TRACE HYDROGEN IN QUARTZ DURING WEATHERING OF GRANITE

Quartz sand is a ubiquitous component of sedimentary rocks, often collected as forensic material. It is also a major component of granitic rocks. Recent studies have shown that provenance of sediments can be determined from the infrared spectrum of trace hydrogen in quartz. This study presents infrared analyses of quartz from a weathering sequence of the Rolesville granitic pluton in North Carolina, to determine the feasibility of hydrogen-in-quartz as a forensic tool, and to determine changes in spectra as a function of weathering. Analyses of fresh granite test the viability of measuring hydrogen in granite as a tool for tracking water during crystallization.

Quartz, and quartz sand were collected from the Mitchell Mill State Natural Area. Specimens range from fresh unaltered granite, friable highly weathered granite, and quartz sand from streams that drain the Rolesville granite. The quartz was analyzed with polarized infrared radiation as doubly polished and unpolished grains. Thickness was determined directly from spectra by peak height at 1794 cm^-1. (Stadler et al. 2017, Am.Min. 102, 1832).

The OH stretching region, (3400 -3600 cm^-1), was analyzed to determine the change in water content. The unaltered granite frequently produced Al³⁺ peaks at 3365 and 3430 cm^-1 and one small, broad molecular water peak at 3400 cm^-1. The highly weathered granite produced one large molecular water peak in the 3400 cm^-1 regions .

Data in hand indicate that there is considerable compositional overlap between quartz from unweathered and weathered granite, and the coarse quartz sand. This finding is promising for the forensic investigations involving quartz. H-in quartz also has potential for studies of igneous rocks rich in quartz.

Some specimens of weathered quartz lack a molecular water component, or any OH signal. We attribute this observation to mechanical and thermal weathering of the quartz. It could be concluded that the variability of the hydrogen signal in quartz, from a single locality, severely limits its use as a forensic tool. However, greater amounts of data elucidate a different picture. Analysis of quartz grains can be automated to produce large data sets, one of the strengths of this method. Given that FTIR is already in use in many criminal laboratories, this technique may have utility in forensic investigations.