| Summary: | Calcium carbonate (CaCO<sub>3</sub>) from the seaweed <i>Lithothamnium calcareum</i> is a suitable dietary supplement for the prevention of osteoporosis, due to its chemical composition. This study compared CaCO<sub>3</sub> from <i>L. calcareum</i> to CaCO<sub>3</sub> from oyster shell and inorganic minerals that are already used in the pharmaceutical industry. The Rietveld refinement of the XRD showed that the mineral fraction of <i>L. calcareum</i> is composed of aragonite (50.3 wt%), magnesian calcite (45.3 wt%), calcite (4.4 wt%), comin contrast to oyster shell and inorganic minerals, which contain only calcite. The morphology of <i>L. calcareum</i> carbonate particles is granular xenomorphic, which is distinct from the scalenohedral form of inorganic calcite and the fibrous and scale-like fragments of oyster shell. The crystal structures of aragonite and magnesian calcite, present in <i>L. calcareum</i>, have higher contents of oligoelements than the pure calcite in other materials. The isotopic composition (stable isotopes of carbon and oxygen) is heavy in the CaCO<sub>3</sub> from <i>L. calcareum</i> (δ<sup>13</sup>C = 1.1‰; δ<sup>18</sup>O = −0.1‰) and oyster shell (δ<sup>13</sup>C = −4‰; δ<sup>18</sup>O = −2.8‰) in marked contrast to the much lighter isotopic composition of inorganic mineral CaCO<sub>3</sub> (δ<sup>13</sup>C = −19.2‰; δ<sup>18</sup>O = −26.3‰). The differences indicated above were determined through principal component analysis, where the first and second principal components are sufficient for the clear distinction and traceability of CaCO<sub>3</sub> sources.
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