Difference - chemical formula and Idealised chemical formula

davenn

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Hi guys
I'm into rocks and minerals but I'm not a chemist
I cant really find a definition between .... chemical formula and Idealised chemical formula ?

example below of a mineral sample I purchased a few days ago

part info from ...
https://www.mindat.org/min-3760.html

Chemical Properties of Stellerite


Formula:
Ca4(Si28Al8)O72 · 28H2O
Idealised Formula:
Ca(Si7Al2)O18 · 7H2O

Now, as a non-chemist, I look at that and wonder about the significance between the two formula ?
how can the upper formula be reduced to the lower idealised formula ?

OK, one light just came on as I was writing this ... I note that everything in the top formula has been divided by 4.
So, my Q still remains, does this really affect the composition of the mineral ?
if not, as I suspect, why list it as the first when the second formula can be used ?


Thanks Lots
Dave
 

Wrichik Basu

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The idealised formula seems to be another name for an empirical formula, which gives the proportions of the elements present in a compound but not the actual numbers or arrangement of atoms.

Maybe for minerals with large structures, empirical formulae become useful as they are able to give you the knowledge of which elements are present and in what proportions. The use of the idealised formula in mineral sciences might be the same as that of empirical forumla in chemistry. I believe one of the uses should be calculation of percentage composition of constituent elements, which is important in metallurgy while choosing an ore.
 

davenn

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The idealised formula seems to be another name for an empirical formula, which gives the proportions of the elements present in a compound but not the actual numbers or arrangement of atoms.

yes, possibly

Maybe for minerals with large structures, empirical formulae become useful as they are able to give you the knowledge of which elements are present and in what proportions.
A good thought :smile:

I don't know who the chemists are on PF
Hopefully some one will chip in and clarify, else I will have to see which mentor I can tag :wink:


Dave
 

Borek

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Being a chemist I have never seen the term, so I have no idea what it means - and googling doesn't help much. Chances are it is used only in some specific trade (and/or is probably not well defined).

English is my second language so there is a slight chance I am missing something.
 

jim mcnamara

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Apparently the term is is commonly used in discussing minerals.
In Biology -specifically algology:
Carageenan is a linear sulfated polysaccharide that has a varying and huge molecular weight.
It is derived from red algae, and is used to change consistency of foods with high liquid content. A gel agent.

So the idealized formula attempts to show a hypothetical base configuration/molar relationship of atoms - one that may not really exist in nature. Carageenan is, I think, an extreme example. Because of the range of numbers of the repeating subunits of the molecules varies A LOT in a given glob of substance. So you have sort of a "family" of molecules, with an identical repeating component.

https://en.wikipedia.org/wiki/Carrageenan

I'm not sure that helps very much. I tried....
 

Bystander

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If you google "silicates" you'll start to get an idea.
 

davenn

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So the idealized formula attempts to show a hypothetical base configuration/molar relationship of atoms - one that may not really exist in nature. Carageenan is, I think, an extreme example. Because of the range of numbers of the repeating subunits of the molecules varies A LOT in a given glob of substance. So you have sort of a "family" of molecules, with an identical repeating component.

Thanks Jim

It's common usage in mineralogy. I have seen it for years as in the link I provided and in mineral identification books
and have only just recently started to wonder about why it is done.
It's almost like they are stating the full "complex" formula and then a simplified one ( idealised)


Thanks for responses everyone :smile:


Dave
 

TeethWhitener

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The idealized formula = empirical formula. The reason the formula above is quadrupled can be seen by looking at the mineral's unit cell:
http://www.iza-online.org/natural/Datasheets/Stellerite/Stellerite.html
You can see that each unit cell is made up of 4 subunits that are identical except that they've been rotated/reflected relative to one another. So even though the idealized formula gives the ratios of the elements, the full formula imparts the notion that the unit cell is made up of 4 symmetry inequivalent but chemically identical subunits. It's like saying that your hands are "finger-ly identical" but symmetrically inequivalent (being related by a reflection in plane).
 

DrDu

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I only know the idealized formula concept in mineralogy as refering to a formula where typical substitutions are ignored, i.e. writing Mg2 SiO4 instead of [Mg,Fe]2 SiO4 for Forsterite.
In chemistry you sometimes see P2O5 and sometimes P4O10 without the former sum formula being called idealized. What do you do if a unit cell contains several identical molecules, like for example glucose, which contains 4 molecules per unit cell? Would you also write C24H48O24 instead of C6H12O6?
 

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