The rapid increase in mining and processing of frac sand has led to concerns about potential health hazards, particularly silicosis from airborne respirable silica. A regional petrographic analysis, focused on the two primary ‘frac’ sand targets in the upper Mississippi River Valley, the Wonewoc (n=17) and Jordan (n=27) formations, has been undertaken to evaluate potential sources of respirable silica. Regional petrographic analysis provides insight into both detrital and matrix compositions, and constrains the types of airborne particles that can be liberated during the production process. Petrographic examination illustrates similarities and differences between the Wonewoc and Jordan formations, and suggests geographic variations in sandstone mineralogy. Both units contain primarily detrital monocrystalline quartz (68-67%) and void space (20-21%). Cement composition varies geographically both within and between formations. Cement in the Wonewoc Formation is dominated by hematite (~16%) with lesser authigenic feldspar (Kspar, 9%) and sericite (4%). Jordan Formation cement contains up to 8% calcite with lesser authigenic Kspar (7%) and hematite (10%). The Wonewoc Formation displays minor geographic variations in grain composition, with a slight decrease in detrital Kspar from northwest to southeast (5% to 2%), but a distinct difference in cement composition, varying from 16% authigenic Kspar cement in the northwest to 2% in the southeast. The Jordan Formation shows similar variation in authigenic Kspar cement (10% northwest vs. 5% southeast), but also shows a distinct geographic variation in the hematite (13% northwest vs. 6% southeast) and calcite (10% northwest vs. 6% southeast) components. Silica cement is essentially absent, with silicified zones <2m thick limited to restricted horizons in the upper Jordan Formation in the southern region.

Textural analysis suggests complex, multi-stage diagenetic events, including an initial generation of authigenic Kspar, followed by carbonate, then hematite in the Jordan Formation. The high amount of pore space is the product of multiple stages of dissolution. These analyses indicate that interstices in the Wonewoc and Jordan formations contain mainly void space, calcite, hematite, authigenic Kspar and sericite. Interstitial silica is rare.