Minerals of the montmorillonite group, their origin and relation to soils and clays

Abstract

A detailed study has been made of the montmorillonite group of clay minerals, one of the most widely occurring of all mineral materials.Minerals of this group are exceedingly fine grained crystalline aggregates, and they present an unusually wide range in composition.Essential information on the group was obtained only after study, by all available mineralogical methods, of more than 100 carefully selected specimens.This information has made possible the determination of the limits of variation in composition and the function of various ions in the crystal lattice.The underlying causes for the important property of base exchange have been established.This property plays a prominent part in determining the physical properties of minerals of the montmorillonite group, and in particular their behavior towards water.Interpretations of chemical analyses are based on knowledge of the crystal lattice of montmorillonite and related minerals.This lattice is of a layer type in which silicon and aluminum ions have tetrahedral coordination with respect to oxygen.Aluminum, ferric and ferrous iron, magnesium, lithium, chromium, manganic and manganous manganese^ and some other ions occupy positions having octahedral coordination with respect to oxygen.Exchangeable cations are on the surfaces of the silicate layers, and their amounts are determined by the excess negative charge within the layer.These cations usually are sodium and calcium Exchangeable ions T but potassium, magnesium, hydrogen, and aluminum may be exchangeable.The composition and mineral relations of the clay minerals are intimately related to the conditions under which they form.In general, alkalic feldspars and the micas tend to alter to kaolin minerals, whereas ferromagnesian minerals, calcic feldspar?, and volcanic glasses commonly alter to members of the montmorillonite group.Presence of magnesium ions in the altering system is favorable for the formation of montmorillonite-type minerals, in many of which it is characteristically present.The formation of montmorillonite-type minerals is favored by alkaline conditions, and that of kaolinite by acid conditions.These relations are modified by climatic factors, such as the length of time during which weathering continues.Montmorillonite-type minerals form as weathering products in soils and are formed at moderate!7 high temperatures up to at least 200°C., in association with many types of mineral deposits.Formulas of a special type are used to express the composition of the members of the montmorillonite group.In these formulas the name and the ions forming the crystal lattice are given on one line, and the exchangeable ions on the line above.Arroys are placed over the groups having a deficiency of charge, whi«h require the presence of additional base external to a silicate layer.Name [Ions with octahedral coordination] [Ions with tetrahedral coordination]Oio[OH]2 Na.33 Montmorillonite [AU.erMSi* 0 JO [OH] 2 Beidellite AUi7 [A1.8iSi,.i7]Oi0 [OH] 2 Na.ss or Al 2. 22[Ali.ooSi3.oo]Oio[OH] 2 .+ 3 T Nontronite Fe 2.0o[Al.33Si3.67]Oi 0 [OH] 2 Nontronite (aluminian) Fe 2.i7 [A1.83 Na 3s T ' Hectorite [Mg2.67Li.33]Si4 0 10 [OH] 2 Saponite Saponite (aluminian) Oio[OHj 2 1930