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The atomic structure of bakerite and its relationship to datolite

¹ Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, 56126 Pisa, Italy
² Dipartimento di Scienze della Terra, Università di Modena e Reggio Emilia, Via S. Eufemia 19, 41100 Modena, Italy
³ Natural History Museum of Los Angeles County, 900 Exposition Blvd., Los Angeles, California 90007, U.S.A.

Correspondence: ^* E-mail: natale{at}dst.unipi.it

Four samples of bakerite, Ca₄B₅Si₃O₁₅(OH)₅, were studied by means of IR, TG/DSC, EPMA, and X-ray and neutron diffraction. Chemical analyses can readily discriminate bakerite from datolite, Ca₄B₄Si₄O₁₆(OH)₄, by the distinctly lower SiO₂ content of the former. The Rietveld refinement of the combined neutron and X-ray powder data allow the determination of the crystal structure of bakerite to R_p 3.09%. The crystal structure can be derived from that of datolite by the substitution 0.25Si⁴⁺ + 0.25O^2– 0.25B³⁺ + 0.25(OH)^– at the Si-centered tetrahedral site. This leads to the crystal chemical formula Ca₄B₅Si₃O₁₅(OH)₅. Both of the two hydrogen positions in the bakerite structure, one with full occupancy, the other with 25% occupancy, were precisely located. The water molecule generally thought to be present in bakerite structure is actually absent. The excess water detected in chemical analyses can be attributed to the presence of minor impurities. Given the consistent composition of bakerite from various localities and no evidence for substitution of B for Si in datolite, bakerite is retained as a distinct species. A possible explanation of the peculiar 5:3 boron to silicon ratio in bakerite is provided.

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