Mineral and their composition

AI Thread Summary
Cummingtonite and anthophyllite, both classified as commercial asbestos fibers, share the same chemical composition of [Mg,Fe]7[Si8O22](OH)2 but differ in their crystalline structures—cummingtonite is monoclinic while anthophyllite is orthorhombic. The notation [Mg,Fe] indicates a continuous variability in the substitution of magnesium and iron within the mineral's structure, allowing for compositions that can be purely magnesium, purely iron, or a mix of both. This distinction is crucial in mineral classification despite their identical chemical formulas. The discussion also touches on the importance of understanding unit cells in crystallography, noting that the empirical formula may not always reflect the unit cell's composition. Additionally, the forum participants explore the implications of ionic radii on crystal formation and share insights on basic resources for learning about X-ray crystallography.
jbowers9
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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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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
 
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 ?
 
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.
 
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:

Mg_7 (Si_4 O_{11} )_2 (OH)_2

or

Fe_7 (Si_4 O_{11} )_2 (OH)_2

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.
 
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.
 
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?
 
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.
 
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?
 
Last edited:
  • #10
Yes, that is correct
 
  • #11
chemisttree said:
Yes, that is correct
All the three questions?
 
  • #12
It won't allow g'+''+'. but it will allow e++. How about Fe++? Apparently, yes.
 
  • #13
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:
 

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