CLAY MINERALOGY OF A SAPROLITE DEPOSIT NEAR LANCASTER, NEW HAMPSHIRE

A recently discovered saprolite exposure near Lancaster, NH likely represents a preserved remnant of preglacial or interglacial weathering. The deposit is located at an elevation of 460m approximately 4km west of the summit of Mt Cabot in the Pliny Range. The deposit covers at least 2ha in an area mapped as Jurassic medium-grained granite. However, no exposed outcrops have been found in the immediate vicinity.

Oriented <1µm clay mineral samples were analyzed using a Rigaku Smart Lab SE X-ray diffractometer equipped with a D/tex Ultra 250 solid state detector and configured to utilize the divergent beam method. Samples were scanned from 2˚ - 40˚ two theta at 5˚/min after the following treatments; air dried, glycol solvation, heat to 350˚C and 500˚C. Diffraction patterns were compared to simulated patterns created using NewMod II software.

Randomly interstratified kaolinite/smectite (kaolinite(0.95)/smectite R=0) makes up over 90% of the <1µ fraction with the remainder composed of a mixed layer illite/smectite that is mainly smectite (illite(0.1)/smectite R=0). No discrete illite was detected. Residual sand sized material in the deposit was composed of quartz with only minor amounts of k-feldspar.

Deeply weathered bedrock, while relatively rare, does occur at a number of localities in New England. “Rottenstone” localities occur within the Jurassic Conway Granite, although it is unclear if these are the result of subaerial weathering or hydrothermal alteration. Breakdown of the Conway Granite is driven primarily by the alteration of biotite to vermiculite, with the associated expansion of the mineral accounting for the crumbling of the rock. Some kaolinite also occurs from feldspar weathering. Vermiculite and kaolinite are also observed in New England till deposits with the vermiculite generally forming as the result of breakdown of biotite and chlorite. Kaolinite is less abundant but also due to the breakdown of feldspar.

Weathering in the Lancaster saprolite is distinctly different from that in both the “rottenstone” and till. The dominance of kaolinite together with the alteration of most of the primary reactive minerals suggests nearly complete weathering of the underlying granite and suggests that the saprolite is of great age, at least prior to the last glaciation and perhaps even pre-Pleistocene.