Is foliation a physical, or chemical property? Why

That first answer is a bit hefty!


So, simple answer.

A foliation in a rock is simply a layering caused by the alignment of minerals. Like this:

_ _ _ _ _ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _ _ _ _ _

rather than random alignment, like this:

_-//---___|||\\\\|-_

If that makes sense... Not the easiest thing to draw here!

Pressure makes all the long, flat minerals align on a plane.

So, in that sense, it's a Physical property.

In high grade metamorphic rocks, you can have he rock foliated by chemistry, as well. All the light, felsic minerals and the darker, mafic minerals in bands as well. So, you could argue it's partly chemical as well.

The chemistry of the rock controlling the physical properties and appearances?

Foliation is usually formed by the preferred orientation of minerals within a rock. It is a physical property that is controlled by the chemical composition of the minerals.

Foliation is common to rocks affected by regional metamorphic compression typical of orogenic belts. Rocks exhibiting foliation include the standard sequence formed by the prograde metamorphism of mudrocks; slate, phyllite, schist and gneiss.

Typically this is a result of some physical force, and its effect upon the growth of minerals. The planar fabric of a foliation typically forms at right angles to the minimum principal strain direction. In sheared zones, however, planar fabric within a rock may not be directly perpendicular to the principal stress direction due to rotation, mass transport and shortening.

Foliation may be formed by realignment of micas and clays via physical rotation of the minerals within the rock. Often this foliation is associated with diagenetic metamorphism and low-grade burial metamorphism. Foliation may parallel original sedimentary bedding, but more often is oriented at some angle to it.