Behavior of Kimberlitic Garnets Exposed to Chemical Weathering in Laterite Profiles: An Experimental Study and Stability Implications (Experimental Studies of the Kimberlitic Garnets Behavior at Conditions of the Laterite Chemical Weathering)

Abstract

The behavior of kimberlitic garnets involved in chemical weathering in lateritic provinces were studied experimentally using garnets of different paragenetic groups. In natural lateritic environments, garnets experience dissolution showing similar patterns worldwide. The tests were performed with the etching agents of hydrofluoric acid (HF), which never occurs in the free form in nature, and ATP-Na₂, an organic solvent common to biological systems and soils; the two etchants produce the same effects on grain surfaces. The dissolution rate (chemical stability) of garnets depends on CaO and Cr₂O₃ contents: Cr₂O₃ controls more strongly the behavior of peridotitic high-Cr garnets (from lherzolite and harzburgite-dunite) while CaO is the main stability control in eclogitic garnets with Cr₂O₃< 0.5 wt. %. Unit cell size is another characteristic control of chemical stability of garnets. Garnets with bigger unit cell parameters tolerate better chemical weathering but the stability decreases in the garnet series from harzburgite-dunite to eclogite parageneses. The weathering stability difference among the garnet varieties leads to changes in initial proportions of kimberlite mineral assemblages during the weathering processes: low-Cr garnets disappear rapidly while the percentages of diamond-related garnets increase. The applied experimental conditions faithfully reproduce natural processes and provide explanation to effects observed in the field. The reported results have important implications for diamond exploration.