St. Peter Sandstone
Left: St. Peter Sandstone hand sample is four inches across. Right: St. Peter sand grains (20x magnification). Both
specimens are from Pacific, Missouri.
Commodity: St. Peter Sandstone, a sedimentary rock formation belonging to the Ordovician System, is composed almost entirely of silica in the form of quartz sand grains.
Economic Importance: St. Peter Sandstone is used in a variety of ways that enrich the modern way of life. It has been produced in Missouri for the last 140 years. More than 65 million short tons of St. Peter, having an estimated present value of $2 billion, have been mined in Missouri from the 1870s to present. In 2008, more than 700,000 short tons of St. Peter at a value in excess of 20 million dollars were produced from Missouri. St. Peter Sandstone was originally used for the manufacture of glass. Its dominant use recently has been as a proppant in oil and gas formation stimulation nationwide. There is an estimated 3.8 trillion short tons of St. Peter Sandstone reserves in Missouri.
Mineral and Chemical Composition: St. Peter Sandstone is composed almost entirely of silica, otherwise known as silicon dioxide (SiO2) or quartz. In Missouri, the silica content is typically not less than 96 percent, with many areas higher than 99 percent. The iron oxide content is often less than 0.1 percent. The following table is an average chemical composition (weight percent composition) of 22 unwashed samples from different locations in Missouri (Dake, 1918).
Occurrence: The St. Peter is continuously present in the subsurface in the northern half of the state and the southeastern edge of the state. The St. Peter crops out (occurs at the surface) in a narrow band that starts in western Montgomery County and runs southeastward, along the Missouri River, to just west of St. Louis and continues south, just west of the Mississippi River, through Scott County. The outcrop width varies from less than one mile wide to over ten miles wide, and spans more than 150 miles in length. The formation dips into the subsurface radially away from the outcrop band. A notable isolated outcrop occurs in Lincoln County to the north of the main outcrop band. The St. Peter is discontinuously present in the subsurface along a band in the central western portion of the state. The St. Peter is not present at the surface or in subsurface in the remainder of the state. The formation averages 80–100 feet thick.Missouri Geology Store (online), visit our publications counter at 111 Fairgrounds Rd., Rolla, Mo., or call 573-368-2100.
- OFM-11-594-GS: Structure Contour Map on top of St. Peter Sandstone Mineral Resource in Missouri
- OFM-11-595-GS: Isochore (Drill Thickness) Map of St. Peter Sandstone Mineral Resource in Missouri
- OFM-11-596-GS: Overburden Thickness Map above St. Peter Sandstone Mineral Resource in Missouri
Primary Use: St. Peter Sandstone has been dominantly used for glass manufacturing since first being mined. The high silica content and subsequent lack of impurities make it ideal for use as glass sand. More recently, it is rapidly being utilized as a proppant in hydraulic fracturing to enhance oil and gas extraction. It is employed when a rock formation beneath the surface is hydraulically fractured to create or improve the flow of natural gas and oil to wells. The sand is pumped into the fractures to hold them open, thus increasing the yield from wells.
Other Uses of St. Peter Sandstone:
- Refractory material — used as foundry sand, molding sand, and core sand.
- Abrasive — used for polishing compounds, grinding compounds, stone sawing, and blasting sand.
- Metallurgical silica — used as a component in the preparation of silicon alloys and as a flux.
- Finely ground silica:
- Ceramic industry — used in whiteware, earthenware, chinaware, and glazed wall tile.
- Chemical industry — used in the manufacture of soluble silicates and silicon carbide.
- Used as an additive in paint, adhesives, and investment castings.
- Used as an anti-caking agent in table salt.
- Catalytic converters — used to filter exhaust particulates.
- Engine sand — used to improve locomotive traction on slippery rails.
- Filter media — used in drinking water purification and wastewater treatment.
- Filler material — used due to its relatively inert chemical properties.
- Desiccant — in the form of silica gel.
Properties: The St. Peter Sandstone is typically a well-sorted, friable, ultra-pure, fine-to medium-grained, quartzose sandstone with silica content higher than 99 weight percent in places. The sand grains are rounded, highly spherical and characteristically frosted. They typically vary in size from two millimeters (No. 10 U.S. Standard Sieve Series size) to less than 0.075 millimeters (No. 200 U.S. Standard Sieve Series size). Bedding is indistinct, and the formation appears massive throughout. The rock is cross-bedded and ripple-marked locally. The formation is generally porous, permeable and mostly nonfossiliferous in Missouri.
- Color: White, with occasional shades of pink and green. Weathered surfaces are a dirty gray or brown and are case-hardened at many localities.
- Hardness: Individual sand grains are hard, seven on Mohs scale.
- Cementation: Soft, friable, easily disaggregated.
- Specific Gravity: Estimated 2.24 assuming 15 percent porosity (S.G. of quartz is 2.65)
- Porosity: Estimated 10–15 percent
- Solubility: Quartz has a very low solubility under normal conditions, thus St. Peter Sandstone is stable in ordinary situations. The solubility of quartz increases rapidly when pH exceeds nine. Quartz is insoluble in acids except hydrofluoric.
General Geology: The St. Peter Sandstone in Missouri belongs to the Mohawkian Series of the Ordovician System and comprises the following four members, listed from top to bottom:
Starved Rock Member
Kingdom Shale Member
Not all of the members are present in the St. Peter throughout its distribution. In southeastern Missouri the Tonti member of the St. Peter rests, possibly conformably, on the Everton, and is conformably overlain by the Joachim Dolomite. North of Jefferson County, the Kress Member (“detrital zone”) is disconformable on the eroded surface of the Ibexian (Canadian) Series strata. In some parts of the subsurface of northern Missouri two members, the Starved Rock Member and Tonti Member are present and sometimes separated by the Kingdom Shale Member. The Starved Rock is an elongate barrier bar sequence, oriented northeast to southwest, overlying the more wide-spread “sheet-sand” of the Tonti Member. The depositional environment of the St. Peter is much debated. It has characteristics of both eolian and marine origin. It is likely a combination of both, beginning as a massive dune field that was overtaken and reworked by a marine transgression (that is, transgressive barrier bar sequence) and transformed into a series of offshore bar deposits, oriented northeast to southwest.
Mining: The St. Peter Sandstone was first mined in Missouri in the 1870s for glass sand. Early mining was conducted at the surface and underground, and was concentrated near the town of Pacific. Presently (2012) there are four companies producing St. Peter in Missouri. These sites are located in eastern and southeastern Missouri along the St. Peter outcrop band. Current production is from surface quarries using controlled blasting at the quarry face to break the sandstone into pieces. It is then disaggregated by crushing or high pressure water-jet. The product is then washed, separated, and dried by various methods depending on desired end product.
Photos of St. Peter Sandstone abandoned mining location in Pacific, Missouri.
- Recorded production of silica sand (St. Peter Sandstone) began in 1910.
- Missouri’s highest annual production was in 1974, at 1.4 million short tons.
- Cumulate production of St. Peter is now greater than 65 million short tons, with an estimated present value of $2 billion.
Missouri Geology Bibliography: Search the electronic bibliography, published by the Missouri Geological Survey and predecessor agencies, contains references from 1804 to present.
Chemical composition data adapted from the following reference:
- Dake, C.L., 1918, The sand and gravel resources of Missouri: Missouri Bureau of Geology and Mines, Volume 15 (2nd Series), 274 p.
Lithologic and stratigraphic descriptions adapted from the following references:
- Thompson, Thomas L., 1991, Paleozoic Succession in Missouri, Part 2 – Ordovician System: Missouri Department of Natural Resources, Missouri Geological Survey, Report of Investigations 70 Part 2, 202 p., 163 figs.
- Thompson, Thomas L., 1995, The Stratigraphic Succession in Missouri (Revised 1995): Missouri Department of Natural Resources, Missouri Geological Survey, Volume 40 (2nd Series) Revised, 188 p., 42 figs., and 1 tbl.