# Mineral and their composition

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1. Mar 13, 2008

### jbowers9

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.

2. Mar 13, 2008

### lightarrow

Polymorphism:
http://en.wikipedia.org/wiki/Polymorphism_(materials_science)

Cummingtonite is monoclinic, anthophyllite is orthorhombic:
http://en.wikipedia.org/wiki/Cummingtonite

3. Mar 13, 2008

### jbowers9

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 ?

4. Mar 13, 2008

### mgb_phys

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.

5. Mar 13, 2008

### lightarrow

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.

6. Mar 13, 2008

### LtStorm

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.

7. Mar 14, 2008

### jbowers9

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?

8. Mar 14, 2008

### chemisttree

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.

9. Mar 14, 2008

### lightarrow

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: Mar 14, 2008
10. Mar 14, 2008

### chemisttree

Yes, that is correct

11. Mar 14, 2008

### lightarrow

All the three questions?

12. Mar 14, 2008

### chemisttree

It won't allow g'+''+'. but it will allow e++. How about Fe++? Apparently, yes.

13. Mar 15, 2008

### lightarrow

Very interesting, probably we have discovered the "g" point of the site