Analysis of Ca1-xSrxCO3 phases generated by competitive Sr2+ replacement in pre-formed aragonite

Abstract

The ratio of Sr/Ca ions in marine biogenic minerals is considered advantageous for tracking geochemical and biomineralization processes that occur in the oceans. It is debatable, though, whether the ratio in biominerals such as coral skeleton is simply related to values in the seawater environment or controlled by the organism. Recent data show that coral larvae produce partially disordered immature aragonite in Mg-containing Sr-poor calcifying fluids, which transforms into well-ordered aragonite in Mg-depleted Sr-enriched environments, upon animal metamorphosis into the sessile polyp state. Inspired by the process in young coral, we explored in vitro substitution of Ca by Sr in aragonite by exposing aragonite crystals precipitated a priori to Sr solutions with variable concentrations. The resulting biphasic material, comprised of Sr-doped aragonite and Ca-doped strontianite, was carefully analyzed for foreign cation substitution in each polymorph. This allowed to establish a linear correlation between Sr levels in mineralizing solutions and Sr in aragonite as well as Ca in strontianite. It indicated that ca. 5-fold higher Sr solution concentration is needed for substitution in the crystal to reach the level found in corals. It also provided with Sr levels required for a putative strontianite phase to form.

Keywords: Competitive cation substitution in crystals; Coral skeleton; Minor elements in aragonite; Rietveld analysis; Vital effect.

Graphical abstract

Fig. 1

Propagation of XRD patterns of Ca_1-xSr_xCO₃ minerals with increasing X (side view and top view). At X = 0 and X = 1, pure aragonite and strontianite diffractograms are respectively shown. Reflections from the reference material (LaB₆) are marked by asterisks.

Fig. 2

(A) Dependence of the relative molar fraction of Sr in the crystalline phases, $X_{Sr}^{crystal}$, on the relative molar fraction used in the reaction, $X_{Sr}^{solution}$. (B) Weight fraction of the Ca_1-xSr_xCO₃ phases (Sr-aragonite – purple circles, Ca-strontianite – red circles) as function of solution molar fraction of Sr, $X_{Sr}^{solution}$. (C) The variation of the lattice parameters (a, b, and c) and the volume (V) of Ca_1-xSr_xCO₃ unit cell as a function of solution Sr molar fraction. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 3

Scanning electron micrographs showing morphologies of resultant biphasic Ca_1-xSr_xCO₃ minerals formed using various $X_{Sr}^{solution}$ from high to low. (A) $0.818$ (B) $0.5$ (C) $0.13$ (D) $0.1$ (E) $0.047$ (F) $0.0044$. Inset in (A) shows the encircled region in the image, magnified two-fold.

Fig. 4

(A, C, E) Active vibrational regions in the Raman spectra on the Ca_1-xSr_xCO₃ precipitates with increasing $X_{Sr}^{solution}$ from $X$ = 0 (bottom) to $X$ = 1 (top), recorded using a 633 nm laser. The peak vibrational modes (three stretching, three bending, and five lattice) are annotated by color coded dots at the bottom of A, C, E. (B, D, F) Peak positions (in cm⁻¹) in the corresponding spectral regions (A, C, E) are plotted as a function of $X_{Sr}^{solution}$, showing changes in vibration frequency with increase in Sr substitution in aragonite and decrease in Ca substitution in strontianite. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 5

(A) 1D ¹³C CP spectrum and (B) 1D ¹³C DP spectrum of ¹³C labeled Ca_1-xSr_xCO₃ prepared using $X_{Sr}^{solution}$ of 0.0044 (black), 0.13 (green), 0.23 (pink), 0.26 (orange), 0.33 (purple). Analogous spectra of pure aragonite (blue) and pure strontianite (red) are also shown. (C) Transmission electron image of mineral prepared using $X=0.33$. (D) Electron diffraction pattern taken at the region encircled in (C) marked as eds3. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)