Aragonite is the second most abundant polymorph of calcium carbonate in the natural environment, after calcite. It forms the shells and skeletons of countless marine organisms, from corals to pteropods, and it also precipitates inorganically in caves, hot springs, and seawater. Crucially, aragonite rarely grows in chemical isolation. Its crystal lattice readily accepts strontium and barium ions in place of calcium, and under the right conditions strontium and barium can crystallize independently as their own carbonate minerals, strontianite and witherite. All three minerals share the same orthorhombic crystal structure, forming what mineralogists call the aragonite group. Because the degree to which strontium and barium enter aragonite reflects the temperature, chemistry, and rate of the precipitation process, knowing exactly how much strontianite or witherite accompanies aragonite in a sediment is a direct window into the environmental conditions under which that sediment formed.

The tipping point between incorporation and independent crystallization is surprisingly sharp. Previous work, dating back to studies from the 1960s, established that when the strontium-to-calcium ratio in the parent solution exceeds roughly 0.67, strontianite begins to precipitate as a separate mineral phase rather than being incorporated into the aragonite structure. Below that threshold, strontium substitutes into the aragonite lattice. This makes quantitative detection of strontianite contamination a sensitive diagnostic tool: its presence signals that precipitation conditions crossed a specific chemical boundary. Until now, however, quantifying these minor phases reliably meant X-ray diffraction, a technique that is accurate but slow, expensive, and demanding in terms of instrument availability and operator expertise.

The Al-Quds team’s method rests on attenuated total reflection Fourier transform infrared spectroscopy, or ATR-FTIR, a technique in which infrared light is directed into a crystal in contact with the sample and reflects internally, evanescently probing the first few micrometers of material. No grinding into pellets, no dissolution, no dilution is needed; a pinch of powder pressed against the crystal is enough. The researchers first synthesized their three minerals from scratch under controlled conditions. Pure aragonite was precipitated at 25 degrees Celsius from a solution containing ammonium chloride, calcium chloride, and magnesium chloride at a magnesium-to-calcium ratio of 3 to 1, a recipe that steers crystallization toward aragonite rather than calcite. Pure strontianite was grown from a strontium chloride solution, and commercial barium carbonate served as witherite. Purity was verified independently by X-ray diffraction and inductively coupled plasma mass spectrometry before any spectroscopic work began.

With pure endmembers in hand, the team prepared two families of binary mixtures, aragonite with strontianite and aragonite with witherite, spanning a wide range of compositions. Each one-gram mixture was ground thoroughly in a mortar and pestle for thirty minutes to guarantee homogeneity. The spectra were collected on a Bruker Tensor II instrument at a resolution of one wavenumber, with sixty-second sample and background scans, and each sample was measured three times so that average absorbance values could be used. The resulting spectra revealed a beautifully systematic pattern. All three minerals show four principal absorption bands from carbonate vibrations, but the positions of those bands march steadily downward as the metal cation grows heavier. Aragonite’s strongest bands sit at 1474.6 and 853.7 wavenumbers; strontianite’s at 1460.4 and 855.1; witherite’s at 1413.3 and 856.5. The physical explanation is elegant: heavier ions form weaker metal-oxygen bonds, which vibrate at lower frequencies, shifting the absorption peaks to lower wavenumbers.

Those small but consistent shifts are the key to the entire method. Because the peaks of the three minerals overlap so closely, the authors could not simply measure the height of one peak and read off a concentration. Instead, they exploited a normalization strategy: rather than using absolute absorbance, which drifts with sample contact, instrument settings, and measurement technique, they took ratios of absorbances at carefully chosen wavenumbers. For strontianite quantification, for example, the absorbance of aragonite at 1474.6 wavenumbers divided by the absorbance of strontianite at 855.1 wavenumbers produced a linear relationship with weight percent, yielding the calibration equation weight percent of strontianite equals negative 425.81 times the ratio plus 433.2, with a coefficient of determination of 0.9932. For witherite, the ratio of absorbances at 853.7 and 856.5 wavenumbers gave weight percent of witherite equals negative 138.7 times the ratio plus 200.09, with a coefficient of determination of 0.9783.

In total, the team derived eight calibration equations, four for each binary system, with coefficients of determination ranging from 0.953 to 0.993. To quantify the reliability of these models, the authors performed least-squares linear regression and calculated the standard errors of the slopes and intercepts along with the standard error of the estimate. The standard errors of the slopes ranged from 2.65 to 38.59, while the standard error of the estimate ranged from 3.05 to 7.37 weight percent. Those figures mean that a typical prediction from these calibration curves carries an uncertainty of only a few weight percent, which is more than adequate for rapid screening applications. The ratio-based approach is what makes the method portable between instruments and operators: because ratios cancel out many systematic errors, a calibration built on one machine should transfer far more gracefully than one built on raw absorbance values.

The authors are careful to spell out the limits of their work. The calibration equations were derived from pure, physically mixed synthetic minerals, not from natural samples or solid solutions. Natural aragonite commonly hosts a menagerie of trace elements, including magnesium, lithium, boron, cadmium, uranium, and thorium, and even small amounts of these foreign ions within the crystal lattice can nudge absorption peaks slightly and alter their intensities. That means the equations as published may not transfer directly to natural sediments without recalibration or a suitable normalization procedure. The team’s future work will address this gap by synthesizing minerals under conditions that more closely mimic natural precipitation environments and by systematically investigating how trace elements shift wavenumber positions and absorption maxima.

Even with those caveats, the implications are considerable. For geochemists studying carbonate sedimentation, the method offers a rapid way to assess the distribution of strontium and barium carbonates within aragonitic sediments, sharpening interpretations of past precipitation conditions and their environmental significance. For laboratories that process large numbers of samples, from marine sediment cores to industrial carbonate feedstocks, ATR-FTIR screening could replace hours of X-ray diffraction queue time with minutes of measurement. The authors also point toward medical and industrial applications, where calcium carbonate phases must be identified and quantified quickly and cheaply. What began as a study of three look-alike minerals ends as a demonstration that a careful choice of measurement ratios can turn an ordinary infrared spectrometer into a quantitative mineralogical instrument, one that asks for almost nothing from the sample except a moment of contact with a crystal.

Subject of Research: Quantitative ATR-FTIR spectroscopic analysis of aragonite-group carbonate mineral mixtures

Article Title: Quantitative analysis of aragonite-group carbonates synthetic mixtures using attenuated total reflection Fourier transform infrared

Article References: Alkhatib, M., Thabteh, N., Al-Rimawi, F., & Qutob, M. (2026). Quantitative analysis of aragonite-group carbonates synthetic mixtures using attenuated total reflection Fourier transform infrared. European Journal of Mineralogy, 38(5), 557-565. https://doi.org/10.5194/ejm-38-557-2026

Image Credits: AI Generated

DOI: 10.5194/ejm-38-557-2026

Keywords: aragonite, strontianite, witherite, ATR-FTIR, infrared spectroscopy, carbonate minerals, X-ray diffraction, calibration curves, absorbance ratios, geochemistry, mineralogy, calcium carbonate