Volume 103, Issue 5

Biomineralization is an evolutionarily ancient phenomenon and one of the fundamental biological processes by which living organisms produce minerals with multifunctional properties. Among the more general biomineralization processes, those involving silica (biosilicification), calcium-based biominerals (calcification), and iron-based biominerals (biomagnetism) have been described in a wide pattern of living organisms, from single cells to higher plants, animals, and even humans. After 25 yr of extensive studies of biosilicification, diverse biomacromolucules have been proposed and confirmed as active players in this special field of biomineralization. Despite these discoveries, biosilicification is still a paradigm and a cause of scientific controversy. This review has the ambitious goal of providing thorough and comprehensive coverage of biosilicification as a multifaceted topic with intriguing hypotheses and numerous challenging open questions. The structural diversity, chemistry, and biochemistry of biosilica in viruses, bacteria, plants, diatoms, and sponges are analyzed and discussed here. Special attention is paid to prospects and trends in applications of biosilica for technology, materials science and biomedicine.

In this ab initio study, we expand previous investigations of charge-balanced hydrous Mg ((2H)MgX$\begin{array}{} \displaystyle \rm (2H)_{Mg}^{X} \end{array}$) and Si ((4H)SiX$\begin{array}{} \displaystyle \rm (4H)_{Si}^{X} \end{array}$) defects in forsterite, the Mg end-member of olivine, to address the relative stability of these two defects. First, we systematically search for (2H)MgX$\begin{array}{} \displaystyle \rm (2H)_{Mg}^{X} \end{array}$ configurations to find possible defect states; second, we include the contribution of vibrational energy and defect configurational entropy in the calculation of formation energies of both defects; third, we address the effect of pressure and temperature simultaneously on their relative stability. Based on these considerations, we demonstrate that hydrous Mg defects ((2H)MgX$\begin{array}{} \displaystyle \rm (2H)_{Mg}^{X} \end{array}$) can be stabilized with respect to hydrous Si defects ((4H)SiX$\begin{array}{} \displaystyle \rm (4H)_{Si}^{X} \end{array}$) at relevant mantle conditions and that configurational entropy and vibrational free energy play key roles in this stabilization. Our results reveal that water speciation in olivine is influenced by temperature and pressure. As mantle physical and chemical properties may be affected by the speciation of water in olivine, application of experimental results to the mantle should account for the temperature- and pressure-dependent changes in water speciation.

The encapsulation and delivery of drugs often involves the use of expensive microporous materials, and we have investigated the potential for natural zeolites from the widespread volcanic formations of southern Italy as alternatives to these carriers. Surface-modified natural zeolites (SMNZs) with diverse micellar structures (patchy and complete bilayers) were obtained by using different cationic surfactants [cetylpyridinium chloride (CP-Cl), benzalkonium chloride (BC-Cl), hexadecyltrimethylammonium chloride (HDTMA-Cl), and bromide (HDTMA-Br) with phillipsite-rich tuff from the Campania region (southern Italy)]. Loading and release kinetics tests of sodium ibuprofen (IBU) were carried out with organo-phillipsite composites using Fourier transform infrared spectroscopy (FTIR) and thermal analysis coupled with evolved gas analysis (EGA). Results from these tests were mathematically modeled to evaluate IBU adsorption and release mechanisms. The maximum loaded amount of IBU was attained for organo-phillipsite modified with HDTMA-Br (PHB), which showed a complete bilayer micellar structure. Whenever a patchy bilayer micellar structure formed, the lowest adsorptions of IBU were observed. Equilibrium adsorption results were fit using Langmuir, Sips, and Toth models. Pseudo-first-order and pseudo-second-order fits to the loading kinetic data provided significant goodness of fit. Good fits to the release kinetic data were obtained using first-order and Weibull equations, shedding new light on the release mechanism of IBU from phillipsite. The active amount of IBU on the modified zeolite surface was almost totally available for pharmaceutical purposes.

Microbes play a fundamental role in the precipitation of silicate biominerals, thereby affecting the Si geochemical cycle. The fine mechanisms ruling biomineralization are not yet fully understood, and their microscopic structures can offer deep insight into their processes of formation, reactivity and stability. In this study, a Zn silicate biomineral, extracellularly produced by cyanobacterium Leptolingbya frigida , was investigated combining nuclear magnetic resonance (NMR), Zn K -edge X-ray absorption spectroscopy (XAS) and other complementary techniques. 29 Si magic angle spinning and 29 Si/ 1 H cross polarization magic angle spinning analysis, Fourier transform infrared spectroscopy (FTIR) and XAS analysis revealed a poorly crystalline phase closely resembling hemimorphite [Zn 4 Si 2 O 7 (OH) 2 ·H 2 O]. Zn K -edge extended X-ray absorption fine structure (EXAFS) provided further structural details, revealing that the Zn-O-Si interatomic distances were 7–8% shorter than the abiotic mineral. 13 C NMR spectra analysis was conducted to investigate the composition of the Zn silicate biomineral organic matrix, and results revealed that C atoms occurred in several functional groups such as carbonyl carbons, C rings, O-aliphatic chains, N-aliphatic chains, and aliphatic chains. Under slightly alkaline conditions, bacterial cell walls exhibited fundamental control on the biomineralization process by binding Zn ions and forming Zn–O–Si bonds. In this way, L. frigida cell walls served as a reactive surface for the precipitation of this Zn sorosilicate, hindering the condensation of silicon dimers. Moreover, we found a 29 Si NMR band at 85 ppm that could be attributed to a (C 3 H 6 O 3 ) 2 Si complex. This complex could play a role in the control of silicon polymerization, with implications for Si biomineralization processes.

It is very difficult to synthesize dolomite under Earth-surface conditions in the laboratory. However, multiple carbonate phases, including low-Mg-calcite (LMC), high-Mg-calcite (HMC), and dolomite, have been discovered in authigenic carbonate deposits that precipitated at cold methane seeps. The formation of such seep carbonates is triggered by the sulfate-driven anaerobic oxidation of methane (SD-AOM), which is mediated by a consortium of methane-oxidizing archaea and sulfate-reducing bacteria; this process releases bicarbonate and dissolved sulfide. Thus, the formation of Ca-Mg carbonate phases and, particularly, their respective MgCO 3 contents are likely to be intimately related to SD-AOM and the methane supply at cold seeps. Yet, the driving forces for MgCO 3 enrichment and the actual mechanism responsible for the incorporation of Mg 2+ into the crystal lattice are not fully understood. Interestingly, recent laboratory experiments succeeded in synthesizing disordered dolomite under the catalysis of dissolved sulfide and extracellular polymeric substances (EPS) at low temperatures. To characterize the effect of these catalyzes on the formation of seep carbonates, we investigated mineral phases, microstructure, and contents of Ca, Mg, and rare earth elements of seep carbonates from the Shenhu area and the Southwest (SW) Taiwan basin of the northern South China Sea (SCS). The studied carbonates are composed of multiple Ca-Mg carbonate phases, including HMC, weakly ordered dolomite, and dolomite with a wide range of MgCO 3 contents. Transmission electron microscopy indicates that the microstructure of some Shenhu dolomite is almost stoichiometric, only a few domains exhibit the structure of Mg-calcite. Weakly ordered dolomite from the SW Taiwan basin contains less MgCO 3 than the Shenhu dolomite, and is composed of heterogeneously distributed domains of Mg-calcite and dolomite. A positive correlation between MgCO 3 contents, cerium anomalies, Nd N to Yb N ratios, and 13 C-depletion suggests that Mg 2+ incorporation into the crystal lattice is favored by reducing conditions produced by pronounced SD-AOM. Based on previous studies, we put forward that SD-AOM derived sulfide and EPS produced by the SD-AOM consortium are the most plausible drivers for Ca-Mg carbonate formation at cold seeps. Precipitated under conditions similar to laboratory experiments, the initial Ca-Mg carbonates are apparently disordered nano-crystals with various MgCO 3 contents. In the course of maturation and recrystallization, the Ca-Mg carbonates evolve into weakly ordered dolomite or dolomite. This study contributes to the understanding of dolomite formation at cold seeps and the relationship between carbonate mineralogy, the supply of methane, and microbial activity.

Olivine fabrics are crucial for deciphering anisotropy of many physical properties in the upper mantle, such as seismic velocity and thermal diffusivity. Under circumstances where crystallized mantle rocks do not suffer strong modification from later deformation, olivine fabrics can reflect the results of anisotropic crystal growth and viscous magmatic flow. Currently little is known about the crystallization habits of olivine in nature. Here we report crystallization habits of olivine phenocrysts in two Hawaiian tholeiitic basalts. The low Mg numbers (=Mg/[Mg+Fe total ] × 100 atomic ratios; <88.1) of olivine grains suggest that they are crystallized products from the host magma rather than captured ones from peridotite xenoliths. Many olivine grains have non-equidimensional euhedral crystal shapes due to anisotropic crystal growth rates. Electron backscatter diffraction (EBSD) analyses show that of the 115 analyzed olivine grains, 84 grains are oriented with their long axes parallel to the [001] axis, while 24 and 7 grains have their long axes parallel to the [100] axis and the [010] axis, respectively. This growth habit is different from that reported for olivine grown at subsolidus conditions without fluid/melt. Our results imply that olivine will most likely form the AG-type fabric (i.e., a point maximum of the [010] axis normal to the foliation plane, and a girdle distribution of the [100] and the [001] axis in the foliation plane) during crystallization from magma chambers undergoing compaction, and the B-type fabric (i.e., a point maximum of the [010] and the [001] axis normal to the foliation plane and parallel to the lineation, respectively) during pure or simple shear mode magmatic flow, provided that no or weak later plastic deformation is superimposed upon these magmatic fabrics. The AG-type and the B-type olivine fabrics in natural cumulates can be interpreted as results of anisotropic growth of olivine and viscous magmatic flow. Anisotropic olivine growth is expected to strongly affect the fabric development in the upper mantle portion where there is extensive partial melting (e.g., the mantle wedge above subduction zones or the upper mantle beneath the middle ocean ridges) or reactive melt percolation leading to crystallization of olivine at the consumption of pyroxene.

Melting and subsolidus experiments were carried out on Fe–Si–S alloys (2.2–2.7 wt% Si + 2.0–2.1 wt% S) up to 146 GPa in a laser-heated diamond-anvil cell (DAC). The melting and subsolidus phase relations were examined on the basis of in situ synchrotron X-ray diffraction measurements and ex situ textural and chemical characterizations of recovered samples. The subsolidus phase assemblage changed from Fe-rich hexagonal closed-packed (hcp) phase + Fe 3 S into a single phase of hcp Fe–Si–S alloy above 80 GPa at ~2500 K. The melting curve was obtained on the basis of the appearance of diffuse X-ray scattering and/or melting texture found in the cross section of a recovered sample. Microprobe analyses of quenched molten samples showed that liquid Fe–Si–S coexisted with Fe-alloy solid, which is depleted in sulfur but enriched in silicon compared to the liquid. This result indicates that the liquid evolves toward a Si-poor and S-rich composition upon crystallization. Our data further suggest that the ternary eutectic liquid composition is Si-deficient and close to the tie line between the eutectic points in the Fe–Si and Fe–S binary systems at each pressure. The composition of Fe–Si–S liquid that accounts for the outer core density is outside the liquidus field of solid Fe at the inner core boundary (ICB) pressure. Accordingly, the solid alloy crystallizing from such an outer core liquid must be more enriched in silicon/sulfur than the coexisting liquid and thus cannot form the denser inner core required from seismic observations. Furthermore, neither liquid Fe–Si–C nor Fe–Si–O can crystallize a dense solid at the ICB. These results reinforce the conclusion that silicon is not an important light element in the core.

ThO 2 is an important material for understanding the heat budget of Earth’s mantle, as well as the stability of nuclear fuels at extreme conditions. We measured the in situ high-pressure, high-temperature phase behavior of ThO 2 to ~60 GPa and ~2500 K. It undergoes a transition from the cubic fluorite-type structure (thorianite) to the orthorhombic α-PbCl 2 cotunnite-type structure between 20 and 30 GPa at room temperature. Prior to the transition at room temperature, an increase in unit-cell volume is observed, which we interpret as anion sub-lattice disorder or pre-transformation “melting” (Boulfelfel et al. 2006). The thermal equation of state parameters for both thorianite [ V 0 = 26.379(7), K 0 = 204(2), α K T = 0.0035(3)] and the high-pressure cotunnite-type phase [ V 0 = 24.75(6), K 0 = 190(3), α K T = 0.0037(4)] are reported, holding K0′$\begin{array}{} K_0^{'} \end{array} $ fixed at 4. The similarity of these parameters suggests that the two phases behave similarly within the deep Earth. The lattice parameter ratios for the cotunnite-type phase change significantly with pressure, suggesting a different structure is stable at higher pressure.

The silicic (broadly dacitic to rhyolitic) magmatic systems that feed supereruptions show great diversity, but have in common a role for mafic (broadly basaltic to andesitic) magmas as drivers of the systems. Here we document the mafic component in the rhyolitic magmatic system of the 2.08 Ma Huckleberry Ridge Tuff (HRT), Yellowstone, and compare it to mafic materials erupted prior to and following the HRT eruption in the area within and immediately around its associated caldera. The HRT eruption generated initial fall deposits, then three ignimbrite members A, B, and C, with further fall deposits locally separating B and C. A “scoria” component was previously known from the upper B ignimbrite, but we additionally recognize juvenile mafic material as a sparse component in early A, locally abundant in upper A and sparsely in lower B. It has not been found in the C ignimbrite. In upper B the mafic material is vesicular, black to oxidized red-brown scoria, but at other sites is overwhelmingly non-vesicular, and sparsely porphyritic to aphyric. Despite their contrasting appearances and occurrences, the mafic components form a coherent compositional suite from 49.3–63.3 wt% SiO 2 , with high alkalis (Na 2 O+K 2 O = 4.5–7.3 wt%), high P 2 O 5 (0.52–1.80 wt%), and notably high concentrations of both high field strength and large-ion lithophile elements (e.g., Zr = 790–1830 ppm; Ba = 2650–3800 ppm). Coupled with the trace-element data, Sr-Nd-Pb isotopic systematics show influences from Archean age lower crust and lithospheric mantle modified by metasomatism during the late Cretaceous to Eocene, as previously proposed for extensive Eocene magmatism/volcanism around the Yellowstone area. The HRT mafic compositions contrast markedly with the Snake River Plain olivine tholeiites erupted before and after the HRT eruption, but are broadly similar in several respects to the generally small-volume Craters of the Moon-type mafic to intermediate lavas erupted recently just west of the HRT caldera, as well as farther west in their type area. The combination of trace element and isotopic data on the HRT mafics are only consistent with an origin for their parental magma as melts from mantle enriched by high temperature and pressure melts, most likely from the underlying Farallon slab. Subsequent interaction of the HRT mafic magmas occurred with the Archean lower crust and lithospheric mantle, but not the highly radiogenic upper crust in this area. The close temporal and spatial relationships of the HRT mafic compositions and the preceding Snake River Plain olivine tholeiite eruptives suggest a high degree of spatial heterogeneity in the mantle beneath the Yellowstone area during the early (and subsequent) development of its modern magmatic system.

In studies of the structures of silicate minerals and glasses, 29 Si NMR spectroscopy has been applied almost exclusively to materials containing relatively low concentrations of ions with unpaired electrons spins, such as most transition metals and rare earths, because of sometimes severe broadening and shifting of resonances in such strongly paramagnetic systems. However, by adapting experimental methods to allow detection of very broad signals, and by examining a much expanded range of frequencies, we show here that accurate spectra can indeed be measured for a series of pure-phase transition metal silicates, including olivines (Fe 2 SiO 4 , Co 2 SiO 4 , Ni 2 SiO 4 ), Co-akermanite (CoCa 2 Si 2 O 7 ), and clinopyroxenes (CoCaSi 2 O 6 , NiCaSi 2 O 6 ). For the latter two, we also present data for glasses of the same nominal compositions. For all of these phases, NMR peak broadening is large, in many cases to the point where magic angle spinning (MAS) does not enhance resolution; in all cases the observed paramagnetic shifts fall far outside the known range for diamagnetic silicates. There are clearly large effects of local structure on shift, suggesting great potential sensitivity to variations in chemical, electronic, and magnetic structure in both crystalline and amorphous phases. In particular, the spectra for the glasses are very different from those of crystals in both width and position. In most cases, measured spin-spin relaxation times are long enough to avoid major loss of signal during the NMR acquisition, but this may not always be the case in more magnetically dilute solid solutions, where small, broad paramagnetic resonances due to first cation neighbor interactions may in some systems be difficult to detect. To explore this issue, we present new data on a Ni-doped forsterite (Mg 1.9 Ni 0.1 SiO 4 ) and on natural San Carlos olivine (Mg 1.8 Fe 0.2 SiO 4 ), which improve upon the accuracy of our previous studies of these materials. It is clear that applications of NMR to paramagnetic silicates holds great promise not only for empirical studies of structure of a much wider range of compositions of minerals and glasses, but for future testing and application of advanced theoretical methods to more completely interpret such results.

Silica is thought to be present in the Earth’s lower mantle in subducting plates, in addition to being a prototypical solid whose physical properties are of broad interest. It is known to undergo a phase transition from stishovite to the CaCl 2 -type structure at ~50–80 GPa, but the exact location and slope of the phase boundary in pressure-temperature space is unresolved. There have been many previous studies on the equation of state of stishovite, but they span a limited range of pressures and temperatures, and there has been no thermal equation of state of CaCl 2 -type SiO 2 measured under static conditions. We have investigated the phase diagram and equations of state of silica at 21–89 GPa and up to ~3300 K using synchrotron X-ray diffraction in a laser-heated diamond-anvil cell. The phase boundary between stishovite and CaCl 2 -type SiO 2 can be approximately described as T = 64.6(49)· P – 2830(350), with temperature T in Kelvin and pressure P in GPa. The stishovite data imply K0′$\begin{array}{} K_0^{'} \end{array} $ = 5.24(9) and a quasi-anharmonic T 2 dependence of −6.0(4) × 10 −6 GPa·cm 3 /mol/K 2 for a fixed q = 1, γ 0 = 1.71, and K 0 = 302 GPa, while for the CaCl 2 -type phase K 0 = 341(4) GPa, K0′$\begin{array}{} K_0^{'} \end{array} $ = 3.20(16), and γ 0 = 2.14(4) with other parameters equal to their values for stishovite. The behaviors of the a and c axes of stishovite with pressure and temperature were also fit, indicating a much more compressible c axis with a lower thermal expansion as compared to the a axis. The phase transition between stishovite and CaCl 2 -type silica should occur at pressures of 68–78 GPa in the Earth, depending on the temperature in subducting slabs. Silica is denser than surrounding mantle material up to pressures of 58–68 GPa, with uncertainty due to temperature effects; at higher pressures than this, SiO 2 becomes gravitationally buoyant in the lower mantle.

At Lake Tecopa, in California, white play-of-color opals are found in vesicles of a volcanic ash layer from the Huckleberry Ridge Tuff super-eruption (2.1 Ma). They show characteristic traits of opal-AG by X-ray diffraction and scanning electron microscopy (silica spheres of ~330 nm). These properties are not typical for volcanic opals, and are usually associated with opals formed in a sedimentary environment, such as opal-AG from Australia. The conditions under which opal was formed at Lake Tecopa were determined by oxygen and hydrogen isotopic analyses and give a better understanding of the formation of opal in general. Tecopa opal’s δ 18 O is ~30‰, which leads to a formation temperature between 5 and 10 °C from water composition similar to the present spring water composition (δ 18 O = –12‰), or between 15 and 30 °C (the present day spring water temperatures) in water having a δ 18 O between –9.5 and –5.5‰. As a result, opal experienced 25–50% evaporation at the Tecopa basin. Contrary to long-held views, the formation of opal-AG vs. opal-CT (or opal-C) is not determined by the type of deposits, i.e., respectively sedimentary vs. volcanic, but mostly by the temperature of formation, low (≤45 °C for opal-AG) vs. high (≥160 °C for opal-CT) as suggested in most recent papers. The isotopic composition of water contained in Tecopa opals is assessed and results show that water in opal records different stages of opal formation from groundwater. Opal seems to precipitate from groundwater that is undertaking isotopic distillation during its circulation, most likely due to 15% up to 80–95% evaporation. Hydrogen isotopes are poorly documented in opal and require more systematic work, but this study reveals that, in Tecopa opals, molecular water (H 2 O m ) is isotopically heavier than structural water (OH), a phenomena already well known in amorphous volcanic glass. Overall, opal isotopic composition reflects the composition of the water from which it precipitated and for that reason could be (as established for amorphous silicic glass) a useful tool for paleoenvironments, and paleoclimatic reconstitutions on Earth and on other terrestrial planets.

Dickite is a member of the family of 1:1 dioctahedral phyllosilicates known as the kaolin minerals, with composition Al 2 Si 2 O 5 (OH) 4 . The elucidation of the hydrogen-atom positions in dickite, addressed here, and indeed in other hydrated minerals poses particular challenges. The crystal structure of dickite was determined from single-crystal X-ray diffraction at 100(2) K in the non-centrosymmetric Cc monoclinic space group and found to agree closely with previously reported structures (Bish and Johnston 1993; Dera et al. 2003). 27 Al and 29 Si solid-state NMR spectra of unprecedented resolution bear evidence for two distinct Al and Si sites, being consistent with the previously determined structures. Positions of the four independent hydrogen atoms were optimized and the pertinent 1 H chemical shifts calculated using DFT methods (program CASTEP) and compared with high-resolution MAS NMR experimental data obtained at ultra-high sample spinning rates (up to 67 kHz). This work contributes new evidence on the precise hydrogen-atom positions of dickite, and it illustrates how X-ray diffraction, solid-state NMR, and theoretical calculations may be combined to yield an improved mineral crystal structure.

The partition coefficients of V and Sc between clinopyroxene and silicate melt (DVCpx/SMandDScCpx/SM)$\begin{array}{} (D \rm ^{Cpx/SM}_{V}\,and\,\it D \rm ^{Cpx/SM}_{Sc}) \end{array}$ have been determined experimentally at 1200–1400 °C and 0.8–2.3 GPa, using a hornblende- and clinopyroxene-rich mantle rock in graphite-lined Pt 95 Rh 05 capsules. The results show that the DVCpx/SM$\begin{array}{} D \rm ^{Cpx/SM}_{V} \end{array}$ and DScCpx/SM$\begin{array}{} D \rm ^{Cpx/SM}_{Sc} \end{array}$ values decrease from 3.8 to 2.3 and from 2.6 to 1.1, respectively, as the experimental temperature and pressure vary from 1200 °C and 0.8 GPa to 1400 °C and 2.3 GPa. The presence of water in silicate melts may also reduce DVCpx/SMandDScCpx/SM.$\begin{array}{} D \rm ^{Cpx/SM}_{V} and \,\it{D} \rm ^{Cpx/SM}_{Sc}. \end{array} $ These results imply that the effects of temperature, pressure, and melt water content on DVCpx/SM$\begin{array}{} D \rm ^{Cpx/SM}_{V} \end{array}$ should be considered when using V systematics in cratonic mantle peridotites to constrain cratonic mantle oxygen fugacity ( f o 2 ). However, although the dominant V in the present silicate melt is mixed V 3+ and V 4+ , the DVCpx/SM/DScCpx/SM$\begin{array}{} D \rm ^{Cpx/SM}_{V}/\it D \rm ^{Cpx/SM}_{Sc} \end{array}$ together with literature data obtained at similar f o 2 shows a nearly constant value of 1.68 ± 0.26, regardless of temperature, pressure, melt composition, and melt water content, indicating that these factors cannot cause fractionation of Sc 3+ from mixed V 3+ and V 4+ in mantle melts through clinopyroxene/silicate melt partitioning. Therefore, in combination with V/Sc systematics in primitive MORBs and arc basalts, using DVCpx/SMandDScCpx/SM$\begin{array}{} D \rm ^{Cpx/SM}_{V}\,and\,\it D \rm ^{Cpx/SM}_{Sc} \end{array} $ obtained at 1 bar and dry conditions should be valid to constrain mantle f o 2 except for the case that the D Cpx/SM for Sc 3+ can be demonstrated to be fractionated from the D Cpx/SM for mixed V 4+ and V 5+ , which are present in oxidized basalts.

Monazite, a common accessory rare-earth orthophosphate mineral in the continental crust widely used in U-Pb geochronology, holds promise for (U-Th)/He thermochronology and for the immobilization of Pu and minor actinides (MA) coming from spent nuclear fuel reprocessing. Previous results obtained on natural and plutonium-doped monazite have demonstrated the ability of this structure to maintain a crystalline state despite high radiation damage levels. However, the low critical temperature (180 °C), above which amorphization cannot be achieved in natural monazite under ion irradiation, does not explain this old and unsolved paradox: why do natural monazites, independent of their geological history, remain crystalline even when they did not experience any thermal event that could heal the defects? This is what the present study aims to address. Synthetic polycrystals of LaPO 4 -monazite were irradiated sequentially and simultaneously with α particles (He) and gold (Au) ions. Our results demonstrate experimentally for the first time in monazite, the existence of the defect recovery mechanism, called α-healing, acting in this structure due to electronic energy loss of α particles, which explains the absence of amorphization in natural monazite samples. This mechanism is critically important for monazite geo- and thermochronology and to design and predictively model the long-term behavior of ceramic matrices for nuclear waste conditioning.