Ferrierite from Monbetsu, Hokkaido, Japan

Abstract

The thirteen microprobe analyses of ferrierite from Monbetsu are differentiated into two groups due to Na₂O contents. The averages of ten Na2O-free and three Na₂O-bearing analyses are, respectively, SiO₂ 65.78, 66.87; Al₂O₃ 12.89, 12.86; Fe₂O₃–, 0.16; MgO 3.32, 3.30; CaO 1.35, 1.26; BaO 0.78, 0.78; Na₂O–, 0.47; K₂O 0.86, 0.86 wt.%. The absence of intermediate analysis presents an example of compositional variation of ferrierite in a single specimen, although the difference is approximately given as the simple addition of Na₂O contents and the concomitant increase of SiO₂ content to the latter. The H₂O content measured by Karl Fischer method is 14.29 wt. %, giving totals 99.27 and 100.85 wt. %, respectively. Their empirical formulae (basis: total O = 72 in the anhydrous part) are, K_0.49(Ca_0.65Ba_0.14)_Σ0.79Mg_2.21Al_6.77Si_29.31O₇₂·21.23H₂O and (K_0.48Na_0.40)_Σ0.88(Ca_0.59Ba_0.13)_Σ0.72Mg_2.16(Al_6.65Fe_0.05)_Σ6.70Si_29.31O₇₂·20.89H₂O, respectively. The X-ray powder pattern is indexed on a body-centered orthorhombic cell with a=19.239, b=14.142, c=7.513 Å. The relationships a vs Si and a vs Mg contents agree with the existing data. It occurs as minute blades elongated to c-axis and flattened to a-axis grown on pyrite and magnesian siderite perpendicular to the walls of open fractures in an altered baslt intrusive, in which closed fractures with smectite and pyrite without ferrierite had been formed prior to open fractures. Wider ferrierite-bearing open fractures include tabular calcite, which forms independent veinlets in the quarry, this being more common therein. The basalt body belongs to a member of Miocene Kohnomai Formation, in which many hydrothermal gold-silver veins of the Kohnomai mine were formed, and the ferrierite locality is included within th area of metallic mineralization. The geneses of ferrierite and the direct associates are considered. Pyrite and magnesian siderite are the reaction products of incorporated H₂S and CO₂ with a solution containing Mg²⁺ and Fe²⁺ derived from the alteration of mafic minerals in the basalt by ascending hydrothermal solution. Ferrierite is located between siderite and calcite in sequence of formation, alkaline or neutral condition being needed, besides residual Mg and high silica nature of the solution.