Warwickite is an orthorhombic oxyborate mineral with the general formula t6lM2Ot3lBOj (M : Mg2+, Mn2+, Fe3+, Ti4+, Al3+). The studied warwickite samples are from lamproitic rocks (umillites) and associated carbonatite-like veins (Jumilla, Murcia province, southeast Spain). Their chemical compositions are quite variable and different from those of warwickite described in the literature: in particular, CrrO, varies between 0.0 and 13.5 wto/0,t otal wto/oF e as FerO, is as high as 50.6 Mo/oa nd total wto/oT iO, as high as 10.2 wto/0T. wo main substitutions occur in Jumilla warwickite: Mg'* + Tia+ = 2Fe3+a nd Fe3* + Cr3+. The structures of seven crystals of different compositions were refined in space group Pnma, giving R : 0.018-0.038. The structural results show that: (l) The Ml octahedron is smaller and more distorted than the M2 octahedron. (2) Distortion of the octahedral layer is greater in Fe-rich warwickite. (3) Fe preferentially enters the Ml site. (4) The M2- 04 distance decreasesw ith increasingT i, suggestingp referential ordering at the M2 site. TEM studies show planar (100) defects. These defects cause variations in the octahedral chain length.

Crystal chemistry of Fe- and Cr-rich warwichite / Bigi, Simona; Brigatti, Maria Franca; Capedri, Silvio. - In: AMERICAN MINERALOGIST. - ISSN 0003-004X. - STAMPA. - 76:(1991), pp. 1380-1388.

Crystal chemistry of Fe- and Cr-rich warwichite.

BIGI, Simona;BRIGATTI, Maria Franca;CAPEDRI, Silvio
1991

Abstract

Warwickite is an orthorhombic oxyborate mineral with the general formula t6lM2Ot3lBOj (M : Mg2+, Mn2+, Fe3+, Ti4+, Al3+). The studied warwickite samples are from lamproitic rocks (umillites) and associated carbonatite-like veins (Jumilla, Murcia province, southeast Spain). Their chemical compositions are quite variable and different from those of warwickite described in the literature: in particular, CrrO, varies between 0.0 and 13.5 wto/0,t otal wto/oF e as FerO, is as high as 50.6 Mo/oa nd total wto/oT iO, as high as 10.2 wto/0T. wo main substitutions occur in Jumilla warwickite: Mg'* + Tia+ = 2Fe3+a nd Fe3* + Cr3+. The structures of seven crystals of different compositions were refined in space group Pnma, giving R : 0.018-0.038. The structural results show that: (l) The Ml octahedron is smaller and more distorted than the M2 octahedron. (2) Distortion of the octahedral layer is greater in Fe-rich warwickite. (3) Fe preferentially enters the Ml site. (4) The M2- 04 distance decreasesw ith increasingT i, suggestingp referential ordering at the M2 site. TEM studies show planar (100) defects. These defects cause variations in the octahedral chain length.
76
1380
1388
Crystal chemistry of Fe- and Cr-rich warwichite / Bigi, Simona; Brigatti, Maria Franca; Capedri, Silvio. - In: AMERICAN MINERALOGIST. - ISSN 0003-004X. - STAMPA. - 76:(1991), pp. 1380-1388.
Bigi, Simona; Brigatti, Maria Franca; Capedri, Silvio
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11380/14132
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