Cell parameters, optical properties and chemical composition in some natural sodic amphiboles

The chemical composition, optical properties, cell parameters, and modes of occurrence of six amphiboles from Mont St. Hilaire were studied and identified as: richterite, Na1.68K0.33Ca1.14(Mg4.20Fe''0.28Al0.08Feiii0.06Ti0.02Mn0.01)Si8.06O22(OH,F)2, α1.606, β1.625, γ1.633, 2Vα 67°, γλc 19°, optical pl. // (010), b = β, D 2.948; kaersutite, Ca1.73Na0.90K0.15(Mg2.30Fe''1.49Al0.40Fe'''0.06Mn0.05)Ti0.63(Si5.91Al2.09)O22(OH,F)2, α1.679, β1.698, γ1.709, 2Vα 72°, γλc 17°, optical pl.. // (010), b = β, D 3.278; arfvedsonite, Na2.41K0.33Ca0.40(Fe''2.52Fe'''0.92Mg0.70Mn0.44Ti0.17Al0.10)(Si7.41Al0.59)O22(OH,F)2, α1.687, β1.698, γ1.701, 2Vα 52°, γλc 12°, optical pl. _ (010), b=γ, D 3.322; Na2.48K0.32Ca0.35(Fe''2.33Fe'''1.09Mg0.70Mn0.45Ti0.20Al0.05)(Si7.35Al0.65)O22(OH,F)2, α1-688, β1.700, γ1.704, 2Vα 60°, γλ c 13.5° optical pl. _ (010), b=γ, D 3.498; and riebeckitic arfvedsonite, Na2.47K0.48Ca0.19(Fe'''1.45Mg1.41Mn0.84Fe''0.56Al0.19Ti0.03)Si8.00O22(OH,F)2, α1.655, β1.673, γ1.677, 2Vα 62°, γλc17°, optical pl. // (010), b=β, D 3.224; Na2.50K0.47Ca0.15(Mg1.61Fe''1.42Fe'''0.73Mn0.52Al0.17Ti0.02)Si8.03O22(OH,F)2, α1.658' β1.673, γ1.678, 2Vα 61°, γλc 17°, optical pl. // (010), b=β, D 3.273.The regression studies show that in natural sodic amphiboles both refractive indices and density vary directly with the variation of the ratio (Fe" + Fe'' +Mn+Ti)/total Y.The length of the a-axis is most significantly affected by either the smallest or the largest cations in M sites. It varies inversely with the variation of the Alvi content in riebeckite (including glaucophane) and, is independent of the Alvi content in arfvedsonite (including eckermannite). It varies also inversely with the variation of Fe'' +Ti and F/(F+OH) in arfvedsonite, but varies directly with the same chemical parameters in riebeckite. This difference is, probably, due to the different substitutions in the two minerals. The length of the a-axis increases also with increasing Na+Ca+K content, and the rate of change is gradually reduced and is levelled off in the high Na+Ca+K range.The B-angle is affected by all the elements in the M-site. It varies directly with the Ca+K content and, varies inversely with the Alvi+Ti+Fe content in both riebeckite and arfvedsonite.The length of the b-axis is directly correlated to the Fe" content, and also affected by the Alvi content and the ratio, F/(F+OH), in the same manner as the a-axis. It varies also with the variation of the Na+Ca+K content; directly in riebeckite but inversely in arfvedsonite. This difference is interpreted as a result of the A site occupancy.The length of the c-axis is affected by the Fe'' (or Mg) content, and the ratio, Aliv/total Al. Cell volume is most significantly affected by Alvi, the smallest, and Na+Ca+K, the largest cations.Richterite, riebeckite and arfvedsonite are serially related in the substitution of 4Mg = Fe'" Fe"MnNa . Arfvedsonite usually has higher F content than riebeckite, and the sum of F and OH is generally greater than 2.0 in arfvedsonite, close to 2.0 in riebeckite, and less than 2.0 in richterite. The substitution of Fe'" = Alvi is very important in riebeckite and less so in other sodic amphiboles. The high correlation of Ca with Alvi +Ti+Fe''' implies that the substitutions, CaR" = (Na,K)R''', and CaR"Aliv = R''' Si may prevail in sodic amphiboles. Na+K content varies directly with the variation of F