Geology, Chapter 8.3

Conditions that produce marble

Nonfoliated metamorphic rocks typically form in metamorphic environments where compressional stress is minimal and when the parent rock is composed of minerals that develop equidimensional crystals rather than flat or tabular-shaped crystals. For example, when a fine-grained limestone, made of calcite, is metamorphosed in an environment where differential stress is absent, the small calcite grains in limestone recrystallize to form larger uniform-shaped crystals. The resulting metamorphic rock, marble, consists of intergrown calcite crystals that lack banding and are similar in appearance to the crystals in coarse-grained igneous rocks.

porphyroblastic textures

develop in a wide range of rock types when minerals in the parent rock recrystallize to form new minerals. During recrystallization, some metamorphic minerals, such as garnet, tend to develop a small number of very large crystals. By contrast, minerals such as muscovite and biotite typically form a large number of smaller grains. Thus, recrystallization produces relatively common metamorphic rocks that contain large crystals, porphyroblasts, of garnet embedded in a finer-grained matrix of biotite and muscovite Figure 8.12

Look at figure 8.11.

gneissic texture

examples of foliation

include the parallel alignment of platy minerals through rotation, recrystallization, and flattening of mineral grains or pebbles.

Three types of foliation

1. rock, slaty, cleavage 2. schistosity 3. gneissic texture.

foliation

A rock that exhibits a planar, nearly flat, preferred orientation of its mineral grains or crystals is said to possess foliation.

Schistosity

At higher temperatures and pressures, the minute mica and chlorite flakes in slate begin to recrystallize into larger muscovite and biotite crystals. When these platy crystals are large enough to be discernible with the unaided eye, they exhibit planar or layered structures called schistosity. Rocks that have this type of foliation are termed schist. In addition to containing platy minerals, schist may contain deformed quartz and feldspar crystals that appear flattened or shaped like a lens embedded among the mica grains.

Why does slate split across bedding surfaces?

Because slate typically forms during the low-grade metamorphism of shale, evidence of the original sedimentary bedding surfaces is often preserved. However, as Figure 8.10C illustrates, the orientation of slate's cleavage usually develops at an angle to the sedimentary beds. Thus, unlike shale, which splits along bedding planes, slate usually splits across bedding surfaces.

rotation of mineral grains

Differential stress causes elongated mineral crystals such as amphiboles and platy mineral crystals such as micas to recrystallize perpendicular to the plane of greatest force. This results in the layering exhibited by some metamorphic rocks.

gneissic texture

During high-grade metamorphism, ion migration can result in the segregation of minerals, as shown in Figure 8.11. Notice that the dark biotite and amphibole crystals and light silicate minerals, quartz and feldspar, have separated, giving the rock a banded appearance called gneissic texture, or gneissic banding. Metamorphic rocks with this texture are called gneiss, pronounced "nice". Although they are foliated, gneisses do not usually split as easily as slates and some schists.

preferred orientation

The metamorphic texture that exists where platy grains lie parallel to one another and/or elongate grains align in the same direction.

Rock Cleavage

The tendency of rocks to split along parallel, closely spaced surfaces. These surfaces are often highly inclined to the bedding planes in the rock. Slate is a rock that exhibits this type of cleavage, thus this type of cleavage is often called Slaty Cleavage.

Type of foliation

depends on the grade of metamorphosis and the mineralogy of the parent rock.

compressional stress

differential stress that squeezes/shortens rock units causing mineral grains in preexisting rocks to develop parallel or nearly parallel alignment.

solid state flow

involves slippage that disrupts the crystal lattice as atoms shift positions by breaking existing chemical bonds and forming new ones. This is one process that results in flattening of crystalline structures in rocks during metamorphosis. See figure 8.8

hornfels

is another nonfoliated rock generated when clay-rich rocks like shale and mudstone are intruded by a hot magma body. In this environment, the clay minerals are baked, picture clay pottery in a kiln, to produce a tough rock that lacks alignment of its platy minerals.

pressure solution

is significantly aided by hot, ion-rich water. Mineral matter, ions, dissolves where grains are in contact with each other, areas of high stress) and is deposited in pore spaces (areas of low stress). As a result, the mineral grains tend to become shortened in the direction of maximum stress and elongated in the direction of minimum stress. While both of these mechanisms, solid state flow and pressure solution, flatten mineral grains, the mineralogy of the rock does not change.

porphyroblasts

large crystals that form in solid rock by the reorganization of atoms during metamorphism

nonfoliated metamorphic rock

metamorphic rock that does not exhibit a banded or layered appearance