MSCC3 3rd MINERAL SCIENCES IN THE CARPATHIANS ...

Acta Mineralogica-Petrographica, Abstract Series 5, Szeged, 2006on the other side, are similar provided that the layer charge isthe same.The properties of the OH-group also allow to investigatethe tetrahedral cationic distributions (mainly Si, Al), since theOH … O interactions depend on the charge balance on surroundingoxygens, i.e. the oxygens of tetrahedra. A carefulquantitative investigation of OH-stretching band intensitiesfor different first neighbours (octahedral), and secondneighbours (tetrahedral) shows that the Si, Al distributionsare generally not ordered, but follow a homogeneous chargedistribution pattern, in agreement with high-resolution NMRdata. Considering the influence of second cationic neighbourseffects (i.e. tetrahedrally coordinated cations), on band positionsand band intensities, leads to conclude that in manycases the extinction coefficients are constant, which allowquantitative determinations of cationic distributions aroundthe hydroxyl proton.The method is sensitive enough to enlight long distanceeffects, even for subtle substitutions like Ca 2+ /Mg 2+ at M4sites in clinoamphiboles along the tremoliteCa 2 Mg 5 Si 8 O 22 (OH) 2 –cummingtonite Mg 7 Si 8 O 22 (OH) 2join, or Na + /Li + along the ferri-clinoferroholmquistiteLi 2 Fe 2+ 3Fe 3+ 2Si 8 O 22 (OH) 2 –riebeckiteNa 2 Fe 2+ 3Fe 3+ 2Si 8 O 22 (OH) 2 series. Owing to the distances,these long-range effects are not direct. They are inducedthrough the structure by the propagation of tiny distortionsrequired by local charge balance requirements.In addition to this role of privileged observer, the protonH + frequently plays a key role in many situations. Deprotonationin the case of in situ oxidation of a variable chargecation, i.e. Fe 2+ → Fe 3+ , in biotites, hornblendes, and so, noprotonation when high cation (Mn 3+ , Ti 4+ ) are incorporated tocrystal structures, in micas like norrishiteK(Mn 3+ 2Li)Si 4 O 10 O 2 and many high-Ti phlogopites, or amphiboleslike ungarettiite NaNa 2 (Mn 2+ 2Mn 3+ 3)Si 8 O 22 O 2 , obertiiteNaNa 2 (Mg 3 Fe 3+ Ti)Si 8 O 22 O 2 and dellaventuraiteNaNa 2 (MgMn 3+ 2LiTi) Si 8 O 22 O 2 . Additional protonations arealso known in hydrous minerals, for example in partiallydioctahedral micas with a tetrahedrally coordinated cation,Be 2+ , Mg 2+ , Ni 2+ or Co 2+ , or in clinoamphiboles (richterites),with Li + replacing Ca 2+ at the M4 site.The approach is also very useful for identifying unusualcoordinations for cations, for example [4] M 2+ in micas and inamphiboles like joesmithite, PbCa 2 Mg 5 (Si 6 Be 2 )O 22 (OH) 2 , or[4] B 3+ in high pressure olenite (a tourmaline)NaAl 3 Al 6 (BO 3 ) 3 (Si 3 B 3 O 18 )(OH)(OH) 3 .The interactions of the hydroxyl proton with theneighbouring oxygens, influenced by the charge distributionsand the distances, also control the possible OH – /F – exchangeproperties, whatever the fluorine activity in the environment.When the hydroxyl proton has no or little interactions with itsneighbours, it acts as a point charge in the structure and theOH – /F – is easy. It is the case in most trioctahedral layer silicates,in all amphiboles and at the inner OH – site of tourmalines.By contrast, if the hydroxyl proton is involved in H-bonds with surrounding oxygens, its replacement by fluorineis more energetically difficult, and becomes impossible ifOH … O bonds are strong. It is why dioctahedral layer silicatesdo not trap much fluorine, and why outer hydroxyl groupscannot be replaced by fluorine in tourmalines.www.sci.u-szeged.hu/asvanytan/acta.htm 103