Serpentine is a common group of magnesium silicate minerals found in many parts of the world. This collection of minerals shares a similar chemical makeup and geological origin. The name derives from the Latin word serpens, or snake, due to the mottled green appearance and scaly texture of the rock they form, which often resembles a snake’s skin.
Defining the Serpentine Group
Serpentine minerals are scientifically classified as hydrous magnesium iron phyllosilicates. Their generalized chemical formula, (Mg,Fe)\(_3\)Si\(_2\)O\(_5\)(OH)\(_4\), indicates they contain magnesium, silicon, oxygen, and hydroxyl groups, with iron often substituting for some of the magnesium. The serpentine group is part of the larger kaolinite-serpentine group of minerals.
This mineral family is primarily composed of three main polymorphs: chrysotile, lizardite, and antigorite. These forms share the same basic chemical composition but differ in their internal crystal structure, or habit. Lizardite is often the most abundant, while antigorite forms tough, pleated layers, and chrysotile is structurally distinct because its layers curl into hollow tubes, giving it a fibrous appearance.
Geological Formation and Occurrence
The formation of serpentine minerals occurs through serpentinization, a low-temperature, hydrothermal alteration involving the addition of water to existing rock. The process transforms ultramafic rocks, which are rich in iron and magnesium-containing silicates like olivine and pyroxene. Water percolates through fractures and reacts with these primary minerals, causing them to break down and reform into serpentine minerals. This reaction is exothermic, releasing heat, and results in a significant volume increase, often between 30 and 40%.
This process is most common in deep-sea environments near tectonic plate boundaries, such as mid-ocean ridges and subduction zones. In these settings, mantle rock is exposed to seawater, which acts as the hydrating fluid. The resulting rock, predominantly composed of serpentine minerals, is known as serpentinite, which can be brought to the surface through tectonic activity.
Distinct Physical Properties
Serpentine minerals and the serpentinite rock they form possess several recognizable physical characteristics. A defining trait is the distinctive luster, often described as greasy, waxy, or silky. Serpentine typically exhibits a wide range of colors, most commonly various shades of green, but it can also be yellow, brown, or black.
The rock is relatively soft compared to many other stones, registering between 3 and 6 on the Mohs hardness scale. This softness, combined with its ability to take a high polish, makes it a popular material for architectural and decorative use. Serpentinite is often cut and sold as a decorative stone, sometimes referred to commercially as “serpentine marble” due to its visual similarity to true marble.
Serpentine and Chrysotile Asbestos
The connection between serpentine and asbestos is based on specific mineral structure, which is important for public health and safety. Asbestos is a commercial and regulatory term used for six types of natural silicate fibers hazardous when inhaled. Only one of these types is a serpentine mineral: chrysotile. Chrysotile is unique among the serpentine polymorphs because its mineral layers curl into microscopic, hollow tubes, accounting for the vast majority of commercially used asbestos, often called “white asbestos.”
In contrast, the other common serpentine varieties, lizardite and antigorite, form massive or platy structures that are non-fibrous and do not pose the same health risk. Regulatory bodies classify chrysotile as a carcinogen due to the risk of respiratory diseases like mesothelioma and lung cancer when its microscopic fibers become airborne and are inhaled. This distinction highlights the difference between the harmful, fibrous chrysotile and the non-fibrous forms of serpentine rock used in construction and carving.