In this article, we will learn about how are minerals formed and a specific group of minerals called silicates.
How are minerals formed?
There are generally four ways in which the minerals are formed. The first one is, that they form from magma or the lava, the second is the process of evaporation, the third one is due to temperature and pressure, and the fourth one is from the living organisms.
The first one is from magma and lava. So there is hot magma. The orange-colored part is the magma that is there inside the earth and when it comes out. We call it the lava.
When this lava or the magma cools down after coming out from the volcano it results in the formation of minerals. Evaporation normally happens when in this process normally the minerals are formed.
When solid crystals remain behind and when the evaporation takes place you can see an example of halite. Now let us proceed toward the next method.
When living organisms die or normally when the outer covering of the organisms is normally considered as the minerals in most cases.
So some living organisms produce inorganic minerals like shells that you can find on the beach.
Crystallization is simply just the growth of a solid from either a gas or a liquid whose constituent atoms come together in a specific arrangement or structure.
This basic arrangement is then repeated in all directions as other atoms come together to make up that mineral. The environment must be suitable for crystal growth to occur.
In other words, the type of mineral you are going to have depends on the concentration of atoms that are available as well as the proper temperature and pressure. Let us understand this with an example.
Halite
It is made up of sodium and chlorine atoms that come together to form a repeating cube-like structure. If minerals could grow in an unrestricted environment, they would tend to grow to their idealized shape or form a nice crystal-like structure.
Most common though, minerals grow in a restricted environment. In other words, the mineral will grow until it encounters another barrier such as a crystal then it will stop growing.
In this case, the crystal structure may take on any shape or the shape of the constraints that surround it.
There are a number of different chemical classes of minerals such as Native elements, silicates, carbonates, oxides, sulfides, and sulfates, among others.
Both silicon and oxygen are very abundant in Earth’s crust and they come together to form a unique geometric shape called a tetrahedron.
A tetrahedron is essentially made up of one silicon atom and four oxygen atoms that come together to form a pyramid-like shape.
These pyramids then come together and stack up to form the various types of silicate minerals. Let’s now examine several of our common silicate minerals starting first with our isolated tetrahedrons. The most common isolated tetrahedron mineral is olivine.
Read about mineral formation.
Olivine
Olivine is primarily composed of single Si-O tetrahedrons, silicon-oxygen tetrahedrons, that are linked by magnesium or iron. A primary constituent of Earth’s mantle and the ocean crust.
It is typically a nice olive green or that green color and it has a glassy luster. Next are our single-chain tetrahedron linkages, the most common are the pyroxenes.
Just like we saw with olivine, pyroxenes also are high-temperature minerals, they are dark green to black in color and are rich in iron and magnesium as well. Now compare that to double-chain linkages of tetrahedrons or what are commonly called amphiboles.
They also are black to dark green in color and look very similar to pyroxenes. In other words, they leave at 120 or 60-degree angles The most common amphibole is a mineral called hornblende which is found commonly in granite.
Minerals that form from sheets of tetrahedrons are known as micas. These are minerals that are rich in potassium and aluminum along with silicate tetrahedrons. Because the tetrahedrons occur in a sheet-like appearance most of these minerals have one direction of cleavage or what’s known as perfect cleavage.
They peel like the pages of a book. There are two types of common micas. the first is muscovite which is a white or colorless mica, and biotite.
A little richer in iron and magnesium that is darker in color, often black to dark brown. Micas are very common to rock granite and are found in many different types of metamorphic rocks.
Let’s now look at common framework tetrahedron minerals The most abundant is a group called the feldspars.
Read: How do we interact with mineral resources in daily life?
Feldspars
Feldspars are very common in granite and are a primary constituent of crustal rock. These are relatively hard minerals because of that rigid tetrahedral framework.
Feldspars typically have a hardness of about 6 Most have a porcelaneous luster or look a bit like porcelain and they have good cleavage typically in two directions.
Two common feldspars are known as potassium feldspar which is rich in potassium obviously. It’s commonly pink or salmon in color and is often known as the mineral orthoclase. And plagioclase is richer in sodium and calcium.
Plagioclase often has striations or little grooves that are part of the crystal on the cleavage planes. The mineral quartz is also an example of a framework silicate mineral. The difference here is that quartz is composed almost entirely of silica-oxygen.
It is relatively hard. For example, purple is amethyst orange which is known as citrine smoky quartz which is gray milky quartz which is white, etc. Therefore often color is a difficult determinant of quartz, instead, you should use the streak of the mineral.
All of those have a streak color of white. Again, quartz is very hard. It’s mostly composed of tetrahedrons in a tight framework where all the bonds are between silica and oxygen and are pretty much equal strength.
Therefore it does not have any cleavage or weakness upon which it breaks. Instead, quartz has a unique property called conchoidal fracture which looks like chipped glass.
So now we know about the specific class of silicate minerals where we examined the different arrangements of silica-oxygen tetrahedrons and the different types of minerals.
They form from isolated tetrahedrons such as olivine to single chains that were pyroxenes double chains, amphibole sheets, which were the micas, and frameworks which included feldspars and quartz.