Why doesnt quartzite contain lots of biotite

Rocks with only minor deformation may be called "schistose," for example "schistose sandstone," "schistose rhyolite," etc. Phyllite is intermediate between slate and schist. Phyllonite : a phyllite owing its fine grain to mylonitization. These rocks have a gneissose, streaked, or irregular structure produced by intimate mixing of metamorphic and magmatic materials.

When they can be recognized as "mixed rock," they are called migmatite or migmatite gneiss. They may originate by injection injection migmatite, injection gneiss, or lit-par-lit gneiss or by differential fusion. Many so-called migmatites probably originate by partial granitization or by metamorphic differentiation. There are two main types of metamorphic rocks: those that are foliated because they have formed in an environment with either directed pressure or shear stress, and those that are not foliated because they have formed in an environment without directed pressure or relatively near the surface with very little pressure at all.

Some types of metamorphic rocks, such as quartzite and marble, which also form in directed-pressure situations, do not necessarily exhibit foliation because their minerals quartz and calcite respectively do not tend to show alignment see Figure 7.

When a rock is squeezed under directed pressure during metamorphism it is likely to be deformed, and this can result in a textural change such that the minerals are elongated in the direction perpendicular to the main stress Figure 7. This contributes to the formation of foliation. When a rock is both heated and squeezed during metamorphism, and the temperature change is enough for new minerals to form from existing ones, there is a likelihood that the new minerals will be forced to grow with their long axes perpendicular to the direction of squeezing.

This is illustrated in Figure 7. After both heating and squeezing, new minerals have formed within the rock, generally parallel to each other, and the original bedding has been largely obliterated. Figure 7. This large boulder has bedding still visible as dark and light bands sloping steeply down to the right. The rock also has a strong slaty foliation, which is horizontal in this view, and has developed because the rock was being squeezed during metamorphism.

The rock has split from bedrock along this foliation plane, and you can see that other weaknesses are present in the same orientation. Squeezing and heating alone as shown in Figure 7. This effect is especially strong if the new minerals are platy like mica or elongated like amphibole.

Slate, for example, is characterized by aligned flakes of mica that are too small to see. The various types of foliated metamorphic rocks, listed in order of the grade or intensity of metamorphism and the type of foliation are slate , phyllite , schist , and gneiss Figure 7. As already noted, slate is formed from the low-grade metamorphism of shale, and has microscopic clay and mica crystals that have grown perpendicular to the stress.

Slate tends to break into flat sheets. Phyllite is similar to slate, but has typically been heated to a higher temperature; the micas have grown larger and are visible as a sheen on the surface.

Where slate is typically planar, phyllite can form in wavy layers. In the formation of schist, the temperature has been hot enough so that individual mica crystals are visible, and other mineral crystals, such as quartz, feldspar, or garnet may also be visible.

Biotite is in most cases not difficult to identify. It is micaceous composed of cleavable sheets. Muscovite has the same property, but it is usually much lighter in color.

The micaceous character, however, may not be immediately obvious if the rock is fine-grained. Hornblende is a common amphibole mineral that may occur in very similar rocks both are common in granitoids and may look alike. In this case a needle is a handy tool that will quickly solve the question. These minerals are notably soft. Their hardness on the Mohs scale is only about 2. Nothing even remotely similar can be done to hornblende which is a much harder material.

Phlogopite is usually more brownish or even fiery looking. It is among the most common and widespread minerals. It occurs in a wide variety of igneous rocks which includes both felsic and mafic rocks. It has different roles in these rocks, though.

It acts as the primary iron-bearing phase in many granitic rocks and as a hydrated potassium-bearing mineral in mafic rocks. Biotite is present in both silica under- and oversaturated rocks. Biotite, because it is the host of relatively uncommon chemical elements in a given magma, is a very common ingredient of pegmatites which crystallize from late-magmatic fluids.

Biotite in felsic rocks tends to be Fe-rich. Phlogopite in igneous rocks occurs in ultramafic rocks, especially diamond-bearing kimberlites.

It is least common in mafic rocks, but these too host it sometimes. Especially if the magma is contaminated by the material from pelitic crustal rocks.

It is much less frequent in volcanic rocks and generally absent in basalts except some K-rich varieties. If present, biotite may be partially altered to other minerals. The reason why biotite is abundant in intrusive but usually resorbed in volcanic rocks is that it is not a stable phase in magma at lower pressures. In metamorphic rocks too, biotite is present in a wide variety of rocks which formed under various temperature and pressure conditions.

Biotite in the majority of cases forms when clay-rich sedimentary rocks are buried deep enough for the clay minerals to metamorphose to it. Biotite also forms in impure metamorphosed carbonate rocks and in metabasic rocks. There are many different geochemical pathways that can lead to the formation of biotite which explains its ubiquitousness in metamorphic systems.

It remains the companion of several metamorphic minerals muscovite, garnet , staurolite , Al-silicates, cordierite that inhabit pelitic rocks at various depths in the crust.

Biotite finally disappears at the granulite facies conditions where hydrous minerals which it is are not stable. Metapelitic rocks that contain lots of biotite are various schists biotite schist, biotite-chlorite schist, albite-biotite schist, garnet-mica schist. In metamorphosed mafic rocks biotite forms as a replacement of low-grade greenschist facies metamorphic rocks containing amphiboles and muscovite. Biotite also occurs in skarns which are contact-metasomatized rocks which form when silicate magma reacts with carbonate rocks.

Biotite in clastic sediments is a common mineral when biotite-bearing rocks are exposed nearby. It is not as resistant in the weathering environment as muscovite and gets less frequent with extended transport. Biotite is usually altered to clay minerals montmorillonite and vermiculite.

Biotite and muscovite are the two most common mica minerals that often occur together in Al-rich igneous rock. Peraluminous two-mica granitoid. Width of sample 9 cm. Monomineralic rock that is composed of almost pure biotite is known as glimmerite. It is usually part of a larger pegmatitic assemblage. Width of sample 25 cm. Tonalite pegmatite white-gray on the right and garnet amphibolite on the left. Black in pegmatite is biotite, white is plagioclase, gray is quartz.

Width of sample 10 cm. Biotite gneiss. Black mineral forming bands in the rock is biotite. White feldspar is plagioclase. Trondhjemite is a leucocratic igneous rock tonalite in which the sole mafic mineral is biotite hornblende is rare. Width of sample from Norway is 10 cm. Phlogopite with apatite greenish yellow and enstatite dark green in pegmatite.

Width of sample 22 cm. Nepheline syenite.