Rock Fabric in Fine-grained Homogeneous Rocks
For homogeneous, fine-grained rocks, the third digit in the Ferm code represents grain alignment. Alignment in fine-grained rocks ranges from highly parallel to highly irregular. This property can be observed on the side of the core, but the best way to see it is to break the core and inspect the butt end of the specimen. Four degrees of mineral alignment are recognized:
- XX4 Massive: grains are highly aligned, parallel to bedding
- XX5 Massive-churned: slightly irregular grain alignment
- XX6 Churned: distinctly irregular grain alignment
- XX7 3X8 Bioturbated: high degree of irregular alignment, often accompanied by slickenslides
Upper right: Massive fabric (XX4) showing smooth break parallel to bedding.
Upper left: Massive churned fabric (XX5) showing very slight irregularities to the fracture.
Lower left: Churned fabric (XX6) showing distinctly irregular fracture on the end of the core.
Lower right: Bioturbated (root-penetrated) fabric (XX7) showing highly irregular fabric accompanied by slickenside structures.
Bioturbated fabric can result from either root penetration or animal burrowing. In homogeneous, fine-grained rocks, it is difficult to discern burrowing due to the absence of distinct burrow fillings of sand or silt. Root structures are often lined with organic material (coal) or may be filled with siderite, but this can be the case with burrows as well. In rocks where it is difficult to distinguish roots from burrows, or both are present, the rock should be classified as churned (XX6). For more information go to the Biogenic Structures page for examples.
Fabric and rock strengthFabric is a very important indicator of rock strength and durability in fine-grained rocks. Massive varieties have high strength perpendicular to the long axis of the clays (usually vertical), but poor strength parallel to the long axis. The more fabric disorientation that is present, the weaker the rock tends to be and the more susceptible it is to moisture.
To learn more about weaknesses caused by churned and root-penetrated clays in mine roofs go to Weakly-bedded shale roofs for cores that exhibit shale and clay (but no coal) above the mined coal, and Coal rider roofs for core with shale, clay and coal rider in the expected roof rock above the mined coal.