2005 年 12 巻 Supplement1 号 p. 13-20
Illite-smectite (I-S) mixed-layer minerals (MLMs) are ubiquitous in low-temperature geological environments: weathering, diagenesis, low-grade metamorphism, hydrothermal alteration, and even contact metamorphism. We have studied I-S MLMs in hydrothermally altered volcanic rocks at Kakkonda, and the integration of the results provides a clue to longstanding controversy about the crystal chemical nature of the component layers, the formation mechanism, and the controlling factors. The Kakkonda I-S MLMs consist of basically the two component layers with few vermiculite layers. Changes in the I/5 ratio and the Reichweite (R) parameter occur with a continuous manner as a function of temperature from 150-200°C. The actual stacking of layers in each I-S crystallite is represented by stacking of Mn sub-units rather than individual I and S layers, where Mn denotes a sub-unit that is composed of central illitic interlayers with the number of n sandwiched by two half smectitic interlayers at the outermost surface. The number of n increases with temperature in turn from n=0 for pure smectite, n=1 for R1 I-S, to n>2 for R3 I-S. According to the present model, the formation of I-S resulted from the precipitation of various types of Mn without the precursor smectite, and it is neither the layer-by-layer transformation nor the dissolution-crystallization. The chemistry of reacting fl uid affected the composition of product I-S and resultantly the formation temperature under the conditions of constant composition of precursor rocks and constant water/rock ratio.
