Common Name: Serpentine – “Serpent-like” is an allusion to the colors of a snake’s skin; green and gray is the hues of the predominant mineral combinations although whites and yellows may also be present. The rock that contains serpentine minerals is serpentinite.
Scientific Name: Magnesium silicate Mg3Si2O5 (OH) 4
Potpourri: Serpentine is a mineral that results from the foundational geologic progression of plate tectonics. When two plates move in opposition, the resultant compression forces cause the uplift of mountain orogeny followed by subduction earthquakes and volcanism as one plate slides under (subducts) the other. When two plates move apart, the molten magma from the lower mantle comes to the surface as newly formed crust, essentially recycling the used, previously subducted plate. Plate tectonics are the engine of geology, the continents forming and reforming into different configurations over (geologic) time, sometimes combining into a single large (super) continent like Pangaea that coalesced about 225 million years ago (mya) and separated at the end of the Mesozoic Era 65 mya. Serpentine results from the chemical reaction that occurs when mafic magma comes in contact with water. Mafic magmas are those that have high percentages of magnesium (ma) and iron (Fe is its elemental symbol) and low levels of silica. As plate separation normally occurs in ocean basins such as the mid-Atlantic ridge, the magma-water interaction is inherent. The geologic process that forms serpentinite is called serpentinization.
The Appalachian Mountains are postulated to have resulted from a complex sequence of two or more orogenies due to interaction of the North American Plate with the Eurasian and African Plates punctuated by intervening periods of separation and erosion. The Appalachian Blue Ridge province is the epicenter falling away to the Piedmont (foot of the mountain) province moving east. The accordion effect of opening and closing resulted in the superposition of what was ocean crust up and onto the continental crust as the gap narrowed in closure. The resultant geologic formation is sometimes referred to as an ophiolite from the Greek ophio meaning snake and lithos meaning rock, a tautology of the gray-green serpent-like leitmotif of its rock composition. Serpentine from the oceanic crust located on the continental crust is therefore a direct indication of uplifting and superposition that is the weltanschauung of making terra firma; literally a rock signpost to indicate where the plates at one time met.
Serpentinization is global and it is found everywhere on earth where plate interactions have occurred from Norway to New Zealand. The active geology of the westward movement of the North American Plate into the Pacific Plate is manifest in California. Serpentine comprises about one percent of the surface; it became the California state rock in 1965. This is something of a misnomer, as serpentine is a mineral whereas the rock serpentinite is composed of various serpentine minerals. They are frequently used interchangeably however. Serpentinization is not only global but cosmic. Methane detected in the atmosphere of Mars in 2004 invited speculation as to possible biologic bacterial origination and fueled the perdurable ‘life on Mars’ thesis. An alternative methane source has been suggested. Serpentinization produces methane as a by-product when carbon dioxide is present – and this is plausible from the Mars perspective.
According to Webster, rock is a combination of minerals “formed in large quantities in the earth’s crust by the action of water, heat and/or pressure;” a stone is usually considered to be a piece of a rock. Rocks are subdivided into three groups: igneous from the cooling and hardening of molten magma; sedimentary from the erosive comminution and subsequent layering by neritic deposition; and metamorphic from the transformation of any rock by temperature and pressure. About 80 percent of all rocks in the earth’s crust are igneous. Serpentinite is considered metamorphic but is in a sense quasi-igneous as it transformed magma. A mineral is a rock component which has a composition that can be expressed as a chemical formula which imbues it with consistent distinctive physical properties such as hardness and color. While there are over 2,000 minerals, only about 30 of them are important for igneous and metamorphosed igneous rocks (like serpentinite). In elemental terms, the igneous rock minerals are comprised in whole or in large part by the mass abundance of the predominant elements of the mantle from which they were formed: oxygen (45%), magnesium (23%), silicon (22%) and iron (5.8%). The earth’s core is primarily iron and solid as it is at high pressure consistent with its high temperature.
Serpentinite formation begins with olivine, a magnesium rich iron silicate mineral that comprises about 50 percent of the mantle; typically olive green in color (hence the name). It comes in two varieties according to whether the primary constituent is magnesium (known as forsterite Mg2SiO4) or iron (known as fayalite Fe2SiO4). When the predominant forsterite comes in contact with the water and silicon dioxide in ocean basins, the reaction is serpentinization:
Forsterite 3Mg2SiO4 + Aqueous silica SiO2 + Water 4H2O à Serpentine 2Mg3Si2O5 (OH)4
This reaction is exothermic, elevating the water temperature to more than 500°F creating an oceanic hydrothermal vent for the nurture of thermophile life forms such as some archaea (formerly archaebacteria). However, fluid flow, chemical interactions, and geologic forces are chaotic by nature; it is actually not as simple as one formula. Serpentinite rock can contain any of about 20 different varieties called polymorphs known as the serpentine subgroup; the differences are in the crystal lattice structure of the molecules, not the chemical composition. The three most important are antigorite, chrysolite, and, whimsically, lizardite.
Serpentinite is not a good substrate for soil generation; areas with substantial serpentine are called barrens due to the paucity of vegetative growth. The soil of serpentine barrens is characterized by a low calcium to magnesium ratio, a lack of nutrients necessary for plant growth (fertilizer is nitrogen-phosphorus-potassium in three NPK digit code percentages), and high concentrations of inimical metals such as chromium and nickel. Of these deleterious parameters, the low calcium to magnesium ratio is considered the most significant. Calcium is a key constituent in plant cell walls and in the production of meristematic cells which are undifferentiated to promote organ growth much like human stem cells. Stunted growth is syllogistic with low calcium.
Serpentine barrens are widely scattered in regions with tectonically active histories that included mafic ophiolites. They are characterized by xeric grasslands punctuated by scattered resilient trees like red cedar, post oak and Virginia pine and a petrichor smell after rain. The 2,000 acre Soldier’s Delight Natural Environmental Area just west of Baltimore is the largest expanse of serpentine barrens in the Eastern United States. It is the only remaining preserve of what was once the Soldier’s Delight Hundred that extended from the Patapsco River to the Pennsylvania border established in 18th Century colonial Maryland. In 1810 extensive deposits of chromite were discovered. The chromium extracted from the ore was at that time used in the manufacture of dyes, paints and chemicals; Soldier’s delight was largest supplier in the world for most of the 19th Century. The origin of the name Soldier’s Delight is obscure, but possibly due to the use of the area by the soldiers from a nearby fort who patrolled there in the colonial era; other more prurient etymologies have been advanced.