Illuvial Versus Inherited Clays in a Cryoboralf of the Boulder Batholith, Montana

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Abstract
Some forested Mollic Cryoboralfs of the granitic Boulder batholith have smectite‐rich B horizons. A representative soil was studied to determine processes responsible for morphologically well‐developed argillic horizons in relatively young soils formed in coarsegrained quartz monzonite parent material. X‐ray diffraction and particle‐size analysis indicated that B horizons of this soil differ greatly from surface eluvial horizons in both type and amount of clay minerals present and in the degree of weathering of plagioclase feldspars. Illite is the dominant clay mineral in the andy loam eluvial horizons and plagioclase grains appear only slightly weathered when examined in thin‐section. In Bt horizons, smectite is the dominant clay mineral and few identifiable plagioclase grains are seen in thinsection. Soil fabric analysis indicated illite rather than smectite as the dominant clay mineral being translocated within the soil profile. Results suggest that smectite content of B horizons is most likely a result of in situ alteration of plagioclase by hydrothermal processes prior to soil formation. Thus, smectite inherited from parent material is primarily responsible for the mineralogical and morphological properties of the argillic horizon. Although some illuviation of clay has occurred, this process has played a relatively small role in influencing these properties.

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