Opal-A speleothems from a variety of caves show a wide range of composition, containing up to 5% Al2O3 and MgO. The infrared spectra of the samples show that the Al is substituting for Si, with OH groups compensating for the resultant charge imbalance. The Mg occurs as interstitial ions to which OH groups are bonded to balance the charge. Thus the hydroxyls in opal are present not only as surface silanol groups, but also as OH groups within the opal structure to balance the charge of the substituted Al and as OH groups bonded to interstitial elements like Mg.

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