Efficacy of Lithium Non-exchangeable Method to Determine Vermiculite in Shrink-swell Soil Clays

Abstract

Vermiculite in soils and clays was determined based on its ability to fix potassium using a chemical method developed by Alexiades and Jackson (1965, 1966). This was done by cation exchange capacity (CEC) determinations using CaCl2 and subsequently replacing Ca2+ by MgCl2, Mg2+ by KCl and K+ by NH4Cl. The overall scheme involved quantitative mineralogical analysis of vermiculate and smectite in soils and sediments which are essentially composed of minerals that are crystalline in nature. Further, the assumption is that amorphous minerals do not fix K and certain value of CEC is assigned to the amorphous minerals, defies what actually exist in nature. Several authors attempted to solve this problem with different assumptions. Rengasamy et al. (1974) employed the Hashimoto and Jackson (1960) method to extract amorphous alumino silicates from clays which involved boiling the samples in 0.5 N NaOH for 2.5 minutes and cooling immediately. But a chance of disruption of mineral structure due to such strong alkaline treatments persists. In view of this, an alternate chemical method to determine vermiculite in shrink-swell soil clays is proposed here. Twenty soil fine clay samples from some typical Vertisols were taken and Greene-Kelly test (modified Lim and Jackson, 1986 method), was performed. The Li-Clay was exchanged with MgCl2 to get the Mg-Clay. The Mg-clay was further treated with mild acid buffer to get the non-exchangeable Li (LiNE). This non-exchangeable Li accounts for the vermiculite content. Vermiculite was also estimated semi-quantitatively from XRD graphs. The present study compares the vermiculite content by four methods, viz Alexiades and Jackson, Rengasamy and associates., non-exchangeable Li and semi-quantitative estimates from XED graphs. Statistical relationships were developed among the four methods by paired t-test and Bland Altman Plots . The study indicated that vermiculite content determined by LiNE, Rengasamy and semi-quantitative methods have good agreement among them and are comparable. The semi-quantitative estimates and Alexiades and Jackson methods are not reliable because of its semi-quantitative nature and over estimation due to amorphous minerals, respectively. However methods developed by Rengasamy and associates and LiNE are reliable, though the former method has to be executed very efficiently. Although the size of Li+ (0.76 Å) is much smaller compared to the size of K+ (1.38 Å), it appear that Li+ was effected in replacing K+ from vermiculites in shrink-swell soils. Furthermore, mild acid buffer is an effective medium to replace K+ ions from vermiculite through hydrolysis when Mg2+ is used as a counter ion. Therefore Li+ ions with higher hydration energy compared to K+ ions would be easily replaced by Mg2+ in acid condition, thereby enabling the determination of vermiculite. The study also demonstrated the wider applicability of Greene-Kelly test as a method to determine vermiculite from shrink-swell soils.