Abstract

The breaking characteristics of a mudrock can be represented on a triangular diagram with massive, flaky-fissile, and flaggy-fissile as end members. Fissility in shales is usually associated with a parallel arrangement of the micaceous clay particles and nonfissility with a random arrangement. Experiments and observations indicate that the clay minerals attain a parallel arrangement by gravity settling or by flocculation and compaction, unless the particles are adsorbed on irregularly shaped sesquioxide or silica particles or grow randomly in a gel. The nature of the cementing agent determines whether a shale will be flaky or flaggy. If the cementing agent can hold the material in a large slab, the shale will be flaggy. If the amount or the tenacity of the cementing agent is small, the shale will be flaky. Cementing agents other than organic matter tend to hinder cleavage parallel to the clay particles causing a decrease in fissility and an increase in massiveness. Moderate weathering increases the fissility of a shale. In general the type of fissility does not correlate with the type of clay minerals present in a random collection of mudrocks.

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