Lunar apatite has been studied in the last few decades to estimate the water content of lunar mare basalts. Quantifying the volatiles within apatite can be used to interpret the evolution of the volcanic rock and estimate the water content in the coexisting melt. We collected geochemical data with an electron probe microanalyzer on apatites from 5 samples of mare basalt from Apollo 12, 15, and 17. We analyzed for 30 major and trace elements including REEs, fluorine, and chlorine, and calculated the OH contents of the phosphates based on mineral stoichiometry with the goal to understand more about the crystallization patterns for these elements. Most apatites from these basalt samples have >70% F in the halogen site, which is more than in most terrestrial magmatic apatites. During progressive crystallization, the REEs and volatiles would become enriched in the britholite rims of apatite crystals. The Cl-F-OH contents of the intermediate and endmember britholites do not show the expected decrease in F and an increase in OH and Cl. A potential explanation is that the evolved residual melts formed a separate fluid phase rich in water and Cl, leaving F in the melt (degassing). Such open system evolution may explain the overall richness in F of both apatites and britholites in lunar mare basalts. Partition coefficients for each cation has been used in the past to constrain volatile budgets and estimate parent magma water content, but there is a large amount of error in these numbers. Another approach with less error uses an exchange coefficient, which requires the concentration of Cl in glass from the same sample. One sample was available with appropriate Cl in glass measurements, and we estimated the water contents of the associated melt through the use of temperature-dependent exchange coefficients, which in the case of open system evolution would only provide a minimum water content for the mare basalt of ~10 ppm. A short survey of terrestrial apatites with metasomatic influences shows a different texture than that of the lunar basalt apatite crystals.