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Hot answers tagged crystallography

38

Ice grows in many forms. As mentioned in the other answer, all of the ice we are going to observe is Ice Ih, but there are many other forms. See this phase diagram of water: Image courtesy of Cmglee on wikipedia The different ice regimes grow different crystalline shapes. Ice Ih grows hexagonal crystals and in certain regimes you can find triangular and ...

26

H2O ice on Earth crystallizes with a structure called "Ice-Ih" which is hexagonal. The structure is dependent on the dipole properties of H2O-molecules. Similar to what water does with ions to bring them into solution, the crystal structure is dependent on the energetically favourable alignment of the dipoles. In Ice-Ih the most favourable alignment is a (...

18

To add to both Spießbürger's and casey's excellent answers, hydrogen bonds are the reason why some snowflakes are six-sided. His was alluded to, but I think it could use a bit more extrapolation. www.physicsofmatter.com The image above shows an oxygen atom bonding with two hydrogen atoms (water). We can call these covalent bonds for our purposes, although ...

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The hardness of minerals is diagnostic because the hardness is determined by the strength of bonds and the structure of the mineral lattice. Hardness is basically the stress required to create and grow extended lattice defects such as micro-fractures, stress twins, and dislocations. Diamond, quartz, and framework silicates, such as feldspar, are hard ...

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The reason minerals like quarts and diamonds vary in color is generally caused by the chemical elements involved while the crystal is being formed. Chemicals Different colors can be created by different chemicals. Amethyst for example has traces of iron built into its crystalline structure giving it a purple hue. Iron can also give crystals a yellow hue. [...

10

Double-terminated crystals can from by crystallizing from a melt. The crystallization nucleus has to float freely in the magma chamber. As long as no other crystals obstruct the growth the crystal will grow in its own characteristic form (euhedral). This happens for example with feldspar crystals. (Example: http://www.erdwissen.ch/wp-content/uploads/2010/12/...

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To answer the first part, there are many minerals on earth which do not involve silicate bonded structures. For instance, non-silicates (minerals of carbonates, sulfides, sulfates, phosphates, and oxides are common non-silicate varieties). For a detailed list of mineral species, you should check out the Dana Classification here: Minerals Arranged by the New ...

9

I'd like to elaborate of the Chemicals issue of Azzie Rogers' answer. You can divide the chemical coloring into three main parts (there may be more, but these are the important ones): Inclusions A large, solid crystal can have tiny inclusions of other solid minerals. Commonly these inclusions are too small to individually observe by the naked eye. ...

7

All unit cells are parallel-sided hexahedra. These are six sided shapes with parallel opposite sides. Their three principle angles may or may not be 90 degrees. And the three side lengths may or may not be equal. All of these unit cells can be uniformly stacked. Using these building blocks it is only possible to produce planes of reflection, diads (axis of ...

6

This is not a four- digit number but four separare numbers called Bravais-Miller indices. Bravais-Miller indices descrive the orientation of a crystal plane relative to the symmetry axes of a crystal, as described in Wikipedia. Put very briefly, a zero index means the plane is parallel to that axis, while nonzero indices encode the relative values of the ...

5

Now all of the 230 space groups are realized by a natural occurring mineral. I found this blog: crystalsymmetry.wordpress.com where space groups are listed with at least one chemical compound is listed for each space group. Minerals are listed for all of the space groups. For example, Benitoite has fairly unusual crystal symmetry. I think it was first ...

5

There are some subtleties that I'd like to add, in addition to Mark's answer. When talking about the hardness of a mineral, the nature of the chemical bonds in the crystal structure (e.g. covalent vs ionic) are not the only important thing. Crystal morphology is also important. For example, Si-O-Si and Al-O-Al bonds usually cause minerals to be hard, such ...

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I'm pretty sure a more rigorous answer deserves to come along, but I can give a simple overview of some of the important factors. Cleavage planes have to do with bond strength and bond geometry. If there isn't a plane of bonds that can be cut through, then you won't get cleavage. When a mineral is fractured, the fracture "wants" to take the path where the ...

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Not sure this is appropriate for Earth Science SE (Chemistry SE would be a better fit), but the answer is "maybe". quoting from the same Wikipedia article: Three oxides of xenon are known: xenon trioxide ($\small\mathsf{XeO_3)}$ and xenon tetroxide $\small\mathsf{(XeO_4)}$, both of which are dangerously explosive and powerful oxidizing agents, ...

4

The color of a mineral can be caused by a variety of mechanisms. This is also true of amethyst, which is a variety of quartz ($\ce{SiO2}$), and can be found in many colors. The major factors responsible for the production of color in minerals fall into five categories: The presence of an element essential to the mineral composition The presence of a minor ...

4

Your question is much more complicated from mathematical point of view than it seems to be. First, I'll start with a nice photo: (source: Wikipedia). What you see is really a photo and it is almost a mono-crystal. The only problem is that we all know that it cannot be a monocrystal since it cannot tile the space. So what is it? A quasicrystal -- matter with ...

4

The white "patina" is most likely some form of opal as you suggest. I did some research and I'll explain how the agate becomes opal and why it's more common in the eroded rock. Some background: agate and opal are both forms of silica with the chemical formula SiO2 opal is a hydrated amorphous form of silica (1) You would find the opal more often on ...

4

Although this is a somewhat poorly researched question, I think there is an opportunity to make a nice point here. First of all, let's look at the Mohs scale: Talc Gypsum Calcite Fluorite (fluor)Apatite K-Feldspar Quartz Topaz Corundum Diamond Their (simplified) chemical formulae are: $\ce{Mg3Si4O10(OH)2}$ $\ce{CaSO4\cdot2H2O}$ $\ce{CaCO3}$ $\ce{CaF2}$ $\... 4 Botryoidal habit describes the arrangement of a large group of hematite crystals. While hematite's trigonal crystal system describes the geometry of a single unit crystal of hematite, the two terms are really not related at all. Many minerals with different crystal systems can have a botryoidal habit. The difference between a Botryoidal habit and ... 4 They are not synonymous. Cleavage means breaking along planes defined by crystallographic directions. For example, cubic crystals like halite, NaCl, often cleave along directions that follow the cubic form. However, with a different bond strength, like fluorite$\ce{CaF2}\$, the crystals cleave most easily along octahedral directions, similar to two ...

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The question is in regard to pressure confining a rare, deep-mantle formed mineral visible within a diamond inclusion. The pressure on the inclusion within the diamond crystal is really the pressure of confinement within the crystalline lattice of carbon that makes a diamond what it is. Let's digress for a moment. A diamond is a covalent crystal. Each ...

3

The unit cell space and reciprocal space are fourier transforms of each other. The unit cell indicates the stacking space between crystal elements. The reciprocal space is a similar kind of vectorial representation of the diffracted X-rays. The following reference explains it quite well, although it is not a simple concept. Unless you are smarter than me, ...

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To put things in perspective, as Tachylite says, there are about 5150 known natural mineral species on Earth. Despite more mineralogists and better analytical equipment than ever before, the discovery of new minerals has dropped to a mere trickle. In some years there are no new mineral discoveries, although four new minerals were discovered both last year ...

3

The references you and @farrenthrope cited (or a good introductory mineralogy text) should have answers. In brief: The crystallographic class is significant to mineralogy because it is direct result of the mineral's crystalline lattice (its most fundamental property.) Each mineral can belong to only one crystallographic class. How can I determine the [...

3

If you can get your hands on a copy of Fegley's book Practical Chemical Thermodynamics for Geoscientists, he gives a good description in Chapter 07, which I'll summarize here. An enantiotrope is a polymorph that undergoes a reversible transformation into another polymorph at atmospheric pressure. If you're not familiar with the concept of reversibility you ...

3

If "oblique square prism" is this, where the top and bottom planes are parallel, then yes, there are. Carbonates (members of the Calcite and Dolomite, but not Aragonite, groups) have perfect cleavages that leave them with such habit. But be warned that they do not crystallize with such habit, it is a product of an effort that forced it to break accordingly ...

3

Should one assume that the mineral petitjeanite and the chemical discussed in the recent Chemical & Engineering News article Photocatalyst shreds drinking water contaminant PFOA are probably the same crystal configuration and unit cell, or can there be some variety? No you cannot assume the structure from the formula, many materials have polymorphs (...

2

Double terminated crystals normally form in free floating pockets of liquid that slowly evaporated, leaving perfectly formed crystals with terminations on both sides. Unlike usual quartz formations which grow in igneous rock, double terminated crystals are more than likely found in sedimentary rock laid down by subterranean marine deposits a long time ago. ...

2

The question asked is "why do snowflakes form into hexagonal structures". I see snowflakes as following the framework of a flattened cuboctahedron.

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