Mineral and their composition

In summary, polymorphism refers to the existence of different chemical compositions between two minerals that have the same name. The nomenclature of minerology is new to the chemist, but the terminology is in use to describe the chemical makeup of minerals. Polymorphism is important to geologists because it affects the crystalline structure of the minerals.
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jbowers9
89
1
I'm currently doing a little research in PCM for asbestos ID. My background is in chemistry and I've had to wade through a good deal of mineral classification terminology. My question is this; If cummingtonite and anthophyllite - two of the six recognized species of commercial asbestos fiber - have identical chemical compositions -[Mg,Fe]7[Si8O22](OH)2 - why are they classified as different minerals? Again, I must stress that my background is in chemistry so the nomenclature of minerology is new to me.
 
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  • #2
jbowers9 said:
I'm currently doing a little research in PCM for asbestos ID. My background is in chemistry and I've had to wade through a good deal of mineral classification terminology. My question is this; If cummingtonite and anthophyllite - two of the six recognized species of commercial asbestos fiber - have identical chemical compositions -[Mg,Fe]7[Si8O22](OH)2 - why are they classified as different minerals? Again, I must stress that my background is in chemistry so the nomenclature of minerology is new to me.
Polymorphism:
http://en.wikipedia.org/wiki/Polymorphism_(materials_science)

Cummingtonite is monoclinic, anthophyllite is orthorhombic:
http://en.wikipedia.org/wiki/Cummingtonite
 
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When the nomenclature as written is -[Mg,Fe]7[Si8O22](OH)2 - does this mean that seven magnesium and seven iron atoms occur in each molecule, or is it a variation between the two, ie --[Mg]7[Si8O22](OH)2 - [Mg,Fe]7[Si8O22](OH)2 - -[Fe]7[Si8O22](OH)2 ?
 
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Remember coal and diamond are the same chemical compound - but the difference is important to geologists!
Sorry I haven't seen the [Mg,Fe] nomenclature before.
 
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jbowers9 said:
When the nomenclature as written is -[Mg,Fe]7[Si8O22](OH)2 - does this mean that seven magnesium and seven iron atoms occur in each molecule, or is it a variation between the two, ie --[Mg]7[Si8O22](OH)2 - [Mg,Fe]7[Si8O22](OH)2 - -[Fe]7[Si8O22](OH)2 ?
It means Fe or Mg, so:

[tex]Mg_7 (Si_4 O_{11} )_2 (OH)_2 [/tex]

or

[tex]Fe_7 (Si_4 O_{11} )_2 (OH)_2 [/tex]

but I don't remember if, in the same macroscopic crystal, it means there are both Fe and Mg or if there can be Fe only or Mg only.
 
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I think those formulas describe the unit cell of the crystals. What each of the very smallest crystals contain, so, the microscopic crystals. And yeah, the difference between them is entirely related to their crystalline structure.

I think the two formulas mean each unit cell of the crystal either has magnesium or iron in it, but a Mg crystal can be next to a Fe.
 
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Wow

Thanks for the heads up guys. This minerAlogy stuff is scary. Luckily, I'm in NYC. So I get to tool around the Museum of Natural History's mineral hall, in addition to checking out other stuff in the permanent collection, which is cool too. Anyone got any hints on what I could read as a VERY BASIC primer on X-ray chrystallography?
 
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The [Fe,Mg]7 refers to a continuous variability of substitution of these two cations in the mineral. The mineral can be composed of pure fractions of the iron-substituted variety or of the magnesium-substituted variety or of random cosubstitution of both cations. The forumula is simply the empirical formula for the mineral. The unit cell may be different than the empirical formula description. Halite, for example, has a unit cell chemical description of Na4Cl4 and an empirical formula of NaCl. Quartz's unit cell is composed of Si3O6 while it's empirical formula description is SiO2.
 
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chemisttree said:
The [Fe,Mg]7 refers to a continuous variability of substitution of these two cations in the mineral. The mineral can be composed of pure fractions of the iron-substituted variety or of the magnesium-substituted variety or of random cosubstitution of both cations. The forumula is simply the empirical formula for the mineral. The unit cell may be different than the empirical formula description. Halite, for example, has a unit cell chemical description of Na4Cl4 and an empirical formula of NaCl. Quartz's unit cell is composed of Si3O6 while it's empirical formula description is SiO2.

That is to say that Fe2+ and Mg2+ behaves as indistinguishables, in that crystal? Possibly their difference in ionic radius (Fe2+ = 83.5 pm; Mg2+ = 86 pm) is not enough to create a different crystal?

P.S. Why I can't see the preview or save the post if I write Mg2+ with two "+"? Did I discover some hidden command?
 
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Yes, that is correct
 
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chemisttree said:
Yes, that is correct
All the three questions?
 
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It won't allow g'+''+'. but it will allow e++. How about Fe++? Apparently, yes.
 
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chemisttree said:
It won't allow g'+''+'. but it will allow e++. How about Fe++? Apparently, yes.
Very interesting, probably we have discovered the "g" point of the site :biggrin:
 

1. What is a mineral?

A mineral is a naturally occurring, inorganic solid substance with a specific chemical composition and a characteristic atomic structure. It is typically formed through geological processes and has a defined physical and chemical properties.

2. How are minerals classified?

Minerals are classified based on their chemical composition and crystal structure. There are several mineral classification systems, but the most commonly used is the Dana classification, which categorizes minerals into classes, subclasses, and groups based on their chemical composition and crystal structure.

3. What is the composition of a mineral?

The composition of a mineral refers to the elements and chemical compounds that make up the mineral's chemical formula. It is determined by analyzing the mineral's physical properties, such as color, hardness, and streak, and conducting various laboratory tests, such as X-ray diffraction and spectroscopy.

4. How do minerals form?

Minerals form through various geological processes, such as crystallization from molten rock (igneous rocks), precipitation from a solution (sedimentary rocks), and solid-state changes due to heat and pressure (metamorphic rocks). Some minerals also form from biological processes, such as the formation of shells and bones.

5. What is the importance of mineral composition?

The composition of minerals plays a crucial role in determining their physical and chemical properties, which have significant implications in various fields such as geology, mining, and industry. Understanding mineral composition also helps in identifying and categorizing minerals, as well as predicting their behavior and potential uses.

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