北海道岩内郡小沢産濁沸石凝灰岩

抄録

Two different occurrences of laumontite are found at Kozawa, Iwanai-district, Hokkaido. One is the altered materials of volcanic glasses of Miocene green tuffs. Another is its concentration on the surface of the specimen. The latter laumontite is the pure material, and its structural formulae as follows,
(Ca_3.128Mg_0.055Na_0.622K_0.183)(Al_6.95 Fe^III_0.073) Si_16.939.O₄₈13.260 H₂O.
The chemical composition agrees well with the druze-type laumontites of the igneous rocks, but Na₂O is slightly abundant. The cation exchange capacity by Shollenberger's method is less than 10 m. e./100 g. It shows that his method is not suitable to measure the C. E. C. of laumontite.
For the purpose of the estimation of the quantity of laumontite involved in the specimen, X-ray intensity method and chemical composition method have been applied. The former result is 19% and the latter 25.05%.
Clinoptilolite-tuff and mordenite-tuff were found near the laumontite-tuffs. No regulations were recognized in their distribution. It is probable that the genesis of laumontite is not by the diagenesis or the low-grade metamorphism, but by the local metamorphism (alteration) by the ore-solution and the like.