Formation and Stability of Hydroxy-Mg Interlayers in Phyllosilicates

A study was conducted to (1) determine the conditions of hydroxy-Mg interlayer formation with respect to type of clay mineral, acidity, and time; (2) evaluate the stability of this interlayer to dissolution treatments; and (3) ascertain the effects of such treatments upon the determination of clay minerals in soils and sediments. Hydroxy-Mg interlayers were formed in montmorillonite and vermicu- lite by adding MgCI~ and NaOH in amounts to give a wide range of pH. The resulting chloritic intergrades were examined after 10 days, 6 months, and 1 yr. Alkaline conditions favored the formation of hydroxy magnesium interlayers in phyllosilicates. Hydroxy-Mg interlayered montmoriUonite which resulted from 10 days equilibration at pH 10.4 did not expand upon solvation with ethylene glycol and exhibited practically no collapse after K-saturation and heating at 550~ A small amount of interlayer was formed between pH 6.8 and 9.8 (10 days). In contrast, vermiculite exhibited no evidence of interlayer formation at pH values up to 9-7 (10 days). Chloritic intergrades formed at pH 10-7 did not collapse after K-saturation and heating at 300~ but did so at 550~ Hydroxy-Mg interlayers were not formed in either mineral by using a drying method. This method apparently failed to provide the required alkaline conditions for interlayer formation. The amount of magnesium interlayers present in the phyllosilicate systems decreased with time. The interlayers formed in vermiculite decreased more sharply than those in montmorillonite. Sequential dissolution treatments included boiling 2 per cent Na~CO3, buffered sodium citrate- dithionite, a second citrate-dithionite treatment, and boiling NaOH. Hydroxy-Mg interlayers in montmorillonite exhibited a higher stability to sequential treatments than the interlayers formed in vermiculite. A stable 14 ,~ line was observed in interlayered montmorillonite after the dithionite- citrate and NaOH treatments. The interlayers in montmorillonite showed a relatively high stability to HCI dissolution treatments. In contrast, most of the magnesium interlayer in vermiculite was removed by two HC1 washings. The reagents used in this study are sometimes used to remove coatings and cementing agents from soil surfaces prior to particle size and clay analysis. The present data show that these treatments also remove some hydroxy-Mg interlayers and produce changes in properties of clays, A proper interpre- tation of data for clay mineral identification and characterization must recognize these changes due to treatment. HYDROXY-Mg interlayers in phyllosilicates would be expected to occur in soils (Brydon, et al., 1961) and marine sediments (Rich, 1968), but they have received little attention compared to aluminum interlayers. Singleton (1965) reported the presence of hydroxy-Mg in addition to hydroxy-Al and -Fe in the interlayer space of clays from the Lookout series in Oregon. The formation of secondary chlorite in the dark magnesium clay soils of Hawaii and in Grumusols (Ladybrook Series) of Queens- land, Australia, was related to the hydroxy-Mg interlayering in montmorillonite (Jackson, 1959).