. The measured density is 3.32(1) g/cm 3 . In plane-polarised light it is strongly pleochroic, X = light greenish brown, Y = dark greyish brown and Z = dark greyish olive green. Absorption (very strong) is Z > Y > X. The orientation is: Z ∥ b , and X forms a small angle with [001]. Ferro-ferri-katophorite is biaxial (–), with α = 1.688(3), β = 1.697(3), γ = 1.698(3) and 2V(calc) = 36.7°. It is monoclinic, space group C 2/ m , a = 9.8270(7), b = 18.0300(8), c = 5.316(4) Å, β = 104.626(4)°, V = 911.4(6) Å 3 and Z = 2. The strongest five lines in the powder X-ray diffraction pattern [ d in Å ( I )( hkl )] are: 8.416(100)(110), 3.135(50)(310), 2.815(26)(330), 2.720(18)(151) and 1.4422(15)( $\bar{6}$ 61). The chemical composition is SiO 2 43.08, TiO 2 2.76, ZrO 2 0.15, Al 2 O 3 8.76, V 2 O 3 0.07, Fe 2 O 3 9.28, FeO 13.85, MnO 0.43, MgO 6.88, CaO 6.58, ZnO 0.06, Na 2 O 5.55, K 2 O 1.18, Cl 0.01, H 2 O calc 1.36, total 99.95 wt.%. The formula unit (confirmed by single-crystal structural analysis) is (Na 0.74 K 0.23 ) Σ0.97 (Ca 1.08 Na 0.91 Mn 0.01 ) Σ2.00 (Fe 2+ 1.78 Mg 1.57 Fe 3+ 1.07 Ti 4+ 0.32 Al 0.19 Mn 2+ 0.04 Zr 0.01 V 3+ 0.01 Zn 0.01 ) Σ5.00 (Si 6.61 Al 1.39 ) Σ8.00 O 22 (OH 1.59 O 0.61 ) Σ2.00 . Aluminium is strongly ordered at the T (1) site. Ferro-ferri-katophorite is the 9 th species carrying the katophorite root name and is related to katophorite by the Fe 2+ + Fe 3+ → Mg 2+ + Al 3+ substitution. Type material was deposited at the Museo de Mineralogía “Stelzner”, Universidad Nacional de Córdoba, Argentina, under catalogue number MS003341.