Common rock-forming minerals

The following is a very basic discussion of minerals, which is critical for the understanding the classification of igneous rocks, the topic of the next post.

Atoms (electrons, protons, and neutrons) make up elements (92 natural ones; (examples: oxygen, silicon, potassium).

Elements make up minerals (3,500 kinds).

     Minerals consisting of pure (native) single elements are rare     (e.g., gold, diamond, copper).

     Nearly all minerals consist of combinations of elements.

Only 20 minerals are common, and they are the rock-forming minerals. Seven of these are shown below.

white ("milky") quartz, 3.3 cm in height

Quartz (made up of the elements silicon + oxygen) is the most common mineral in the continental crust of Earth. The presence or absence of quartz in a rock is fundamental in the classification of igneous rocks. For more information, see my previous two posts, which review the subject of quartz.

potassium feldspar (K-spar), 3.7 cm in height

One of the largest groups of minerals is referred to as the "feldspar group," which consists of two subgroups. One of these subgroups is the "alkali feldspars,"which contain varying amounts of the elements potassium and sodium in their composition. The mineral pictured above is one of these alkali feldspars, which are generally called "potassium feldspar" or "K-spar" until x-ray and petrographic microscope studies are made to determine the exact mineral. 

plagioclase, 3.3 cm in height

The second subgroup of the "feldspar group" is called the "plagioclase feldspars," which contain varying amounts of the elements sodium and calcium in their composition. Plagioclase is characterized by the presence of striations, which can be seen as closely spaced lineations in the upper half of the specimen shown above.

flakes of muscovite, largest flake 4 cm height

Muscovite and biotite (see below) belong to the "mica group" of minerals. Both have one-dimensional cleavage, which splits into sheets or flakes. For a discussion of what cleavage is, see one of my more recent posts. Because of its clarity, large sheets of muscovite could have served as "windows" for pioneers living in wood cabins. Muscovite is common in quartz-rich igneous rocks (granite), metamorphic rocks like (schists and gneisses), and sedimentary rocks (siltstone and claystone).

large sheet of biotite, 6.5 cm height

Biotite has a more varied composition than does muscovite. For example, biotite has iron in its structure, which causes a dark coloration. Biotite occurs in igneous rocks, as well as a wide variety of metamorphic rocks. 

hornblende, 7 cm height

Hornblende has a very complicated composition and much chemical variability, depending on the chemicals available and the temperature of formation. Hornblende has two directions of cleavage (approximately at 60 degrees and 120 degrees). This mineral is found in a wide variety of igneous and metamorphic rocks. 

olivine, 3 cm height

The "olivine" group minerals contain varying amounts of iron and magnesium in their composition. "Olivine" is most common in quartz-free rocks and commonly has a green color.

In the minerals shown above, three of them occur as solid-solution series, which form when two elements can substitute for one another in a mineral. The three solid-solution series are the
alkali feldspars, plagioclase feldspars, and the "olivine" group.

Most of the minerals in these solid-solution series are not compositionally pure end members; rather they have an in-between composition.

Minerals make up rocks (3 main types: igneous, sedimentary, and metamorphic).

Rocks make up the Earth's crust (two kinds: continental and oceanic).

In the next post, igneous rocks will be discussed and illustrated, and the chart below is presented here to show how quartz is used to help define certain kinds of igneous rocks. For example, granite (think expensive kitchen-counter tops) is rich is quartz but basalt (think lava flows in Hawaii) is low in quartz.  

Labradorite, a beautiful, dark-colored mineral with iridescence

Labradorite, which is derived from "red-hot" molten material called magma, is one of several phases (varieties) of the mineral plagioclase. All these phases have the same general formula (Ca, Na)(Al,Si)3O8. As the magma cools, a solid-solution series of different phases/varieties of plagioclase crystallize out sequentially, with varying amounts of Ca (calcium) and Na (sodium). The calcium and sodium ions mix in a continuous series with their ratio varying from 100% calcium and 0% of sodium, to the extreme opposite.

Labradorite consists of 50 to 70% calcium and occurs as blocky to lath-shaped crystals in calcium-rich igneous (magma-derived) rocks, such as basalt, gabbro, and anorthosites. Labradorite is relatively uncommon, but some rocks consist almost entirely of this mineral.

A 3 foot-high slab of labradorite used as a pedestal for a monument.
You can see how some of the crystals are large and lath-shaped.
 The bluish-iridescence of this mineral is especially evident  in the upper part of the picture.

One of the most memorable features of labradorite is its iridescent play of colors, which results from this mineral’s peculiar reflection of light. The reflection is caused by internal fractures that reflect light back and forth.

Polished piece (width 5.7 cm, 2.5 in.) of labradorite showing its iridescence. To see it, one must tilt the specimen at just the right angle to the prevailing light; a few degrees too much or too little tilt, and the iridescence disappears.

Labradorite-bearing rocks occur worldwide, especially in Labrador, Canada (where this mineral inherited its name) and in Norway.

Labradorite is used for making floor tiles, kitchen counter-tops, tables, and benches. It is also a popular gemstone.

For more information about the solid-solution series that is associated with the formation of plagioclase, please Google the term "Bowens Reaction Series." 

Note: I used to include links to topics covered in my posts, but recent changes in Google Posts now deactivate these links when posts go to the "Archive" file.