Crystallization or Solidifaction

  • #1

Main Question or Discussion Point

I have repeatedly seen that a geologist's terminology regarding the formation of an igneous rock is "crystallization" ; the lava 'crystallizes' & an igneous rock is born... is this terminology correct???

Shouldn't it be ; the lava ' solidifies ' & an igneous rock is born. ???

I have seen quite a lot of people using the first sentence which made me think I might have misunderstood the process of rock formation,or is it just a mass psychology?

If the rocks are crystallized, what makes them different from Minerals... Previously I used to think that the main difference is that the rocks are solidified lava in a random way, & minerals are highly ordered crystallized crystals.

Thanks in advance.
 

Answers and Replies

  • #2
Gokul43201
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I have repeatedly seen that a geologist's terminology regarding the formation of an igneous rock is "crystallization" ; the lava 'crystallizes' & an igneous rock is born... is this terminology correct???
I know nothing of geologist terminology, but in the language of physics and materials science, the terminology would only be strictly correct in some cases. The thing of relevance is whether the solid formed is crystalline or amorphous (glassy). If the rock is essentially glassy (and I believe many igneous rocks indeed are), then the solidification is not really a crystallization process.

If the rocks are crystallized, what makes them different from Minerals... Previously I used to think that the main difference is that the rocks are solidified lava in a random way, & minerals are highly ordered crystallized crystals.

Thanks in advance.
(As an outsider to geology) I remember reading that the difference between rocks and minerals is that a mineral is defined by a single (primary) chemical composition (and possibly even a single crystal symmetry), while a rock may be composed of an agglomeration of several different minerals. In general, I would imagine a rock can be anything from mostly amorphous (more likely among the igneous kind, I think) to mostly crystalline (as many metamorphic rocks are).
 
  • #3
matthyaouw
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Most igneous rocks are crystalline, so that terminology is correct. There are a few amorphous ones, but these are only ones that cool extremely quickly. In rocks such as granite you can see the crystals with the naked eye but with others such as basalt you will need a hand lens or microscope to see them.
Rocks are aggregates of minerals, be it largely one mineral or many different ones. It's mainly sedimentary rocks that consist of almost entirely one mineral. Igneous rocks are genereally several.

Does this clear things up?
 
  • #4
In rocks such as granite you can see the crystals with the naked eye but with others such as basalt you will need a hand lens or microscope to see them.

Rocks are aggregates of minerals, be it largely one mineral or many different ones.
Thank you, I am realizing some stuff :)

But I have another question;

As you have mentioned rocks are aggregates of minerals, and as I understand this means that it isn't composed of a uniform chemical composition through out the rock.

________

The following definition of Crystal is from Wikipedia;

A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions.

_________

What I am trying to understand is that how are the two definitions of both rock and a crystal match...

Rock being an aggregate of minerals without a highly ordered atomic structure while crystals have an orderly repeating patterned atomic structure, &
that are there rocks with definite crystal systems (hexagonal, cubic etc.)
 
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  • #5
The thing of relevance is whether the solid formed is crystalline or amorphous (glassy). If the rock is essentially glassy (and I believe many igneous rocks indeed are), then the solidification is not really a crystallization process.
This is infact what I had thought...
 
  • #6
matthyaouw
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Each crystal, each mineral in the rock has a repeating, regular atomic structure, but it's the rock as a whole that does not. The mineral's atomic structure and composition is exactly the same every time. The rock as a whole can vary though. There may be different proportions of each mineral, crystal sizes or crystal orientations might be different.

Hope this helps, but I'm not entirely sure I understand your question.
 
  • #7
Each crystal, each mineral in the rock has a repeating, regular atomic structure, but it's the rock as a whole that does not.
It sure helps,Thank you once again.

This is exactly what I am trying to say, rocks as a whole don't have a regular structure...

The minerals within the rocks were not crystallized during the cooling process of lava, & that the crystals were solid & crystallized during the eruption of magma. As I have understood, in other words they are trapped in the rocks during the solidification process.

If my understanding of this process is correct rocks are solidified lava rather than crystallized lava, & as a result they are not found in any crystal form or crystal system instead rocks are of an amorphous type which lack a definite crystalline structure as a whole ...

Please correct me if I am wrong.

Thanks in advance.
 
  • #8
The minerals within the rocks were not crystallized during the cooling process of lava, & that the crystals were solid & crystallized during the eruption of magma. As I have understood, in other words they are trapped in the rocks during the solidification process.
No - the crystal do not form before solidification - they formed during. Crystals form when magma cools (lava is magma that is on the surface). The slower the cooling, the larger the crystals will be (pressure also plays a part). Lava cools quickly, as it's at the surface. Therefore, igneous rocks formed from eruptions are fine-grained; basalt is the classic example.

The crystals therefore form at the same time as the rock forms. The term "crystallisation" is therefore very correct to describe the formation of igneous rocks.

The crystallisation may occur in several phases, giving you very different crystal sizes throughout the rock: a porphyritic texture.

If my understanding of this process is correct rocks are solidified lava rather than crystallized lava, & as a result they are not found in any crystal form or crystal system instead rocks are of an amorphous type which lack a definite crystalline structure as a whole ...
Igneous rocks as a whole have no crystalline structure and as a whole are technically amorphous. However, as they are exclusively made from crystals, they still crystallise. Welcome to the wacky world of geology jargon ;)

This page gives a very simple intro the formation of igneous rocks: http://www.geologyrocks.co.uk/tutorials/introduction_to_igneous_petrology
 
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  • #9
No - the crystal do not form before solidification - they formed during. Crystals form when magma cools.

The crystals therefore form at the same time as the rock forms. The term "crystallisation" is therefore very correct to describe the formation of igneous rocks.
Thanks for your detailed reply :) & I apologize for my delayed reply...

You have mentioned very interesting issues which I wasn't aware of... such the crystallization during eruption (as lava cools). What I had in my mind was in fact crystals like garnet, diamond, olivine, spinel, corundum & so on... which are crystallized within Asthenosphere, & even quartz which has an mp of about 1650 C while the max. temp. of lava is about 1200 C during eruption. What I thought was that as most of a rock is generally quartz & feldspar, both of which have a melting point greater than that of erupting lava those crystals should have crystallized within the earth before eruption & later trapped in solidifying lava after eruption...

In other words how is it possible to have quartz & feldspar in molten state during erruption...

Welcome to the wacky world of geology jargon ;)
Thank you jargon :)

It really is a wacky wacky world !!!

Hope to hear from you & once again sorry for taking so long.
 
  • #10
You have mentioned very interesting issues which I wasn't aware of... such the crystallization during eruption (as lava cools). What I had in my mind was in fact crystals like garnet, diamond, olivine, spinel, corundum & so on... which are crystallized within Asthenosphere, & even quartz which has an mp of about 1650 C while the max. temp. of lava is about 1200 C during eruption. What I thought was that as most of a rock is generally quartz & feldspar, both of which have a melting point greater than that of erupting lava those crystals should have crystallized within the earth before eruption & later trapped in solidifying lava after eruption...
All minerals have different melting points, as you may well be aware of. The Asthenosphere is solid (a bit tacky, maybe, but solid). So this leads us to a few issues, but please excuse my briefness here, as covering them in sufficient detail requires an undergrad degree in geology ;)

- melting occurs in the mantle due to either a localised raising of temperature or a decrease in pressure. This can be caused by adding water (subduction) or stretching the mantle (spreading ridge).
- magma forms and has the bulk chemical composition of the stuff that melted. The first things to melt are quartz and K-feldspar (melting points of around 650 deg C). Last things to melt are olivine (1200 deg C). We can classify the magma as silica-rich (very viscous, lots of silica, low melting point) - often called evolved or acidic magma, or silica-ppor (very hot, runny, 20% or less silica) - often called basic or unevolved.
- the order of melting/crystallisation is called "Bowen's Reaction Series". See http://www.geologyrocks.co.uk/images/bowen%26%23039;s_reaction_series?size=_original for a nice diagram explaining this.

- As crystallisation occurs, the composition of the remaining magma changes to become more acidic (i.e. it evolves).

As an example: If we melted some crust completely and cooled it again we would get some basic rocks first (basalt), then some intermediate rocks, then evolved rocks (micro granite). This is all in theory, as in practice, the cooling process is much more complex and the crystallised minerals are in contact with the magma and may re-melt, alter, swap ions, etc.

Most standard textbooks cover this and if you're really keen, it's worth having a read of those.

Hope that helps.

In other words how is it possible to have quartz & feldspar in molten state during erruption...
 
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  • #11
Gokul43201
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Igneous rocks as a whole have no crystalline structure and as a whole are technically amorphous. However, as they are exclusively made from crystals, they still crystallise.
This can't be right. And the first sentence doesn't make a whole lot of sense to me. What do you mean by it?

For something to be amorphous, it should have no crystalline structure at any length scale. Your link clearly says: "Igneous rocks are formed form the cooling of molten rock, magma. They are crystalline, which means they are made up of crystals joined together." Well, if they are crystalline, they clearly are not amorphous.
 
  • #12
All minerals have different melting points, as you may well be aware of. The Asthenosphere is solid (a bit tacky, maybe, but solid). So this leads us to a few issues, but please excuse my briefness here, as covering them in sufficient detail requires an undergrad degree in geology ;)

...

Hope that helps.
Thanks for your kind reply :)

Once again, very interesting...

In fact I am studying Gemology & it's just my 4th semester now, so way too much left to get an idea of what is really going on :)

You are absoloutelty right, but what makes me confused is that once again the crystallization process belongs to the minerals which occur due to varience in temp. of magma/lava, not the rock as a whole.

Why should we consider the mineral aggereates which crystallize as being part of the rock as a whole, rather than some trapped material in a solidified remanants of the cooling process which keeps them all togeather?
 

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