Calcium-lead fluoro-vanadinite apatites. II. Equilibrium structures

Area of Science:

• Materials Science
• Crystallography
• Inorganic Chemistry

Background:

• Vanadinites are apatite-structure compounds with the general formula (Pb,Ca)10(VO4)6(F,O)2.
• Understanding the structural and compositional variations in synthetic vanadinites is crucial for materials design.

Purpose of the Study:

• To synthesize and characterize synthetic vanadinites with controlled lead and calcium content.
• To investigate the site occupancy and partitioning of lead and calcium in the apatite structure.
• To correlate crystallographic parameters with compositional variations.

Main Methods:

• Synthesis of synthetic vanadinites with varying Pb and Ca content (0.57 < x < 9) and F/O ratios (0.10 < y < 0.47).
• X-ray diffraction (XRD) for crystal structure determination and unit cell parameter refinement.
• Analysis of metal site partitioning using crystallographic data.

Main Results:

• Synthetic vanadinites adopt the P6(3)/m apatite structure, with unit cell parameters a and c varying with composition.
• The calcium endmember exhibits a monoclinic P2(1)/m structure.
• Lead preferentially occupies the AII site (kPb(AI/AII) ≈ 0.33 for x < 7), and a miscibility gap exists between x=2 and x=3.
• Crystallographic trends correlate with the AIO6 metaprism twist angle.

Conclusions:

• The study elucidates the structural behavior of synthetic vanadinites as a function of Pb/Ca substitution.
• Site preference and miscibility gaps are key factors governing the crystal chemistry of these apatite compounds.
• The AIO6 metaprism twist angle provides a framework for understanding and predicting cell parameters.