Gypsum:
In Central Texas, damage due to heave is not exclusive to swelling clay. Related to clay swelling is the formation (crystallization) of sulfate rich minerals known as Ettringite (and Thaumasite) and the phenomenon is known as "sulfate-induced heave". When a clay subgrade has significant soluble sulfate content, the addition of calcium-based additives to treat the clay (e.g. lime stabilization) can initiate the formation of these minerals when adequate moisture (water) is present that is at a certain pH environment. Alumina and silica in the clay soil reacts with sulfate, water, and calcium to form the new minerals. The crystallization can increase the volume of the soil mass significantly (i.e. 200% to 250%). Clay soils that have notable gypsum content may be at a high risk of yielding sulfate-induced heave. Near surface exposures of "Eagle Ford" clay and "Del Rio" clay can yield significant soluble sulfate content and gypsum content. Pavement design and construction must be carefully considered when building on top of these geologic formations. Clay mineralogy also influences sulfate heave potential, which is why "Eagle Ford" and "Del Rio" clays can typically exhibit sulfate inducated heave at much lower concentrations of soluble sulfates than "Taylor" clay. Heave is typically noticed within a few months as damage to the pavement becomes visible (i.e. 6 inches vertically within a few months). The crystal growth is therefore relatively rapid. Gypsum is a calcium-sulfate mineral that crystalizes white or colorless (clear). "Selenite" is simply the clear transparent form of gypsum.
Nicholas Kauffman was a project geotechnical engineer during the design of the initial 4 segments (or 15 sections) of the SH130 toll road highway where soluble sulfate detection protocols and pavement subgrade treatment protocols were developed under the expertise of Professor Dallas Little. TxDOT and TTI helped develop the field detection protocol which consists of measuring conductivity rapidly to determine areas of potential risk of high soluble sulfate content.
DAMAGE TO BUILDINGS
Sulfate-induced heave in soils has also damaged buildings. A publication in 1969 by Spanovich & Fewell revealed a case history involving carbonaceous shale. For a shale with 5% pyrite content, 8.3% vertical heave was exhibited under certain chemical reactions, with the swell pressure exceeding 5,000 psf.
A case history in 1970 revealed 3 inches of heave under a slab at the Rideau Health Center in Ottawa, Canada, where shale above the groundwater table exhibited gypsum crystal growth. The shale supposedly produced sulfuric acid when pyrite was exposed and oxidized. It was theorized that specific bacteria in the warm subgrade under the slab contributed to the breakdown of the pyrite.
Another case history in 1970 revealed the Bell Canada building experienced heave in the basement slab. 4 inches of heave was reported, with an estimated growth rate as high as 21.6 mm per year (2.1 feet in 30 years if the environmental conditions were to remain intact).
A 1975 report described heaving of a basement of Saint Luke's Church in Ottawa caused by gypsum crystals.
A 1977 report described 8 inches of heave in mine floors in Kansas City area several years after exposure. Mining was within a shale deposit with significant gypsum content.
A 1986 report described 4 inches of heave in a college building in Quebec. The building was constructed on shale in 1971. Heave was determined to be continuing at a rate of 10 mm per year (12 inches in 30 years).
MASSIVE GYPSUM CRYSTALLIZATION
(photograph credit: Tullio Bernabei)
The most renowned example of gypsum or gypsum-selenite growth was discovered in the silver-lead-zinc mine of Naica in Mexico. Groundwater was pumped out to permit mining. Four caves have been discovered that exhibit very large crystals. The first cave was discovered in 1910 at a depth of 120 m (394 feet). In the year 2000, however, 3 new caves were discovered at a depth of 290 m (951 feet) that exhibited some of the largest crystallization ever recorded on our planet. One of the most amazing geological discoveries, the crystals in the largest cave were as long as 36 feet and over 3 feet in girth. The crystals in these new caves are composed of gypsum.
The near surface geology consists of a limestone rock sedimentary deposit. Conjecture about the origin of the metal deposits includes hydrothermal liquid (superheated groundwater, heated by molten rock below) depositing sulfides rich in silver, lead, and zinc onto the limestone surfaces in the fissures in the limestone. This is similar in context to hydrothermal vents on the ocean floor where the hot fluid is emitted into the ocean from the hydrothermal vents formed from the ocean floor. Sulfides are a class of minerals that include metals (silver, lead, copper). Fissures and interconnected cavity or cave features exist in the limestone due to the solubility of the limestone as the calcium carbonate limestone reacts with hydrogen ions in water flowing through fissures and ultimately cavities and caves. Solubility increases with acidity (i.e. organic acid or acid-forming gasses). At Naica, there did not appear to be any interconnection between the deep caves and the surface.
The crystals include fluid inclusions and pollen has been identified in the fluid. Pollen preserves in environments that lack oxygen and are acidic where organisms that decompose pollen cannot survive. Pollen is transported notable distances in the air, therefore the source of the pollen is always uncertain to a high degree.
Crystallization of selenite requires specific temperature, pressure, geochemistry, and water. According to a report co-authored by Mexican geologist Martin Orozco, the large crystals took at least 30 years to form, and can only occur during a short time span due to the improbability of the perfect environmental conditions to exist for a long period of time. Crystal growth occurs from the addition of new ions, atoms, or polymer strings into the lattice of the crystal. Rapid crystal growth occurs after nucleation. Due to the low density of crystals in the caves, the nucleation environment was brief (not many starter crystals) and the growth phase was very rapid. At a temperature slighlty below 58 degrees Celcius, gypsum is not yet soluble but anhydrite is soluble. One hypothesis is that the groundwater was supersaturated with gypsum due to the solution of anhydrite (dissolved Anhydrite: CaSO4.2H2O), which provided abundant amounts of calcium-sulfate to build the gypsum crystals.
A uranium-thorium measurement was used for hypothetical dating (assumes initial daughter ion was zero at initial point in time) of a core crystal and yielded an "age" of only 34,500 years (Garofalo et al.). Initial guesses by various sources had started at "millions" of years to just 600,000 years, to now just 34,500 years. The true age is likely much less when the narrow band of constant temperature and water chemistry could possibly exist to allow the gypsum to grow. Statistically and physically it appears impossible for the perfect conditions that permit crystallization in the specific deep environment to have occurred over a long period of time. The growth most likely occurred as the original geologist from Mexico stated, or perhaps speculatively 100 to 600 years or so.
Garofalo, P.S., et al., 2007, The Fluids of the Giant Selenite Crystals of Naica (Chiuhahua, Mexico), European Current Research on Fluid Inclusions, University of Bern, Switzerland