Coordination of Boron in Nominally Boron-Free Rock Forming Silicates: Evidence for Incorporation of BO3 groups in Clinopyroxene

To explore mechanisms of B-incorporation in common chain silicates we have investigated synthetic diopside samples produced under boron-saturated conditions by 11B and 29Si magic-angle spinning (MAS) NMR and single-crystal NRA, FTIR, EMP and XRD/SREF techniques. Our samples contain 0.14–0.65 wt.% B2O3. NMR reveals that B is predominantly present in trigonal coordination in the clinopyroxene structure. This observation is supported by vibrational bands characteristic for B–O stretching in BO3 groups in the range 1250–1400 cm−1 in polarised single crystal FTIR-spectra. Single crystal structure refinements suggest that boron replaces Si at the T site. Combined, these results suggest that boron replacement for Si at the T-site leads to disruption of one of the T–O bonds of the nominal clinopyroxene structure resulting in replacement of SiO4 tetrahedra by BO3 groups. Our results show that high concentrations of boron can be incorporated in the nominally boron-free diopside. Elevated B-concentrations in the present calcic clinopyroxenes are accompanied by modifications of the diopside crystal structure involving the breaking of one T–O bond and simultaneous formation of vacancies at the octahedral M2 site. These structural modifications destabilize the structure and constitute thereby limiting factors for incorporating higher boron concentrations in diopside.

Edén, MattiasStockholms universitet,Avdelningen för fysikalisk kemi,Mattias Edén,Stockholm universitet(Swepub:su)eden (författare)
Skogby, HenrikNaturhistoriska riksmuseet,Enheten för geovetenskap(Swepub:nrm)henrskog (författare)
Nazzareni, SabrinaUniversità di Perugia (författare)
Kristiansson, PerLund University,Lunds universitet,Kärnfysik,Fysiska institutionen,Institutioner vid LTH,Lunds Tekniska Högskola,Nuclear physics,Department of Physics,Departments at LTH,Faculty of Engineering, LTH(Swepub:lu)nucl-pkr (författare)
Resmark, JeppaLunds universitet (författare)
Naturhistoriska riksmuseetEnheten för geovetenskap (creator_code:org_t)

Ingår i:Geochimica et Cosmochimica Acta: Elsevier BV74, s. 5672-56790016-70371872-9533

Av författaren/redakt...: Hålenius, Ulf; Edén, Mattias; Skogby, Henrik; Nazzareni, Sabri ...; Kristiansson, Pe ...; Resmark, Jeppa

Om ämnet

Av lärosätet: Stockholms universitet; Lunds universitet; Naturhistoriska riksmuseet

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