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Team Microdot

Is granite granular or not?

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I see granite described as granular all the time but recently ended up in a debate as to whether that was true and whether it was correct to use certain terms together, for example granular with phaneritic.

 

It's increasingly difficult for us enthusiastic amateurs to know which 'facts' to trust.

 

An example - from geology.com - you'd expect that to be fairly trustworthy, right?

 

Quote

Granite is a light-colored igneous rock with grains large enough to be visible with the unaided eye.

 

Wikipedia? Yeah - I know - a bit hit a miss perhaps in the trustworthiness stakes - but you'd think maybe someone educated would edit out any nonsense on such a basic scientific description?

 

Quote

Granite ( /ˈɡrænɪt/) is a common type of felsic intrusive igneous rock that is granular and phaneritic in texture....

 

The word "granite" comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a holocrystalline rock.

 

The term "granitic" means granite-like and is applied to granite and a group of intrusive igneous rocks with similar textures and slight variations in composition and origin. These rocks mainly consist of feldspar, quartz, mica, and amphibole minerals, which form an interlocking, somewhat equigranular matrix of feldspar and quartz with scattered darker biotite mica and amphibole (often hornblende) peppering the lighter color minerals.

 

So it looks like Latin speakers thought granite looked grainy enough to use that fact to name the rock...

 

I was also told that describing granite as granular and phaneritic or granular and holocrystalline was a no-no.

 

The Open University website? Not the oldest university in the world by a long way but I do know people who have worked there and written some of the material and they are far more qualified than I:

 

Quote

Granite is a coarse-grained intrusive igneous rock. It contains large interlocking crystals which are randomly oriented.

 

Oxford English Dictionary? Fairly trustworthy I would have thought?

 

Quote

A very hard, granular, crystalline, igneous rock consisting mainly of quartz, mica, and feldspar and often used as a building stone.

Origin

Mid 17th century: from Italian granito, literally ‘grained’, from grano ‘grain’, from Latin granum.

 

 

What does granular actually mean anyway?

 

Quote
adjective
adjective: granular
  1. 1.
    resembling or consisting of small grains or particles.
    synonyms: powder, powdered, powdery, grainy, granulated, gritty, sandy;
    technicalcomminuted
    "plant food in a new granular form"
    • having a roughened surface or structure.
  2. 2.
    technical
    characterized by a high level of granularity.
    "a granular database"

 

So it's possible and fair then to describe a rock that consists entirely of crystals as grainy or granular based on that rock's appearance? It does seem that Latin speakers thought so at any rate.

 

So what's people's thinking here - is granite granular or not?

 

 

Edited by Team Microdot
Submitted before complete by accident

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I forgot about microgranite on the OU website:

 

Quote

Microgranite is a medium-grained intrusive igneous rock. It contains crystals, smaller than grains of rice, which are interlocking and randomly oriented. It is pale grey and can sometimes be pinkish in colour.

It contains a number of minerals, mostly feldspars, which are pale grey or pinkish, and quartz, which is grey or white. It also contains small specs of mafic (dark coloured) minerals.

How was it formed?
Microgranite is the medium-grained equivalent of granite. The crystals are slightly smaller than granite indicating that the magma cooled more quickly.

 

Medium-grained.

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High School science teacher here with twelve years of teaching rocks and minerals in Earth-space Science class. I also have a non-teaching minor in GeoScience that focused heavily on rocks and minerals.

 

For rocks and minerals granular means "consisting of small grains or particles" --Granite is granular. The grains are so large you can easily see them with your naked eye. One of my students once said "the grains on this rock look like cornflakes" about a piece of granite .  Microgranite you might need to get out a hand magnifier to look at the grains if your eyes are getting old like mine. It's described as a "medium-grained" (1–5 mm grain diameter) rock. I can still see the mm sections on my ruler, but not like I could twenty years ago. Still, if I grab a piece of what someone calls microgranite, I can see that it is made of grains of a mixture of minerals. 

 

The grains form because granite is an intrusive rock; it cools slowly enough for the grains to form. Unlike extrusive igneous rocks such as basalt or rhyolite, which cool so quickly the grains cannot form well.

 

ETA --to ask if that helps at all?

 

 

Edited by Neos2
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13 minutes ago, Neos2 said:

High School science teacher here with twelve years of teaching rocks and minerals in Earth-space Science class. I also have a non-teaching minor in GeoScience that focused heavily on rocks and minerals.

 

For rocks and minerals granular means "consisting of small grains or particles" --Granite is granular. The grains are so large you can easily see them with your naked eye. One of my students once said "the grains on this rock look like cornflakes" about a piece of granite .  Microgranite you might need to get out a hand magnifier to look at the grains if your eyes are getting old like mine. It's described as a "medium-grained" (1–5 mm grain diameter) rock. I can still see the mm sections on my ruler, but not like I could twenty years ago. Still, if I grab a piece of what someone calls microgranite, I can see that it is made of grains of a mixture of minerals. 

 

The grains form because granite is an intrusive rock; it cools slowly enough for the grains to form. Unlike extrusive igneous rocks such as basalt or rhyolite, which cool so quickly the grains cannot form well.

 

ETA --to ask if that helps at all?

 

 

 

Thankyou for that detailed response ^_^

 

The person who told me that using the terms grains, granular etc. would insist that we remove all such references from your text and replace them with crystals, crystalline etc.

 

Is it possible that would be more technically correct?

 

Is it also possible that it's still fair to describe granite as granular because it has a grainy appearance / granular appearance?

 

I think salt and sugar are crystalline in nature - but common parlance we speak of grains of salt and grains of sugar...

 

I'm conscious that Earthcaches must assume no prior knowledge and a reading age of 14 years.  On that basis, would have thought it would be reasonable to invite them to look for the coarse grained rock personally - but I'm told otherwise.

Edited by Team Microdot
typo

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31 minutes ago, Team Microdot said:

The person who told me that using the terms grains, granular etc. would insist that we remove all such references from your text and replace them with crystals, crystalline etc.

 

Is it possible that would be more technically correct?

 

Is it also possible that it's still fair to describe granite as granular because it has a grainy appearance / granular appearance?

 

I think salt and sugar are crystalline in nature - but common parlance we speak of grains of salt and grains of sugar...

 

I'm conscious that Earthcaches must assume no prior knowledge and a reading age of 14 years.  On that basis, would have thought it would be reasonable to invite them to look for the coarse grained rock personally - but I'm told otherwise.

 

Your pedantic friend is more correct, but it's not such a big deal.

 

I think of rocks like quartzite, in which the individual crystals tend to separate easily, as "granular."  In granite, the crystals are locked together as a result of how they formed (in a very hot environment at the base of the crust) and they don't come apart easily.  I think that property is what your pedantic friend was referring to.

 

I mean, technically, steel is made of crystals, but we don't call it "granular."

 

So I think it could go either way, but in any case I would not get on your case about it!

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fizzymagic said it well.

 

It wouldn't be wrong to use crystal instead of grain, and yes, it may be a bit more technically correct. One correct way to  use grain if it makes you more comfortable would be to say "the grain size of the crystals are about 3-5mm" etc. You could look at it as your chance to teach someone to proper vernacular on a subject in a way that isn't hard to do!  I'd feel more comfortable using the word grain that way because I think it's a familiar way to use it for lay people, perhaps your friend would find that acceptable, too.

 

 

 

 

Edited by Neos2

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When choosing between technical terms or the vernacular names, I have often seen (and used) this:  early in the write-up, add a sentence where you define the term and say you will be using one or the other.

Something like "Granite is a multi-colored igneous rock where the different colors are small crystals of various minerals.  These crystals, also known as grains, ....."

 

(Sorry, not my best prose, but I hope it gets the point across.)

 

I wonder if the objection to using "grains" is because a rock like sandstone also is grainy, but those grains of sand are easily abraded from the surface, while the "grains" of granite are not???

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I went looking online for a scientific answer to the question "What is the difference between grain and crystal?" and found a discussion among "professionals"  that debates this same question. It doesn't leave me with a definitive answer, because "it depends" on who is asking...

 

Basically, whether you think there is a difference depends on your background. A petrologist or a metallurgist would probably think differently than a mineralogist or your everyday rockhound.

 

From my (entry level college class) notes:

Extrusive Igneous Rocks- formed by crystallization of magma at the surface of the Earth

~ characterized by fine-grained textures

~ rapid cooling at or near the surface -lack time for large crystals to grow.

~ called aphanetic (fine grained)

~ EX basalt, rhyolite

 

Intrusive Igneous Rocks- formed by crystallization of magma at depth

~ characterized by large crystal sizes, i.e., visible individual crystals interlocked together 

~ slow cooling at depth - large crystals can grow

~called phaneretic (coarse crystals)

~EX granite, gabbro

 

I copied that so you could see how my college professor used the terminology to teach an entry level class. He was a geologist before he became a teacher; his specialties were identifying petroleum bearing rock outcrops and using X-ray diffraction to identify unknown substances. The class he taught in Crystallography had much more specific information about what was meant by crystal and grain etc. but it was a graduate level class, too. It was awesome, but technical far beyond anything we would ever need to know for an EarthCache.

 

(The notes I gave my freshmen high school student weren't that much watered down from those notes; it's pretty basic definition type info).

 

 

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3 hours ago, Neos2 said:

From my (entry level college class) notes:

Extrusive Igneous Rocks- formed by crystallization of magma at the surface of the Earth

~ characterized by fine-grained textures

 

3 hours ago, Neos2 said:

~called phaneretic (coarse crystals)

 

3 hours ago, Neos2 said:

I copied that so you could see how my college professor used the terminology to teach an entry level class. He was a geologist before he became a teacher;

 

Entry level class - sounds like the target market for Earthcaches given that we must assume no prior knowledge and a reading age of 14, would you agree?

 

But then again - there's some slight inconsistency there too - fine-grained textures in the first instance and coarse crystals in the second.

 

Is that an indicator that, at entry level at least, these terms are acceptably interchangeable?

 

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5 hours ago, egroeg said:

I wonder if the objection to using "grains" is because a rock like sandstone also is grainy, but those grains of sand are easily abraded from the surface, while the "grains" of granite are not???

 

I can't actually see how that would be relevant.

 

We might say that the grains in granite are tightly interlocked whereas those in sandstone are less so / cemented together.

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9 hours ago, Neos2 said:

fizzymagic said it well.

 

It wouldn't be wrong to use crystal instead of grain, and yes, it may be a bit more technically correct. One correct way to  use grain if it makes you more comfortable would be to say "the grain size of the crystals are about 3-5mm" etc. You could look at it as your chance to teach someone to proper vernacular on a subject in a way that isn't hard to do!  I'd feel more comfortable using the word grain that way because I think it's a familiar way to use it for lay people, perhaps your friend would find that acceptable, too.

 

 

My friend rejected outright the phrase crystal grains.

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15 hours ago, Team Microdot said:

Wikipedia? Yeah - I know - a bit hit a miss perhaps in the trustworthiness stakes - but you'd think maybe someone educated would edit out any nonsense on such a basic scientific description? 

 

I may need to retract my hit-and-miss judgement.

 

This wikipedia article talks about a particular type of granite thus:

 

Quote

Alkali feldspar granite. Holocrystalline texture, coarse-grained.

 

When I view the history of that page and look at previous edits one of the most recent is by a user called GeoWriter who describes themselves thus:

 

Quote

I'm a geologist, specialising in volcanoes and volcanic rocks, but I'm also very interested in all aspects of geology.

I have been contributing to Wikipedia since October 2006.

 

Surely if a geologist specialising in volcanoes and volcanic rocks is comfortable describing thos rocks as holocrystalline and coarse grained in the same sentence that should be acceptable in an EarthCache?

 

 

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Remember that geology is broad, and there can be some various definitions of the different terms in various studies. 

 

Grains has a specific meaning to many.  A grain is a size measure; silt, sand, cobbles and boulders,  are all grains.    A grain can be one material or many crystals.  Common usage says to describe grains, (in fact I am pretty sure that is where the term granite comes from).  The boundary between crystals is a granular boundary.    Grains also implies more with sedimentary rocks.   It is used a lot in classifying sedimentary materials. 

 

But I think discussing the makeup of most igneous material it is kind of odd.  Kind of like describing cache size with difficulty.  If you asked me the size and I said four star difficulty, it may mean something, kind of, but not really.  You can say car, but the proper term may be SUV. Everyone will ask what car you drive, everyone understands it, but in digging down (pun intended) to the bedrock of the issue (another terrible pun) you would say a SUV, or a Ford SUV, or go into more detail and give the make and model. 

 

Grains is vague, being more specific might work better.  I would not say grain size in a cache page, I would use crystal size, or "size of the feldspar crystal".  I might use grains if discussing the small pieces coming off the rock.  There is a local area here where they break off like small peas forming piles. 

Edited by BlueRajah

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3 minutes ago, BlueRajah said:

I would not say grain size in a cache page

 

What about:

  • Aphanitic - fine grained
  • Phaneritic - coarse grained

?

 

It seems to me that the accepted terminology is disjointed.

 

We can describe holocrystalline rocks in terms of granularity / equigranular / inequigranular but describing their visible constituent parts as grains is completely unacceptable.

 

And yet:

 

Quote

Aphanite, or aphanitic as an adjective (from the Greek αφανης, "invisible"), is a name given to certain igneous rocks that are so fine-grained that their component mineral crystals are not detectable by the unaided eye (as opposed to phaneritic igneous rocks, where the minerals are visible to the unaided eye).

 

And if you read the whole of this table you'll likely end up even more confused 😵

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On 11/4/2018 at 4:22 PM, Neos2 said:

High School science teacher here with twelve years of teaching rocks and minerals in Earth-space Science class. I also have a non-teaching minor in GeoScience that focused heavily on rocks and minerals.

 

For rocks and minerals granular means "consisting of small grains or particles" --Granite is granular. The grains are so large you can easily see them with your naked eye. One of my students once said "the grains on this rock look like cornflakes" about a piece of granite .  Microgranite you might need to get out a hand magnifier to look at the grains if your eyes are getting old like mine. It's described as a "medium-grained" (1–5 mm grain diameter) rock. I can still see the mm sections on my ruler, but not like I could twenty years ago. Still, if I grab a piece of what someone calls microgranite, I can see that it is made of grains of a mixture of minerals. 

 

The grains form because granite is an intrusive rock; it cools slowly enough for the grains to form. Unlike extrusive igneous rocks such as basalt or rhyolite, which cool so quickly the grains cannot form well.

 

ETA --to ask if that helps at all?

 

 

 

Huh, I only have a BS in chemistry so I am far from being an expert, and probably bordering on geologically stupid. But my opinion when I think of granular is solids mixed together (and hardened by time and pressure). For instance, sandstone would be granular. And though granite appears to be granular, what you're really seeing is crystals that were formed by a chemical process. But what do I know.. my ECs are pretty lame. 

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12 hours ago, bflentje said:

And though granite appears to be granular, what you're really seeing is crystals that were formed by a chemical process.

 

Yes.

 

You hit the nail squarely on the head.

 

image.png.05eaabbfdb06a4b3511bbe34126ca346.png

 

which, to me, confirms that something which looks grainy can fairly be described as granular - even though it's made of crystals.

 

I don't have a BS in anything.

 

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On 11/8/2018 at 7:15 PM, bflentje said:

 

Huh, I only have a BS in chemistry so I am far from being an expert, and probably bordering on geologically stupid. But my opinion when I think of granular is solids mixed together (and hardened by time and pressure). For instance, sandstone would be granular. And though granite appears to be granular, what you're really seeing is crystals that were formed by a chemical process. But what do I know.. my ECs are pretty lame. 

That works for me as a defining line.

Although it does get fuzzy again when you get to chemical sedimentary rocks, like rock salt, dolomite, and chemical limestone which form as an evaporate or a precipitant that is chemically cemented together, rather than by heat or pressure.  Rock salt is easily described as crystals formed by a chemical process.

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