Questions tagged [crystallography]

questions relating to the arrangement of atoms in solids, such as the ways in which crystal structure influence material properties, or suitable analytical methods e.g. X-ray diffraction and microscopy

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Why do snowflakes form into hexagonal structures?

Snowflakes are known to form into pretty hexagonal structures. The image below shows a variety of such structures that are possible (although by all means not an exhaustive list): What is the ...
Kenshin's user avatar
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23 votes
2 answers
21k views

Why do crystals, like quartz and diamonds, form in different colors?

Some crystals, like quartz and diamonds, form in a variety of different colors. For example, quartz can be clear, 'foggy', pink, yellow, and even blue. What happens during the crystal's formation to ...
Azzie Rogers's user avatar
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18 votes
3 answers
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What determines a mineral's hardness?

Mohs Hardness Scale is basically a measurement of the hardness of a material, or more specific, a rock or mineral. But I have never been able to figure out why some minerals are harder than others.
Azzie Rogers's user avatar
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17 votes
2 answers
21k views

Why does the colour of amethyst fade if exposed to too much light?

It is well known that the purple hue that is characteristic of the amethyst quartz ($\ce{SiO2}$) will fade if the crystal is exposed to light for a prolonged period of time. What is the underlying ...
Kenshin's user avatar
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14 votes
3 answers
7k views

How and where do double-terminated crystals form?

How and where do double-terminated crystals form? Most terminated crystals form by 'growing' out of a rock. Which causes one side to be flat,(or whatever the shape of the rock it 'grew' out of was ...
Azzie Rogers's user avatar
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13 votes
3 answers
7k views

What is the white coating "patina" we often see on deposited chalcedony/agate?

Me and some of my friends were wondering what exactly is the white coating we can see on chalcedony that has been deposited by a river during quartenary (and so the original material is damaged and ...
Petr Hykš's user avatar
13 votes
1 answer
906 views

Why does halite have perfect cleavage at the (110) plane?

Why doesn't it cleave nicely along the (100) or (010) or any other planes? What determines its cleavage plane? I know it has something to do with the fact that it crystallizes in face centered cubic ...
Tamás's user avatar
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13 votes
1 answer
338 views

Why does aragonite form in one case and an acetate in another?

If I add acetic acid to limestone, I can get calcium acetate crystals to grow but if I add acetic acid to dolomite, supposedly I can grow aragonite which is a form of calcium carbonate. Why would I ...
Chris Winchell's user avatar
10 votes
5 answers
6k views

Are there minerals which are not been found on earth but which are still possible to exist?

On Earth we find many kind of minerals, a lot of them are silicates. How much different kind of minerals would there be on Earth? But are there Fe minerals found in meteorites which do no occur on ...
Marijn 's user avatar
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10 votes
1 answer
6k views

What kind of minerals have no cleavage plane?

Why does quartz have no cleavage plane and halite does? What do I need to know about the specific mineral so that I can decide whether it has cleavage or not? I need some kind of rule.
Tamás's user avatar
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9 votes
3 answers
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Is every space group realized by some mineral?

Every mineral has it's own internal building scheme, a lattice on which it's atoms lie. Mathematically, this can be described by the space group of the mineral. From mathematical considerations it's ...
agb's user avatar
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7 votes
2 answers
19k views

Why are there no crystals with 5-fold symmetry?

They say you can't pack objects like pentagons or octagons such that they fill up space entirely and that that's one reason there is no 5-fold or 8-fold rotational axis. Are you telling me you can ...
Tamás's user avatar
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6 votes
2 answers
230 views

When diamonds "migrate" from deep underground to the surface, do they maintain pressure inside when there is no more pressure outside? If so, how?

From Science News' A mineral found in a diamond’s flaws contains the source of some of Earth’s heat: A tiny bit of rock trapped inside a diamond is now opening a brand-new window into what the planet’...
uhoh's user avatar
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6 votes
1 answer
292 views

Does Xenon really covalently bond to oxygen within quartz?

Wikipedia's Xenon; occurrence and production says Within the Solar System, the nucleon fraction of xenon is $\small\mathsf{1.56 \times 10^{-8}}$, for an abundance of approximately one part in 630 ...
uhoh's user avatar
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6 votes
1 answer
155 views

Identifying trichroic crystal systems

An unknown mineral shows three distinct pleochroic colours. Explain what observations you need to make in order to decide which crystal system it belongs to. If it has three pleochroic colours this ...
bon's user avatar
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6 votes
0 answers
99 views

Why does asbestos grow in a hierarchy of separable fibers of separable fibrils; why don't they stick to each other or grow into each other?

Wikipedia's Asbestos begins: Asbestos (/æsˈbɛstɒs, æz-, -təs/) is a naturally occurring fibrous silicate mineral. There are six types, all of which are composed of long and thin fibrous crystals, ...
uhoh's user avatar
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5 votes
0 answers
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Can humidity changes induce stacking-sequence changes?

My understanding is that some minerals with layered crystal structures will reversibly absorb water with changes in humidity. Are there any minerals where this reversible absorption leads to a ...
Max Radin's user avatar
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4 votes
1 answer
193 views

How can the 'crystal cleavage' of apatite have a four-digit number?

On Wikipedia's page for apatite, the crystal cleavage is listed as [0001] or [0101], which I assume are Miller indices.... But how can there be four numbers/digits instead of two, like there usually ...
Kurt Hikes's user avatar
4 votes
1 answer
200 views

Reciprocal space in the context of x-ray diffraction and a crystal lattice?

Could someone please explain what reciprocal space means in the context of of x-ray diffraction of a crystal lattice?
user11185's user avatar
4 votes
1 answer
900 views

Applications of differential/integral calculus in crystallography (teaching undergraduates)?

I'm a mathematics professor who is seeking to find interesting, application-driven ways of teaching freshmen college students differential/integral calculus. I am in no way versed in crystallography ...
Sinister Cutlass's user avatar
4 votes
0 answers
45 views

What is the name for a ABC hcp lattice that is squeezed along the direction vertical to the ABC planes?

I am trying to classify a bravais lattice. It is a hexagonaly close packed lattice with (ABC) planes, that is fcc, but it is squeezed along the stacking direction, so perpendicular to the planes. Just ...
Dominik's user avatar
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3 votes
1 answer
3k views

What's the difference between cleavage and fracture?

I see both terms refereing to fractures on crystals or minerals, so I wonder if they could be considered synonymous. If no, what's the difference between both terms?
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3 votes
2 answers
5k views

What exactly are enantiotropic and monotropic polymorphic transitions?

Recently, I've been studying the subject of polymorphism and met two important concepts, the monotropic and enantiotropic polymorphic transition types. However, I can't really seem to grasp the ...
Matheus's user avatar
  • 348
3 votes
1 answer
617 views

Significance of crystal classes in crystallography

I am studying crystals and I learned that there are 32 crystal classes with well-described data regarding symmetries. I have visited a few sites and am having difficulty determining the significance ...
kettlecrab's user avatar
3 votes
1 answer
128 views

How do cations control the crystal structure of silicate minerals?

Beyond the obvious fact that in many of the silicate subtypes they play an ionic 'glue' like role by bonding the tetrahedra units together; how do cations control the structure of a silicate and how ...
Lewis's user avatar
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3 votes
1 answer
34 views

What is a good definition for "lattice bound" and is iron within octahedral sites of illite considered lattice bound or interstitial?

The title pretty much says it all. I cannot find a satisfactory definition of "lattice bound", just a number of examples of it being used in literature. For example, in this paper about ...
Josiah's user avatar
  • 123
3 votes
2 answers
112 views

Iron-nickel phase in pallasite meteorites

I have recently been learning and researching about meteorite types and classification. Pallasites are mainly composed of a 'solid-solution' of olivine crystals and an iron-nickel alloy. My question ...
PlanetGazer8360's user avatar
2 votes
1 answer
109 views

How can a crystal habit be round?

I am confused how hematite can have a "botryoidal" habit when its crystal structure is trigonal, which is an angled structure. How can an angled crystal present itself as rounded? If a botryoidal ...
Tyler Durden's user avatar
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2 votes
1 answer
344 views

An oblique-square-prism crystal?

Does any mineral crystallize in the shape of an oblique square prism? If so, what crystal system (monoclinic, cubic, hexagonal, etc.) would such a mineral fall under?
resplaine's user avatar
2 votes
1 answer
431 views

Difference between crystal and mineral

What is the exact difference between a crystal and a mineral?
Jithin Murali's user avatar
2 votes
1 answer
159 views

Voronoi-like pattern in metal samples

What is this pattern in the samples' surface called and how/why is it formed? I remember having seen a similar pattern in galvanized sheets. Photos of metal samples from Wikimedia Commons (here and ...
amyspark's user avatar
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2 votes
0 answers
321 views

What stone/crystal is it? [closed]

I found this stone/crystal when I walked down the street few days ago. I picked it up since it looks very different with other stones in the area, I am really curious about what stone/crystal it is, ...
Gundyam's user avatar
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2 votes
0 answers
45 views

Can crystal lattices be analogue? [closed]

This question might turn out to be more about physics then geology so apologies in advance if that is the case. Years ago I read something that said in crystals "atoms have finite positions along the ...
Tailspin's user avatar
1 vote
2 answers
58 views

What does the unit cell of petitjeanite look like?

The question in Chemistry SE What is the crystal structure of bismuth oxyhydroxyphosphate (BOHP)? is about a catalyst synthesized by accident. A link in this comment points to the page https://www....
uhoh's user avatar
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1 vote
1 answer
44 views

How can crystal habit be determined when rock is worn and rounded?

In many cases when I find interesting rocks they are worn and rounded off so the crystalline structure is not clear. How can I determine what kind of crystal it is in those cases? Is there a simple ...
Tyler Durden's user avatar
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1 vote
0 answers
21 views

Good modern paper demonstrating differentiation of glauconite and berthierine using xrd or FTIR

I'm looking for a recent paper that demonstrates how one would unambiguously differentiate glauconite from berthierine and have yet to find something useful. I'm running across a lot of papers seeming ...
Josiah's user avatar
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1 vote
0 answers
37 views

Help me find a certain type of crystallographic notation

I was looking through wikipedia a few weeks ago while half asleep, reading articles about rocks and their chemical compositions. I then came across a certain crystallographic notation which I had not ...
paulina's user avatar
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1 vote
0 answers
38 views

Crystallography: The shape of axial divergent profiles

I don't know if crystallography related topics go into this stack exchange, so sorry if that's wrong here. My question concerns the characteristic shape of axial divergent powder patterns. According ...
Justanotherchemist's user avatar
-2 votes
1 answer
212 views

To find hardness in Mohrs scale [closed]

Except diamondת which mineral is having high hardness in Mohr scale. A Oxide B silicate C carbonate D phosphate
Girish Kumar Chandora's user avatar