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.  

Varieties of Quartz: Part 2

The previous post showed some common varieties of macrocrystalline (visible crystals) of quartz (silica = SiO2). This post shows some common varieties of quartz that do not show any visible crystals and have a dense structure, referred to as  microcrystalline  [= cryptocrystalline] quartz. Two main groups are shown here: chalcedony and opal.

Chalcedony is the generic named given to materials that are composed of microcrystalline quartz. Chalcedony includes blue chalcedony, chert, "touchstone," agate, jasper, and "tiger eye." Examples of these are shown below: 

height 5 cm, from Nsanje District, Malaw (southeast Africa) 

Blue chalcedony (commonly called blue "quartz"). Chalcedony is a microcrystalline form of quartz (silica, SiO2), composed of intergrowths of quartz and moganite, and each has a different crystalline structure. 

height 3 cm

Chert is a hard, opaque chalcedony with an amorphous microcrystalline texture. Chert has conchoidal fracture, which produces very sharp edges, especially in flint (black or brown forms of chert). That is why chert was commonly used for making arrowheads.

height 7 cm

The image above shows a sedimentary rock consisting of chert layers (in black) alternating with impure white siliceous (silica bearing) material, referred to by some geologists as "porcelaneous chert" (i.e., resembles glazed porcelain).

height 7.5 cm

"Touchstone" is a black, nodular version of chert. For at least 2,500 years, it has been used by goldsmiths to test the purity of gold and silver by the streak left on the stone when rubbed against these metals. "Touchstone" is also called the "lydite stone," also spelled as "lyddite."

sliced and polished specimens, largest one height 7.5 cm

Agate is banded chalcedony and has a wide range of colors. The bands form over time and can have different colors, depending on the impurities. Agates form commonly in areas of volcanic activity or in limestone where waters, rich in dissolved silica flow through fractures and cavities in these rocks.

height 1.7 cm

An agatized snail, whose hollow interior was filled with agate.

petrified wood (14 cm width), from Petrified Forest, Arizona

Petrified wood is, in most cases, wood replaced by chalcedony. Other minerals (like opal), however, are also known to replace wood.

height 5 cm

Jasper is an opaque aggregate of chalcedony and microgranular quartz. Jasper is an impure variety of silica, thus it can have red, yellow, brown, green, or other colors. The red color of jasper is because of iron inclusions.

polished specimen, height 3.7 cm

"Tiger eye" is a metamorphic rock consisting of chalcedony that formed by replacement of an asbestos-group mineral (called crocidolite). "Tiger eye" has a golden brown to very dark brown color and silky luster. 

The other group of microcrystalline silica is opal, which is a mineraloid consisting of hydrated amorphous silica (water content 3 to 21 percent by weight; commonly 6 to 9 percent). Opal is deposited at relatively low temperature, and it can be deposited in the fissures of any kind of rock, especially volcanic rock. Opal is the national gemstone of Australia.

Examples of "precious opal" are shown in the next two images below. "Precious opal" displays flashes of colors when exposed to light (= a form of iridescence). 

height 1 cm

height 2 cm

height 4.5 cm

This last variety of opal is "fire opal," which is bright red, yellow, 
or orange in color.

width 10 cm

Both macrocrystalline and microcrystalline quartz occur commonly in geodes, which are round rocks with a hollow cavity line with crystals. In the example above, the geode has two hollow cavities, both partly filled with druzy (macrocrystalline) quartz alternating with layers of chalcedony.

width 12 cm

Cavities completely filled with chalcedony (e.g., agate, jasper) are called nodules. In the nodule shown above, macrocrystalline amethyst mostly filled a geode. The remaining cavity was filled eventually filled completely with chalcedony. This unusual specimen was collected by my friend, the late George Edward Davis.