Last time we looked at the basic requirements that minerals must meet to be classified as
minerals, but how are minerals even formed?
Keep in mind that most minerals are found within rocks so many minerals are formed when
those rocks are formed.
We’ll look at the rock cycle in depth later, but for now minerals can be formed in a few
main ways: sublimation from volcanic gases, crystallization of lava or magma, and
deposition or evaporation from aqueous solutions.
Sublimation is the process of going from a gas directly to a solid.
When volcanoes erupt, lots of gases are released.
The contrast between the temperature of the extremely hot volcano and the much cooler
atmosphere means that some of those gases are form lower energy solids pretty quickly without even passing
through the liquid phase.
Since sulfur dioxide is released during eruptions, native sulfur is common to find around volcanoes.
Cooling of magma or lava results in crystals and igneous rocks being formed.
Through the Bowen’s reaction series, which we’ll talk about in an igneous rock video,
different minerals are formed at different temperatures of this cooling process.
Another way they can form is after hot water flows
between rocks and dissolves particles.
Then, when this water cools, these dissolved particles return to a solid state and can
deposit, or precipitate, back out.
This can lead to large sections of rocks coated in other minerals or even veins of minerals
inside other rocks.
This can deposition can take place in hot springs or hydrothermal vents and veins.
Evaporation can also lead to minerals being formed.
If seawater has a large salt content, not necessarily just sodium chloride but any salt,
when that seawater evaporates it can leave behind those minerals as they settle to the
bottom.
Rock gypsum can be formed by this process, and it usually occurs in water basins when there's
less water coming into the system than is being evaporated.
Now, some minerals won’t always remain the same mineral.
A mineral can change form, or “morph”, and be replaced by another mineral based on
environmental conditions.
There are many different types of this, but in general it’s just one mineral being replaced
by another because of a change to its chemical composition.
This gradual replacement leads to some pretty cool looking samples called pseudomorphs
meaning "false form". These samples usually have some of both mineral present.
One of the more famous examples of a pseudomorph is the malachite-azurite combo.
Malachite slowly replaces the azurite by losing its hydroxyls in a chemical change, and since
they’re both such bright and beautiful colors, this makes their combination a popular gemstone.
Pseudomorphs aren’t to be confused with polymorphs, which are minerals with the same
chemical formula but different atomic arrangements.
The most commonly used example of this is graphite and diamond since they’re both
made of pure carbon but the different arrangements lead to their different properties.
Graphite is more slippery since it’s arranged in sheets, while diamond is stronger because
of its tetrahedral form.
The last video covers this, so here’s a link to that.
Anyway, there’s one more important thing to discuss about mineral formation.
Since minerals form from crystallization, sometimes crystals can grow abnormally.
Errors in a mineral’s growth because of environmental conditions can result in twinning:
the symmetrical growth of two crystals from similar lattice points.
The way a mineral twins can help to identify it since different minerals have different
twinning patterns.
While there are a ton of types of twinning, different laws, and a whole classification
system, there are two main types: contact and penetration.
Contact twins involve the twinned crystals sharing only one plane between them; it looks
like they’re just touching each other.
Penetration twins involve one of the crystals going through the other one, in other words,
more than one plane is shared.
Calcite is a common contact twin since its crystals share only one plane, while staurolite
is the famous penetration twin that forms little crosses and star-looking shapes.
Those are some ways minerals form and processes that go along with mineral formation.
Next time we’ll dive into the different properties of minerals, starting with hardness.
Thanks for watching, be sure to like and subscribe, and rock on!