What does a comma signify in inorganic chemistry?ida ils Tg lewmr esunBskimrSps N
$\\ce{(Ca{,}Co)CO3}$
I have never ever seen this before. What does this mean exactly?
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1$\\begingroup$ It just means that relative amounts of calcium and cobalt are indeterminate. $\\endgroup$ – MaxW 12 hours ago
3 Answers
Quoting from here:
In accord with general usage, the comma "," should be used to indicate continuous substitution, partial as well as complete, if there is no change of CN. Vacancies (symbol ") are treated here just like atoms. This usage is restricted to chemical formulas for atoms on a specific site. The comma is not appropriate for structure site formulas because different symbols indicate a change in the coordination number. Examples for proper use of the comma include forsterite-fayalite [$\\ce{(Mg,Fe)2(SiO4)}$] and hornblende $\\ce{(Na2,Ca )Ca(Mg,Fe,Al)_5[(Si,Al)4O11]2(OH)2}$. In structure type formulas, the comma is used to mean "or" where substitution may occur. An example is in the zeolites where H2O and M units may both occur.
Comma representation are often used in writing chemical compositions of minerals as the elements and its concentration vary to a certain extent. See here:
A number of minerals contain a varying amount of two or more elements like Aurichalcite, which has a chemical formula of $\\ce{(Zn,Cu)5(CO3)2(OH)6}$ contains an unspecific varying amount of zinc (Zn) and copper (Cu). This is indicated by comma separating the Zn from the Cu. If a chemical formula with two elements in parenthesis is separated by a comma, the number of those elements vary. Aurichalcite has a variable amount of zinc of copper where the combination of both these elements totals five. The more dominant element is usually listed first.
For more information, see this previous chem.SE question and see here to find steps regarding how to write such type of formulae using chemical markups
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$\\begingroup$ For hornblende the formula isn't quite accurate. When you add sodium the calcium is diminished so that you really have $\\ce{(Na_2,Ca)Ca}$ and then the rest. You go between two calcium ions at one and and one calcium plus two sodium at the other. $\\endgroup$ – Oscar Lanzi 8 hours ago
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$\\begingroup$ It must have been some sort of typo. @Oscar $\\endgroup$ – Nilay Ghosh 7 hours ago
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$\\begingroup$ It's all good now. $\\endgroup$ – Oscar Lanzi 6 hours ago
A short “obligatory” reference from IUPAC Recommendations [1, p. 21], section IR-2.2.3.1 regarding use of parentheses in formulae:
(d) In solid-state chemistry, to enclose symbols of atoms occupying the same type of site in a random fashion. The symbols themselves are separated by a comma, with no space.
Example:
- K(Br,Cl)
Related question on chemistry typesetting in $\\mathrm\\LaTeX$: Comma in chemical formula.
References
- IUPAC “Red Book” Nomenclature of Inorganic Chemistry, 1st ed.; Connelly, N. G., Damhus, T., Hartshorn, R. M., Hutton, A. T., Eds.; IUPAC Recommendations; Royal Society of Chemistry: Cambridge, UK, 2005. ISBN 978-0-85404-438-2. PDF
I have seen this nomenclature used as a shorthand to indicate a solid solution of two different ionic compounds that share a common anion. For example:
https://www.ncbi.nlm.nih.gov/pubmed/22011346
and:
https://www.sciencedirect.com/science/article/pii/S0016703714004888
Since it's a solution of two different compounds, its composition can be variable.
