Mineral and their composition

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Discussion Overview

The discussion revolves around the classification of minerals, specifically cummingtonite and anthophyllite, which share the same chemical composition but are categorized differently. Participants explore the implications of mineral nomenclature, polymorphism, and the significance of crystalline structure in mineral identification.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants question why cummingtonite and anthophyllite, despite having identical chemical compositions, are classified as different minerals.
  • There is a discussion about the meaning of the chemical formula -[Mg,Fe]7[Si8O22](OH)2, specifically whether it indicates a fixed number of magnesium and iron atoms or a variation between them.
  • One participant draws a parallel to coal and diamond, emphasizing the importance of structural differences despite identical chemical compositions.
  • Another participant suggests that the formulas describe the unit cell of the crystals and that the differences between the minerals are related to their crystalline structure.
  • Some participants propose that the formula indicates a continuous variability of substitution between magnesium and iron in the mineral structure.
  • There is mention of the ionic radius of Fe2+ and Mg2+ and how it may not significantly affect the crystal structure.
  • Participants express uncertainty about the implications of the chemical formula and its relation to the empirical formula and unit cell descriptions.

Areas of Agreement / Disagreement

Participants generally agree that the classification of these minerals is complex and that structural differences are significant. However, multiple competing views remain regarding the interpretation of the chemical formula and its implications for mineral classification.

Contextual Notes

Participants note limitations in their understanding of mineral nomenclature and the implications of polymorphism, as well as the potential for confusion regarding the chemical formulas used.

Who May Find This Useful

This discussion may be useful for individuals interested in mineralogy, chemistry, and the classification of minerals, particularly those exploring the nuances of mineral identification and nomenclature.

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:

[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.
 
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?
 
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  • #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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