The crystal chemistry of apatite from an experimental and natural perspective

Canali, Andrew (2017) The crystal chemistry of apatite from an experimental and natural perspective. Masters thesis, Memorial University of Newfoundland.

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Abstract

Apatite has a relatively accommodating crystal structure that allows for the incorporation of geochemically important elements (e.g., high field strength elements [HFSE]: Hf, Th and U; and rare earth elements [REE]: lanthanides plus Y and Sc) that can be analyzed with modern microanalytical techniques, making it a useful tracer of petrogenesis, post crystallization alteration, and mineralization. Chapter 2, presents the results from an experimental investigation of REE-doped fluorapatites, which suggest that Si is the preferred charge balancing cation in apatite, whereas Na played a minor role in the charge balancing of the REE under the experimental conditions used. Varying the proportion of the REEs in the starting reagents produced REE distributions in the apatite crystals indicating that the melt composition plays a more important role in the incorporation of REEs in apatite than the crystal chemistry of apatite. However, the enrichment of LREE (e.g., La, Nd, Sm) in the synthetic apatite crystals also suggest a crystal-chemical structural control for the larger LREEs that share similar ionic radii with the Ca1 and Ca2 sites as compared to the ionically smaller HREE. Chapter 3, presents a comparative study of natural apatites including the major, minor, and trace element compositions of reference apatites from well-known geological localities, along with those of the case study apatites from the Gruvberget (northern Sweden) magnetite apatite (MtAp) deposit and related rocks. These specific reference apatite samples were chosen because they provide a range of compositions with distinguishing characteristics for each locality. This range allows for empirical discrimination, which is used to develop a framework for apatite from a given rock type, and/or to constrain the effects of post-crystallization alteration processes. The assemblage of actinolite + scapolite in the Gruvberget host rocks and ore suggests metasomatic alteration of the apatite post- and/or syn-mineralization. The Gruvberget apatites, relative to the reference apatites in general, are: 1) relatively depleted in total REEs, especially LREEs; 2) enriched in Cl and OH compared to the other MtAp ore samples; 3) relatively enriched in Mn; and 4) depleted in REE charge balancing cations Na and Si. It is postulated that the primary composition of the Gruvberget apatites were overprinted via metasomatism by hydrothermal fluids composed of a sodium, calcium, and chlorine rich brine, by metamorphism, or by both.

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