The research team, led by Catarina Ermida of the Laboratory of Forensic Anthropology, together with Eugénia Cunha, Francisco Gil and Maria Teresa Ferreira, drew on a uniquely well-documented resource: identified skeletal collections in Portugal, including the 21st Century Identified Skeletal Collection at Coimbra and clavicles recovered from autopsies at the Portuguese National Institute of Legal Medicine and Forensic Sciences. Because the identities and death dates of these individuals are known with documented precision, the researchers could work with genuinely known postmortem intervals, something that is remarkably rare in this field. In total, 38 unburned human clavicles were analyzed, spanning PMIs from 4 to approximately 500 years. To probe the effects of heat, the team added a subsample of 12 clavicles that had been burned and carried a PMI of only four years, providing a natural experiment in how thermal alteration reshapes bone mineral independently of time.

The technique at the heart of the study, X-ray diffraction, is one of the workhorses of materials science. When a beam of X-rays strikes a crystalline substance, the atoms scatter the radiation in patterns dictated by the regular arrangement of the crystal lattice. Bone mineral, a poorly crystalline form of calcium phosphate closely related to the mineral hydroxyapatite, produces characteristic diffraction peaks. Freshly formed bone contains many small, imperfect, strain-laden crystals, and their disorder broadens and smears the diffraction peaks. As bone ages, and especially as it is altered by the burial environment, crystals tend to recrystallize and coarsen: small crystals dissolve and redeposit onto larger ones, lattice defects are repaired, and the peaks become taller, narrower and sharper. Two parameters capture this maturation. The crystallinity index, or CI, is calculated from the relative sharpness of the diffraction pattern, essentially a ratio comparing the height of key peaks with the depth of the trough between them. Crystallite size, meanwhile, is estimated from the broadening of the diffraction peaks themselves, using the physical principle that smaller crystals produce wider peaks. If bone mineral truly aged like a clockwork, both numbers should climb steadily with time since death.

That is roughly what the study found for the crystallinity index, but not for crystallite size. Across the 38 unburned clavicles, crystallite size showed no consistent relationship with PMI at all; values fluctuated widely from sample to sample, swamped by variability that had nothing to do with elapsed time. The crystallinity index told a different story. It displayed a moderate positive correlation with postmortem interval, with a Spearman rank correlation coefficient of 0.587, a statistically significant result with a p value below 0.001 and a 95 percent confidence interval stretching from 0.194 to 0.810. In plain terms, older bones did tend, on average, to show sharper, more crystalline diffraction patterns, consistent with the long-standing picture of slow diagenetic recrystallization. For archaeologists sorting ancient from modern material, that trend is real and useful. But the confidence interval also reveals how noisy the relationship is, and the devil, as so often in forensic science, was hiding in the scatter.

That scatter is precisely what limits the forensic dream. The authors observed substantial overlap in crystallinity values within the forensic timescale, meaning that a bone dead for five years might produce a CI indistinguishable from one dead for twenty or thirty years. Samples with identical known PMIs returned markedly different crystallinity values, a pattern the researchers attribute to taphonomic factors, the full suite of environmental insults that a body experiences after death: soil chemistry, moisture, pH, temperature swings, microbial attack, roots, and drainage. Two skeletons buried for the same number of years in different microenvironments can follow very different diagenetic trajectories, so the mineral fabric they present to an X-ray beam reflects burial history as much as burial duration. In forensic casework, where the difference between a recent death and a decades-old one can determine whether a crime is prosecuted, that ambiguity is decisive. A method that produces overlapping values across the critical range cannot pin a date, only gesture at one.

The burned-bone subsample added a further warning. Heat is known to drive dramatic recrystallization in bone mineral: as temperatures rise during burning, the small bioapatite crystals grow, merge and reorganize, sharpening diffraction patterns dramatically. The 12 burned clavicles in the study, all with a PMI of just four years, showed thermal alteration of crystallinity that could, in effect, mimic the signature of much older remains. A forensic investigator who analyzed a burned recent bone and interpreted its high crystallinity as a sign of antiquity would be badly misled. Conversely, cremated archaeological material could masquerade as fresh if the thermal signature were misread. The study’s authors stress that thermal history must be recognized and accounted for before any crystallinity measurement is interpreted as a proxy for time, and their data demonstrate concretely how fire can scramble the clock that X-ray diffraction is meant to read.

None of this means X-ray diffraction is useless in forensic contexts. The technique is fast, relatively inexpensive, minimally destructive, and its parameters do track genuine postmortem change in bone mineral. The crystallinity index’s moderate correlation with PMI confirms that mineral maturation is a real, measurable phenomenon, and the study’s use of documented skeletal collections shows how such measurements can be calibrated against known truth rather than assumed ages. What the findings deflate is the more ambitious claim: that CI or crystallite size alone can deliver a precise postmortem interval. The results align with a growing body of literature that has tested physicochemical dating proxies, from earlier X-ray diffraction studies to spectroscopic approaches, and found the same recurring pattern, a broad correlation with time that dissolves into scatter exactly where forensic questions need resolution. The authors conclude that, although XRD-derived crystallinity parameters provide useful insights into postmortem changes in bone mineral structure, their application for PMI estimation remains limited.

There is a broader lesson here about how forensic science progresses, and it is one worth savoring. The temptation in a young and under-resourced field is to chase a single magic number, a lone chemical or crystallographic readout that solves the dating problem once and for all. This study, grounded in 50 human clavicles with known histories, shows why that chase so often disappoints: death does not happen in a laboratory, it happens in soil, in water, in fire, and in a thousand uncontrolled environments that each rewrite the bone record in their own way. The measured effect of time is real but entangled with the effects of everything else. Progress will likely come not from a solitary indicator but from combinations of methods, larger and better-documented reference collections, and models that explicitly incorporate taphonomic variables rather than wishing them away.

For now, the crystals in our bones keep their secrets stubbornly. They do grow and ripen after death, and a diffractometer can watch it happen. But between a fresh forensic case and a centuries-old skeleton, the crystalline clock runs at different speeds in different graves, and fire can knock its hands sideways entirely. The Coimbra team’s careful negative result is, in its way, as valuable as a breakthrough: it tells every future investigator exactly where the limits lie, and redirects the hunt for the forensic clock of death toward approaches humble enough to respect the chaos of the burial world. Until then, the question of how long ago someone died will continue to demand the oldest tools of all, careful context, cautious inference, and the accumulated judgment of forensic anthropologists who know that every bone tells more than one story.

Subject of Research: X-ray diffraction analysis of bone mineral crystallinity as a potential marker of the postmortem interval in human skeletal remains

Article Title: X-ray diffraction analysis of bone crystallinity and its association with postmortem interval in human skeletal remains

Article References: Ermida, C., Cunha, E., Gil, F., & Ferreira, M. T. (2026). X-ray diffraction analysis of bone crystallinity and its association with postmortem interval in human skeletal remains. International Journal of Legal Medicine. https://doi.org/10.1007/s00414-026-04020-8

Image Credits: AI Generated

DOI: 10.1007/s00414-026-04020-8

Keywords: X-ray diffraction, bone crystallinity, postmortem interval, forensic anthropology, human skeletal remains, crystallinity index, crystallite size, taphonomy, bone diagenesis, burned bone, time since death, International Journal of Legal Medicine