Dynamic Compression of Fluorapatite to 120 GPa

Apatite, (Ca5(PO4)3(F,Cl,OH)) is the most abundant phosphate mineral in the Earths crust and a key repository for phosphorous and halide ions in the upper mantle1,2. Recent studies show apatite can be a useful tool in shock barometry and indicator of shock deformation stages within terrestrial impactites, lunar rocks, and meteorites from both Mars and differentiated asteroids3,47. In addition, a high-pressure mineral, tuite , Ca3(PO4)2 has been discovered in a Martian meteorite8, which is proposed to form through shock-induced decomposition of phosphates such as apatite. However, there are very few experimental constraints on the behavior of apatite under shock loading. We have conducted a suite of plate impact experiments, using both a powder gun and a two-stage light gas gun, to determine the response of Durango fluorapatite single crystals to shock compression along the c-axis. Particle velocity profiles and shock velocities were measured using a combination of VISAR and PDV interferometry. Experiments covered a wide stress range from 14.5 to 120 GPa. The Hugoniot elastic limit of apatite was determined to be ~ 8.5 GPa. Between 30 60 GPa, a complex 3-wave structure was observed, likely resulting from a combination of elastic compression, inelastic deformation, and phase transformation. From 76 120 GPa, we observe a single-wave structure. The shock velocity (Us) particle velocity (up) results from the single wave profiles were fit to a linear equation to yield the following relationship for high-pressure fluorapatite: Us (mm/s )= 6.19(14) + 0.87(5)up. Our results place the first constraints on the behavior of apatite under shock loading over a wide stress range. 1 J.V. Smith, Nature 289, 762 (1981) 2 A. Aiuppa, et al., Chem. Geol. 263, 1 (2009) 3 A. Cernok, et al., Meteorit. Planet. Sci. 54, 1262 (2019) 4 G.G. Kenny, et al., 50th LPSC in (2019), Abs# 1357. 5 C.T. Adcock, et al., Nat. Commun. 8, 14667 (2017) 6 A.E. Patiño Douce and M. Roden, Geochim. Cosmochim. Acta 70, 3173 (2006) 7 A.E. Patiño Douce, et al., Chem. Geol. 288, 14 (2011) 8 Z. Xie, et al., Geochim. Cosmochim. Acta 70, 504 (2006)