造岩鉱物の格子定数値を利用した火山灰層の対比の試み

抄録

A new method on the correlation of tephras using the lattice parameters of the rock-forming minerals was examined. Each rock-forming mineral was separated from 172 specimens which were collected from various places of Japan. The lattice constants of titanomagnetites, orthopyroxenes, clinopyroxenes, hornblendes and cummingtonites, 2θ₁₀₄ values of hemoilmenites, d₁₃₀ values of olivines and ⊿2θ=2θ₁₃₁-2θ₁₃₁ values of plagioclases were measured using powder X-ray diffractometer. The lattice constants of titanomagnetites of 166 specimens varied widely from 8.3912(11) to 8.4327(8)Å and the wide distribution of their occurrence was found. The 2θ₁₀₄ values of hemoilmenites of 94 specimens were between 32.68 and 32.91°. The d₁₃₀ values of olivines of 57 specimens ranged from 2.7742 to 2.7846Å and their distribution was fairly narrow. The ⊿2θ values of plagioclases of 139 specimens were largely centralized between 1.90 and 2.05°. The lattice constants of b axis of orthopyroxenes of 127 specimens varied widely from 8.868(1) to 8.946(2)Å and those of clinopyroxenes of 97 specimens varied from 8.909(1) to 8.944(2)Å. The lattice constants of a and b axes of hornblendes of 57 specimens changed from 9.793(3) to 9.852(2)Å, and from 18.059(4) to 18.113(6)Å, respectively. Cummingtonites were found only in 8 specimens among 144 specimens. From the above mineralogical examinations, useful minerals for the correlation of tephras were considered to be (ferromagnetic minerals)＞(amphiboles)＞(pyroxenes)＞(feldspar)＞(olivine) in order. Correlations of some marker tephras were tried using this method. Almost all of marker tephras were able to be easily distinguished from each other using these data. It was concluded that this method was effective to correlate tephras and has advantages to rapidly and precisely obtain various data to correlate tephras compared with previous correlation methods.