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Seeking methodology for identifying crystalline forms.
  
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AngelL




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PostPosted: Jan 03, 2022 11:58    Post subject: Seeking methodology for identifying crystalline forms.  

[A word re: the photos. The first picture is natural under a weak light source. Pictures two thru four are under a better light source but were snapped through the 10x magnifying glass round bubble on a 2x magnifying glass. It made for large unfocused areas but was the only way I could manage close-ups. Picture four was the underside.]

I have been reading and watching videos incessantly these last few days. Additionally, I’m confident that I’ve positively identified another dozen samples. I just ran into a problem with identifying a certain type of sample for which I can’t find a path forward. The group of samples I’m referring to are crystal samples. I’m not attaching photos so that someone can tell me what it is – that’s not the point of the photos. As a matter of fact, I'm pretty sure that I know what it is - I just don't know how to prove it. The point is that a picture is worth a thousand words and I don’t know the words to describe it. The sticky thread, asking “What is it?/Where is it from?” asks for the following:

1. Photo –
2. Locality –
3. Matrix –
4. Color –
5. Luster –
6. Form –
7. Streak –
8. Hardness –
9. Others – Transparency, Fragility, Density, Cleavage.

Mindat offers a couple of others like fracture, optical type, refractive index, relief, birefringence, and crystallography. I can’t tell you its locality. Most of the time there is no matrix and in those cases with (like the one in the photo), it’s not a very significant portion of the sample. I can tell you that it’s light blue, but I don’t know how to determine luster on a crystalline structure. Truth is, I don’t know if it is the crystals themselves that are giving these flashes of light – or if it’s mica or some other mineral I’m unfamiliar with giving that effect.

Moving onto form – I would have guessed cube, like the photo of pyrite in that thread – right up until I looked at rhombohedra. I don’t know what is the determining factor that make one cube and the other rhombohedra – they both look like cubes to me. On to streak and hardness: Do we really risk damaging the sample by doing these tests with fragile crystals? I know how to do these tests but it just seems wrong. As for the rest of the identifying factors – I simply don’t know how to apply them yet. Besides color, the only identifying characteristics I can manage are size and weight – which I can’t imagine is helpful. Nevertheless, 60x54x34 mm and 74.2 grams.

Anyway, if anyone could tell me how to approach identifying crystal samples, I’d be grateful. The reason that I included the picture was so that hopefully, someone would say something along the lines of, “If I didn’t know what this sample was, this is how I would go about identifying it…” Again, any help you can give me on this will be appreciated greatly.



pic1.jpg
 Mineral: unknown
 Locality:
unknown
 Dimensions: 60x54x34 mm
 Description:
 Viewed:  818 Time(s)

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Jordi Fabre
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PostPosted: Jan 03, 2022 14:26    Post subject: Re: Seeking methodology for identifying crystalline forms.  

SteveB wrote:

Yes those photos are useless as is size and weight of a specimen for identification purposes. As for learning Crystallography childrens learning websites are a great place to start as they break things down into simplistic terms everyone can understand. There are major shapes and then there are variations and sub shapes, plus changing conditions during the formation of a crystal can result in alterations and deformations. Important to note is the shape of complete surfaces and angles between edges and faces, these are the big factors in identifying exactly what the crystal shape is. Learn som geometry and learn the terms and use them accurately not just similar looking words as you have in this post. There are very precise definitions so use them so you can record with accuracy and communicate with others with meaning, looking up each word in a good dictionary should explain how the word is made up in the first place which itself should tell you its meaning. When they talk ab out lengths they dont mean get out a ruler and measure, its typically a ratio it refers to. So while you may call something a cube, it may not be because its length, width and height may be different (cube MUST be precisely the same, learn geometry), so pay close attention in your observations and recording of information. Some crystals have well defined precisely measure angles so being able to eyeball angles is important.

Try: https://australian.museum/learn/minerals/what-are-minerals/crystal-shapes/
Click on each crystal diagram for more information about what defines it, shapes and angles, shapes and angles. Calling a specimen by its broader group name doesn’t help anyone, its just a rough starting place as you investigate the shapes the group contains and make observations and measurements in order to narrow it down more precisely , take note of edges and corners of specimen crystals to see if they appear faceted as faces, this is a defining feature so observe and dont ignore. Lastly crystal surfaces are rarely perfectly smooth, the often have tiny pits and ridges/lines from the formation process, If its glass smooth it may be a fake crystal someone has cut and polished into a crystal shape having nothing to do with the material even if it is a mineral to begin with. You need a good light source and magnifier and observe your specimen while gently moving it so crystals reflect the light on all its flat facets,, this can help you see the main face shapes and see if the edges/corners are sharp or facetted, The smaller the crystals the more difficult it can be to observe the edges with accuracy, make good use of the glinting of light also if the crystal formed with facetted corners then every corner should show that way not just one(which could have been chipped off from handling.ALL the geometric features will be present regardless of the size of the specimen, shapes and angles are consistent and due to chemistry a certainty too, though the chemistry can change during crystal formation resulting in a change of crystallographyon the specimen as can the introduction after formation of another mineral that then forms on top.

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James Catmur
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PostPosted: Jan 03, 2022 15:27    Post subject: Re: Seeking methodology for identifying crystalline forms.  

Angles are key! Look at the face and look at the angles at the corners. 90 degrees at all the corners and you have a cubic form, etc.

I worked with an artist some year ago who tried to paint pictures of minerals, and I had to explain that while the colors were great, I knew immediately that the angles were wrong. She learned and then did some great paintings with perfect angles (it is weird you can tell the angle is wrong even when you are looking at the form from a strange angle). See https://www.mineral-forum.com/message-board/viewtopic.php?p=25873#25873
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AngelL




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PostPosted: Jan 03, 2022 15:31    Post subject: Re: Seeking methodology for identifying crystalline forms.  

Jordi: Not sure what happened here. It appears (to me) that you quoted a post that doesn't exist. I've no doubt it did exist, I just don't know if I'm missing something here.

SteveB wrote:

Yes those photos are useless as is size and weight of a specimen for identification purposes. As for learning Crystallography childrens learning websites are a great place to start as they break things down into simplistic terms everyone can understand. Learn som geometry and learn the terms and use them accurately not just similar looking words as you have in this post.


I shall try the children's learning website; thank you for the recommendation. I'm not sure that's going to help though. I have a degree in mathematics and was published as a sophomore - I think my geometry is relatively solid. I am however, autistic. That things were written for people with a different operating system as I have is often a challenge I face.

SteveB wrote:
There are very precise definitions so use them so you can record with accuracy and communicate with others with meaning, looking up each word in a good dictionary should explain how the word is made up in the first place which itself should tell you its meaning.


I just returned to the forum after trying to educate myself on crystal forms. To do so, I went to a website for Dakota Matrix Minerals. They have very good pictures and an excellent data sheet. I compared close-ups of the crystal structure of three different minerals (aragonite, anglesite, and celestine) that the data sheet provided identified as orthorhombic. The working definition I have for orthorhombic includes the following, "The angle between any two edges is always ninety degrees. Edge lengths however, can be unequal."

The five-sided geometric shape that I looked at appears to have three ninety degree angles and two angles that are approximately one hundred-thirty-five degrees. At each of the vertices there is an edge. Now I very well may be missing something - in fact it is likely that I am, but I'm not observing a ninety degree angle between the two edges created at the vertex of that one hundred-thirty-five degree angle and one at the end of each of the arms.

SteveB wrote:
When they talk about lengths they dont mean get out a ruler and measure, its typically a ratio it refers to. So while you may call something a cube, it may not be because its length, width and height may be different (cube MUST be precisely the same, learn geometry), so pay close attention in your observations and recording of information. Some crystals have well defined precisely measure angles so being able to eyeball angles is important.


I have a pretty good understanding of what a cube is. It had occurred to me however, that is clear cubes were stacked, I wouldn't be able to tell the difference with the equipment I have. I don't know enough about the way these crystals grow yet to know if that is possible. I'll assume from your reaction that it doesn't.

SteveB wrote:
Try: australian museum/learn/minerals/what-are-minerals/crystal-shapes/
Click on each crystal diagram for more information about what defines it, shapes and angles, shapes and angles. Calling a specimen by its broader group name doesn’t help anyone, its just a rough starting place as you investigate the shapes the group contains and make observations and measurements in order to narrow it down more precisely , take note of edges and corners of specimen crystals to see if they appear faceted as faces, this is a defining feature so observe and dont ignore.


Thanks for the link. Yeah, I don't ignore anything - it's an autism thing. Sometimes it helps and sometimes, like what you are talking about below, it doesn't.

SteveB wrote:
Lastly crystal surfaces are rarely perfectly smooth...


Since geometric shapes are precise constants, imperfections lead me to walk away thinking, "Clearly, that's not a cube...or a ninety degree angle...or..." That last bit I quoted might turn out to be the most helpful information moving forward. Speaking of angles, I'm not trying to be obtuse; my way of learning is just different.
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James Catmur
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PostPosted: Jan 03, 2022 15:36    Post subject: Re: Seeking methodology for identifying crystalline forms.  

Angel

He was quoting a message he got via email or PM, so it was not a post on this forum. As Admins we do get direct messages that are useful, so we add them as quotes.
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Bob Carnein




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PostPosted: Jan 03, 2022 18:38    Post subject: Re: Seeking methodology for identifying crystalline forms.  

Angell, For many collectors, crystallography is a frustrating subject. If you're serious, don't rely on online simplifications; some of them are awful, and it isn't easy for the beginner to know the bad ones from the good. You need to find a basic, technical, college-level text (e.g. Dana's Manual of Mineralogy) that covers the subject in a systematic way--and talk with someone who understands it to help you wade through all of the new terminology that you'll need to know.
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PostPosted: Jan 03, 2022 19:51    Post subject: Re: Seeking methodology for identifying crystalline forms.  

While I don't disagree with Bob about the value of a textbook, I suggest you look up a set of pages on Mindat.org. Search for Donald B. Peck Mindat or www.mindat.org/user-41199.html#5. There are a series of pages that discuss the 7 different basic symmetries and their subdivisions and more. They're not perfect and not exactly as I would have written them, but they're good enough to recommend as a starting place.

The cubic system involves three axes of fourfold rotational symmetry at 90° to each other (think 3-D Cartesian coordinate system); a cube is a basic form. Actual crystals often deviate from being a perfect cube, however, though the angles should be more perfect than the dimensions. This is true of all symmetries and all natural crystals, hence crystallographers speak of ideal or perfect crystals, recognizing that real crystals meet the ideal to varying degrees.

Most other crystal systems involve either axes of different lengths (i.e. translation steps in different directions are not the same) and/or some or all angles between the axes are not 90°.

Many crystal faces are not parallel to the axes, so, for example, the cube and the octahedron both belong in the cubic or isometric crystal system. The cube faces are parallel to the axes and the octahedron faces cut each axis at the same distance from the center.

Since you asked about rhombohedra, they are in a different crystal system than cubes. Rhombohedra, like cubes, are equal along three dimensions or axes, but unlike the cubic system the angles are not 90°. There's more and it gets complicated, so I will stop with that part now.

Crystallography/symmetry is a very powerful aspect of mineralogy, one which many people fail to understand in any detail. But I can't count the number of times when I told someone with great certainty that the crystal they were holding was not mineral x because the symmetry, the shape of the crystal, was just not possible for mineral x. Check it out!

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AngelL




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PostPosted: Jan 03, 2022 22:23    Post subject: Re: Seeking methodology for identifying crystalline forms.  

Bob Carnein wrote:
Angell, For many collectors, crystallography is a frustrating subject. If you're serious, don't rely on online simplifications; some of them are awful, and it isn't easy for the beginner to know the bad ones from the good. You need to find a basic, technical, college-level text (e.g. Dana's Manual of Mineralogy) that covers the subject in a systematic way--and talk with someone who understands it to help you wade through all of the new terminology that you'll need to know.


Thank you; the book is in my cart now. You're certainly correct regarding beginners unable to tell the good from the bad. I didn't choose the first one I found and yet, ironically, the link to the child-level informational page at the Australian museum showed examples of crystalline structures under each of the seven symmetries that greatly expanded my understanding. That understanding led me see quite clearly where my error was - and how the error was inevitable. The information I got initially wasn't bad - but there was one example. It would never occur to me there might be a range to encompass imperfections. And that the precision would be on the tolerance set by nature. Without explicitly stating that, the thought would never cross my mind. I've got a background in mathematics and physics, and I'm autistic. 'Close enough' is sacrilege.

[quote="Pete Richards"]While I don't disagree with Bob about the value of a textbook, I suggest you look up a set of pages on Mindat. Search for Donald B. Peck Mindat.
Quote:


I shall; thank you.

[quote="Pete Richards"]There are a series of pages that discuss the 7 different basic symmetries and their subdivisions and more. They're not perfect and not exactly as I would have written them, but they're good enough to recommend as a starting place.

The cubic system involves three axes of fourfold rotational symmetry at 90° to each other (think 3-D Cartesian coordinate system); a cube is a basic form. Actual crystals often deviate from being a perfect cube, however, though the angles should be more perfect than the dimensions. This is true of all symmetries and all natural crystals, hence crystallographers speak of ideal or perfect crystals, recognizing that real crystals meet the ideal to varying degrees.


I don't think you'll be able to appreciate how valuable this was for my personal operating system. That paragraph saved me weeks at least - and a lot of aspirin.

Pete Richards wrote:

Many crystal faces are not parallel to the axes, so, for example, the cube and the octahedron both belong in the cubic or isometric crystal system. The cube faces are parallel to the axes and the octahedron faces cut each axis at the same distance from the center.


Again. Wish I had that eight hours ago but sure glad I know it now. :)
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PostPosted: Jan 04, 2022 12:02    Post subject: Re: Seeking methodology for identifying crystalline forms.  

Pete, Maybe I'm quibbling or misunderstood your post, but isometric crystals often have 3 2-fold rotational axes, rather than 4-fold (some may be 4-fold rotary inversion axes, but that's beyond most beginners).
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PostPosted: Jan 04, 2022 13:23    Post subject: Re: Seeking methodology for identifying crystalline forms.  

Bob, right, four 3-fold axis defines the isometric system! it can have three or none 4-fold symmetry axis according to its class.

This is the link to Don Peck article recommended by Pete: The isometric system

PD: Angel, another interesting site of crystal forms: Smorf
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PostPosted: Jan 04, 2022 14:32    Post subject: Re: Seeking methodology for identifying crystalline forms.  

Bob, Josele,

Bob, you are right that some crystal classes in the isometric system have 2-fold axes rather than 4-fold - notably pyrite - and some have four-fold axes with inversion. Josele is right that the definining - but not obvious - criterion is four three-fold axes. I was trying to stay with the most obvious characteristics and may have over-simplified.

If one is willing to blur separation with details, it is interesting to think of the cubic system as a special case of rhombohedral symmetry as defined with three equal axes - the case where the interaxial angle is 90°. In this position, lots of other symmetry elements "emerge", including the four-fold symmetry and three of the four three-fold axes. It is no accident that the symmetry class of calcite, bar-3 2/m, for Americans at least usually considered part of the hexagonal system, and the most symmetric (holohedral) isometric class, 4/m bar-3 2/m, share many of the same symmetry elements.

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PostPosted: Jan 04, 2022 14:52    Post subject: Re: Seeking methodology for identifying crystalline forms.  

I find it most helpful to know the identity of the mineral first if possible, then look up the crystal class to see what faces might be possible. That's because some non-isometric crystals might look like cubes, when they actually are orthorhombic, just as some isometric crystals might have greater growth along one axis and appear to be orthorhombic.

Another help is to look at the face surfaces. On your specimen there appear to be some interesting striations that might give a clue to both the mineral and to its crystal system.
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