Mineralogy of White Venules and Dark Material in Fracture Plans in Sodalitites with Blue Colour of the Itaju do Colônia Stock, Bahia State

Abstract

The Stock Itajú of Colônia is an ellipsoid body, with approximately 2 km², located in the South Bahia Alkaline Province. Currently this stock is the largest reserve for brazilian sodalite-syenite and sodalitites of intense blue color which is exploited as dimenson stones and jewelery. The presence of white venules is an undesirable aspect in these rocks, depreciating its market value. The very fine grained and friable aspect of these white venules were the relevant factors in choosing the procedure called “Powder Method”, of the technique of X-ray diffraction to identify mineral constituents venules, as well as the dark, affanitic and porous material present in these blue rocks. The minerals identified were Tetranatrolite [Na₂(Al₂Si₃O₁₀)2(H₂O)], Paranatrolite [Na₂(Al₂Si₃O₁₀)3(H₂O)], Gonnardite-[Na₂CaAl₄Si₆O₂₀], Halloysite-[Al₂Si₂O₅ (OH)₄], Fluorannite [KFe₃AlSi₃O₁₀F₂], Illite [(K,H₂O) (Al,Mg,Fe)₂ (Si, Al)₄ O₁₀(OH) 2(H₂O)] and Glauconite [(K,Na) (Fe³⁺,Al, Mg)₂ (Si,Al)₄ O₁₀(OH)₂]. This mineralogy, present in the white venules and dark material in the fractures plains, is a low temperature (<300 °C) paragenesis resulting from the precipitation of fluids that interacted with the early alkaline minerals of these rocks (nepheline and sodalite).