Arfvedsonite

ARFVEDSONITE (in honour of the Swedish chemist J.A.Arfwedson * EN: arfvedsonite, soda hornblende; DE: Arfvedsonit; FR: arfvedsonite; ES: arfvedsonita; RU: арфведсонит) is the rock-forming mineral, monoclinic alkaline amphibole, Na2,5Ca0,5(Fe2+,Mg,Fe3+,Al)5[Al0,5Si7,5O22](OH,F)2. The ratios of Mg/Fe2+ and Fe3+/Al vary widely (at Mg >> Fe2+ - magnesio-arfvedsonite); Ca may be practically absent. The impurities are: K, Mn, Ti. There are known the varieties of the intermediate composition between the arfvedsonite and riebeckite (osannite). It forms prismatic, often greatly elongated (up to columnar) crystals, radiant-columnar aggregates, elongated grains of the irregular shape. The colour is black, sometimes with the bluish tint. Unlike aegirite, the colour of the streak is light-bluish-gray or greenish-gray. The lustre is vitreous. The cleavage is along the prism. The hardness is 5.5-6. The density is 3400-3500 kilograms per cubic metre. It is widespread within many massifs of the nephelinic syenites and their pegmatites, within certain alkaline granites, metasomatites and carbonatites.