Crystal chemistry of apatites from the Tapira carbonatite complex, Brazil
Crystal chemistry of apatites from the Tapira carbonatite complex, Brazil
European Journal of Mineralogy (August 2004) 16 (4): 677-685
- activity
- apatite
- Brazil
- carbon dioxide
- carbonate ion
- carbonatites
- chemical composition
- chlorine
- crystal chemistry
- crystal structure
- depth
- electron probe data
- fluorapatite
- formula
- halogens
- hydroxyl ion
- igneous rocks
- infrared spectra
- ion exchange
- ion probe data
- magmas
- mass spectra
- melts
- metals
- mica group
- paragenesis
- partitioning
- phosphates
- rare earths
- refinement
- sheet silicates
- silicates
- South America
- spectra
- substitution
- X-ray diffraction data
- Tapira Complex
The crystal chemistry of 10 hydroxylapatites from the Tapira alkaline carbonatite complex, SE Brazil, has been determined by EPMA, ion microprobe, IR spectroscopy and single-crystal XRD; their compositional range (atoms pfu) is 9.935 < or = Ca < or = 10.024, 5.926 < or = P < or = 6.006, 0.005 < or = REE < or = 0.046, 0.042 < or = F < or = 0.914, 0.003 < or = Cl < or = 0.008, 0.003 < or = Si < or = 0.038, with Na present only as a trace. Crystal structure refinements were carried out in space group P6 (sub 3) /m to R values of 0.016- 0.028. The REE for Ca substitution requires two different exchange mechanisms producing an increase in bond valence on the X site: the deprotonation of OH and/or the substitution of CO (sub 3) (super 2-) (sub ) for OH. The CO (sub 3) (super 2-) substitution in the anionic sites was found to be related to great CO (sub 2) activity of the melt whereas deprotonation of the apatite crystals could be related to the highly oxidizing nature of the Tapira carbonatitic magma. The very low Cl/F ratio suggests that the crystallization of the Tapira pluton was relatively shallow. The determination of REE partitioning between the Ca1 and Ca2 sites is difficult because of the low REE content; a proportional increase in the size of the Ca1 and Ca2 sites was noted with increasing REE content as well as an overall increase of the a cell parameter.