Crystal-chemical variations of celadonite: versus glauconite

Abstract The distinction between celadonite and glauconite remains controversial due to their crystal-chemical similarity, their occurrence in a variety of environments, and their frequent association in mixtures with other phases. Using celadonite from the Janggi area (SE Korea), we report its X-ray diffraction (XRD) and scanning electron microscopy (SEM)-energy-dispersive spectrometry (EDS) characteristics and compare them with previously reported celadonite and glauconite to clarify their similarities and differences. A series of concentration steps for the Janggi celadonite was monitored using XRD patterns, Rietveld refinement and compositional characteristics. Celadonite from this area is classified as celadonite–aluminoceladonite with unit-cell parameters of a = 5.233–5.241 Å, b = 9.402–9.058 Å, c = 10.105–10.145 Å, β = 100.666–100.773°, V = 469.824–473.155 Å3. The compositional variation of celadonite can be explained by a three-site heterovalent substitution of ivAl3+ivSi4+−1vi(Fe3+total,Al3+)1.58viMg2+−1.5xiiNa++0.25xiiK+−1. Here, we (i) confirm that ivAl3+ serves as a simple yet robust first-order criterion to distinguish celadonite (<0.2 phfu) from glauconite (>0.2 phfu), (ii) find that celadonite accommodates the misfit between tetrahedral and octahedral sheets in a different style from that of glauconite due to the wide range of ionic sizes among the octahedral cations, (iii) predict that celadonite may have a broader range of b axis variations than previously recognized, and (iv) suggest that the key differences lie in the octahedral cation composition, substitution trends, charge distribution, and misfit accommodation styles and unit-cell parameters. These findings suggest that the distinction between celadonite and glauconite should be made by crystal-chemical differences rather than their occurrence modes. Therefore, using these minerals as indicators for paleoenvironmental reconstruction may be misleading