Structure and chemistry of pyromorphite, Pb5(PO4)3Cl, mimetite, Pb5(AsO4)3Cl, vanadinite, Pb5(VO4)3Cl, and erythrite, Co3(AsO4)2!8H2O
Date
2018-02-27
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Abstract
This study is based on some lead-apatite minerals of the general formula M5(BO4)3Z, including pyromorphite, Pb5(PO4)3Cl, mimetite, Pb5(AsO4)3Cl, and vanadinite, Pb5(VO4)3Cl. Emphasis is placed on their crystal structure and chemistry. The lead-apatite minerals belong to the hexagonal space group P63/m. In pyromorphite, mimetite, and vanadinite, the BO4 tetrahedra is occupied by P5+, As5+, and V5+ cations, respectively. The variations in size of these cations result in different B-O bond distances in each mineral, thus influencing their unit-cell dimensions. Two samples of pyromorphite were studied, of which one is from China and the other is from Ontario. The mimetite sample is also from China, while the vanadinite sample is from Morocco. To examine the structural trends among these samples, synchrotron high-resolution powder X-ray diffraction (HRPXRD) and electron microprobe analysis (EMPA) was used. EMPA indicated that all samples were chemically homogeneous and represented near end-member compositions. Rietveld refinement of the HRPXRD data indicated that all samples were single-phase. Additionally, it was observed that as the ionic radius of the atom occupying the B site of the BO4 tetrahedra increased, the cell volume also increased. Vanadinite had the greatest cell volume as V5+ has the highest ionic radius of 0.355 Å, while pyromorphite had the smallest cell volume as P5+ has an ionic radius of 0.17 Å. Mimetite and vanadinite had similar cell volumes as As5+ has an ionic radius of 0.335 Å, which is similar to that of V5+. Bond lengths also linearly increased with cell volume, with vanadinite and mimetite having significantly longer bonds than pyromorphite. This relationship was the most pronounced in the B-O bond and the Pb2-Cl bond. The longer bond lengths were associated with larger unit-cell parameters. Vanadinite had the largest a unit-cell parameter, and pyromorphite the smallest. Mimetite had the largest c unit-cell parameter. The structure of the lead-apatite minerals is able to distort and accommodate various atoms in its tetrahedra. A sample of zoned erythrite, generally M3(TO4)2*8H2O, ideally Co3(AsO4)2*8H2O, from Morocco was also studied. It belongs to monoclinic space group C2/m. EMPA of this sample indicated chemical heterogeneity, as observed by the oscillatory zoning. The chemical composition of this sample was {Co2.78Fe0.04Ni0.07Zn0.11}Σ3.00(AsO4)2*8H2O. However, Rietveld refinement indicated a single-phase sample. This is likely because of similar ionic radii of elements occupying the M site of this mineral. The T site of vivianite-group minerals can be occupied by As or P. When structural trends of vivianite-group minerals were examined, two distinct series were observed: the As-series and the P-series. Solid solutions between the two series were not witnessed. It was observed that the As-series had greater a and c unit-cell parameters than the P-series, because of a larger ionic radius of As. The <M1-O> bond length is greater than the <M2-O> bond length in vivianite-group minerals. The atoms occupying the M site have a greater effect on the <As-O> bond than the <P-O> bond as the <P-O> bond is shorter and stronger. A V-series, such as what is observed in lead-apatite minerals, may be structurally possible in the vivianite group and would be an interesting topic to explore.
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Keywords
pyromorphite, mimetite, vanadinite, erythrite
Citation
Dhaliwal, I. (2018). Structure and chemistry of pyromorphite, Pb5(PO4)3Cl, mimetite, Pb5(AsO4)3Cl, vanadinite, Pb5(VO4)3Cl, and erythrite, Co3(AsO4)2*8H2O (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/13064