Strontium-substituted Ca₁₀(PO₄)₆(OH)₂ hydroxyapatite (HAp) powders, with Sr wt% concentrations of 2.5, 5.6 and 10%, were prepared by a solid-state synthesis method. The chemical composition of the samples was accurately evaluated by using inductively coupled plasma (ICP) spectroscopy. The morphology of the samples was analyzed via optical microscopy, while structural characterization was achieved through powder X-ray diffraction (PXRD) and infrared (FTIR) and Raman spectroscopy. The PXRD structural characterization showed the presence of the Sr dopant in the Ca1 structural site for HAp with a lower Sr concentration and in the Ca2 site for the sample with a higher Sr concentration. FTIR and Raman spectra showed slight band shifts and minor modifications of the (PO₄) bands with increasing the Sr doping rate.

Europium-doped hydroxyapatite Ca₁₀(PO₄)₆(OH)₂ (3% mol) powders were synthesized by an optimized chemical precipitation method at 25 °C, followed by drying at 120 °C and calcination at 450 °C and 900 °C. The obtained nanosized crystallite samples were investigated by means of a combination of inductively coupled plasma (ICP) spectroscopy, powder X-ray diffraction (PXRD), Fourier Transform Infrared (FTIR), Raman and photoluminescence (PL) spectroscopies. The Rietveld refinement in the hexagonal P6₃/m space group showed europium ordered at the Ca2 site at high temperature (900 °C), and at the Ca1 site for lower temperatures (120 °C and 450 °C). FTIR and Raman spectra showed slight band shifts and minor modifications of the (PO₄) bands with increasing annealing temperature. PL spectra and decay curves revealed significant luminescence emission for the phase obtained at 900 °C and highlighted the migration of Eu from the Ca1 to Ca2 site as a result of increasing calcinating temperature.

The crystal structures of tobelite and NH₄⁺-rich muscovite from the sedimentary rocks of the Armorican sandstones (Brittany, France) have been solved for the first time by single-crystal X-ray diffraction. The structural study was integrated by electron probe microanalyses, and X-ray photoelectron and micro-Fourier transform infrared spectroscopy. The crystals belong to the 2M₂ polytype with the following unit-cell parameters: a = 9.024(1), b = 5.2055(6), c = 20.825(3) Å, and β = 99.995(8)° for tobelite and a = 9.027(1), b = 5.1999(5), c = 20.616(3) Å, and β = 100.113(8)° for NH₄⁺-rich muscovite. Structure refinements in the space group C2/c converged at R₁ = 8.01%, wR₂ = 8.84% and R₁ = 5.59%, wR₂ = 5.63% for tobelite and NH₄⁺-rich muscovite, respectively. X-ray photoelectron spectroscopy revealed nitrogen environments associated either with inorganic (B.E. 401.31 eV) or organic (B.E. 398.67 eV) compounds. Infrared spectra showed, in the OH⁻-stretching region (3700–3575 cm⁻¹), two prominent bands, centered at ~3629 and ~3646 cm⁻¹, and two shoulders at ~3664 and ~3615 cm⁻¹ that were assigned to Al³⁺Al³⁺□-OH⁻ arrangements having OH⁻ groups affected by different local configurations. In addition, a series of overlapping bands from about 3500 to 2700 cm⁻¹ characteristic of the NH₄⁺-stretching vibrations, a main band at ~1430 and a shoulder at ~1460 cm⁻¹ that were associated to the NH₄⁺-bending vibration (ν₄) were also present. The ammonium concentration was semi-quantitatively estimated in both crystals from the absorbance of the OH⁻-stretching and NH₄⁺-bending vibrations in the infrared spectra. An additional estimate was obtained for the NH₄⁺-rich muscovite by considering the normalized peak area between K2p_3/2 and N1s in the X-ray photoelectron spectrum. The obtained values are in agreement with those derived from the interlayer spacing in the simulated X-ray powder diffraction spectra. The results of this integrated approach converged to (K_0.18Na_0.01NH₄⁺_0.62)_∑=0.81(Al_1.98Fe²⁺_0.02)_∑=2.00 (Si_3.19Al_0.81)_∑=4.00O_10.00OH_2.00 for tobelite and to (K_0.46Na_0.03Ba_0.01NH₄⁺_0.36)_∑=0.86(Al_1.98Mg_0.01Fe²⁺_0.01V³⁺_0.01)_∑=2.01 (Si_3.13Al_0.87)_∑=4.00O_10.00F_0.08OH_1.92 for NH₄⁺-rich muscovite.

A suite of Ti-bearing garnets from magmatic, carbonatitic, and metamorphic rocks was studied by electron probe microanalysis (EPMA), X-ray powder diffraction (XRPD), single-crystal X-ray diffraction (SCXRD), Mössbauer spectroscopy, and secondary ion mass spectrometry (SIMS) to better characterize their crystal chemistry. The studied garnets show TiO₂ varying in the range of 4.9(1) to 17.1(2) wt% and variable Fe³⁺/∑Fe content. SIMS analyses allowed quantification of light elements yielding H₂O in the range 0.091(7)–0.46(4), F in the range 0.004(1)–0.040(4), and Li₂O in the range 0.0038(2)–0.014(2) wt%. Mössbauer analysis provided spectra with different complexity, which could be fitted to several components variable from one (^YFe³⁺) to four (^YFe²⁺, ^ZFe²⁺, ^YFe³⁺, ^ZFe³⁺). A good correlation was found between the Fe³⁺/∑Fe resulting from the Mössbauer analysis and that derived from the Flank method. X-ray powder analysis revealed that the studied samples are a mixture of different garnet phases with very close cubic unit-cell parameters as recently found by other authors. Single-crystal X-ray refinement using anisotropic displacement parameters were performed in the Ia3̄d space group and converged to 1.65 ≤ R₁ ≤ 2.09% and 2.35 ≤ wR₂ ≤ 3.02%. Unit-cell parameters vary in the range 12.0641(1) ≤ a ≤ 12.1447(1) Å, reflecting different Ti contents and extent of substitutions at tetrahedral site. The main substitution mechanisms affecting the studied garnets are: ^YR⁴⁺ + ^ZR³⁺ ↔ ^ZSi + ^YR³⁺ (schorlomite substitution); ^YR²⁺ + ^ZR⁴⁺ ↔ 2^YR³⁺ (morimotoite substitution); ^YR³⁺ ↔ ^YFe³⁺ (andradite substitution); in the above substitutions ^YR²⁺ = Fe²⁺, Mg²⁺, Mn²⁺; ^ZR⁴⁺ = Ti; ^YR³⁺ = Fe³⁺, Al³⁺, Cr³⁺; ^ZR³⁺ = Fe³⁺, Al³⁺. Minor substitutions, such as 2^YTi⁴⁺+ ^ZFe²⁺ ↔ 2^YFe³⁺ + ^ZSi, (SiO₄)⁴⁻ ↔ (O₄H₄)⁴⁻, F⁻ ↔ OH⁻, and ^YR⁴⁺ + ^XR⁺ ↔ ^YR³⁺ + ^XCa²⁺, with ^YR⁴⁺ = Ti, Zr; ^YR³⁺ = Fe³⁺, Al, Cr³⁺; ^XR⁺ = Na, Li also occur.