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The structure of Borax has really baffled me. Its formula is $\ce{Na2B4O7.10H2O}$ and it is formulated as $\ce{Na2[B4O5(OH)4].8H2O}$.
2 structures of Borax given on the internet are

Image by ironchemical.com

Image by mpbio.com

Can anybody explain that why the formulas mentioned on different websites are different.Something is strange about this compound because even Wikipedia does not give the structure.

It only says

there are two four-coordinate boron atoms (two $\ce{BO4}$ tetrahedra) and two three-coordinate boron atoms (two $\ce{BO3}$ triangles).

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Greenwood and Earnshaw's Chemistry of the Elements, 2nd ed. (p 206) writes that the structures of the borates are rather complicated:

It is clear [...] that, without structural data, the stoichiometry of these borate minerals gives little indication of their constitution.

As such, for example, $\ce{KB5O8.4H2O}$ is more properly formulated as $\ce{K[B5O6(OH)4].2H2O}$. You've also noticed that borax, $\ce{Na2B4O7.10H2O}$ is more properly formulated as $\ce{Na2[B4O5(OH)4].8H2O}$. This means that the structure of borax does not simply consist of $\ce{Na+}$ ions and $\ce{B4O7^2-}$ anions with waters of crystallisation.

Therefore I would guess that both structures are, more likely than not, incomplete.

A further illustration is afforded by borax which is normally formulated $\ce{Na2B4O7.10H2O}$, but which contains tetranuclear units $\ce{B4O5(OH)4]^2-}$ formed by fusing 2 $\ce{B3O3}$ rings which each contain 2 $\ce{BO4}$ (shared) and 1 $\ce{BO3}$ unit; borax should therefore be written as $\ce{Na2[B4O5(OH)4] .8H2O}$.

enter image description here

(grey - boron; white - oxygen; black - hydrogen).

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  • $\begingroup$ SO, is the structure you mentioned is the true anion part of Borax?Also,is the structure verified by means like spectroscopy etc?Thanks for your answer. $\endgroup$
    – Amritansh Singhal
    Commented Sep 18, 2016 at 14:07
  • $\begingroup$ Yes, I assume this is the true anion. Don't know, a bit too lazy to look up. $\endgroup$
    – orthocresol
    Commented Sep 18, 2016 at 14:16
  • $\begingroup$ Just wondering, if the true anion is known, then why do most of the first few structures in google image search depict the incorrect ones?And why wikipedia avoids to provide its version?(sorry if the question appears silly) $\endgroup$
    – Amritansh Singhal
    Commented Sep 18, 2016 at 14:27
  • $\begingroup$ I don't know why that would be the case. $\endgroup$
    – orthocresol
    Commented Sep 18, 2016 at 14:30
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    $\begingroup$ @AmritanshSinghal This structure was verified by X-ray structure analysis, I have a paper link in my A: What is the IUPAC name of borax? $\endgroup$
    – mykhal
    Commented Sep 24, 2018 at 2:04

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