Friday, June 14, 2013
I have a couple of specimens mainly composed of the mineral serpentine, more properly termed by the rock name serpentinite. One was received as a CSMS Christmas party and I know little, well nothing, about its provenance. The second is a smaller slab that was collected on Staten Island, New York.
Serpentine itself is not really a mineral but refers to a group of minerals (maybe 16) with a common chemistry but different crystal structures---they are polymorphs. A generic formula is (Mg,Fe,Ni,Al,Zn,Mn)3(Si,Al,Fe)2O5(OH)4 where only two or three of the first group of elements are present at any one time. Generally these minerals tend to look similar, are green to yellow-green in color, have a waxy luster, mostly non-crystalline but massive compact in habit, sometimes fibrous to splintery to platy, with a hardness of about 4+/- (Mohs). The Serpentine Group includes four major groups: 1) antigorite (mostly platy), 2) chrysotile (mostly fibrous; asbestos); 3) lizardite (mostly small plates); 4) amesite (platy or columnar). Most serpentines are rare minerals except antigorite and chrysotile.
Serpentines are secondary minerals formed by a process termed serpentinization—a low grade metamorphism in the presence of water. Olivine minerals, amphiboles, and pyroxenes are the main source minerals.
Serpentinites are common world-wide, especially in areas where continental plates have interacted along orogenic margins. Perhaps the most “famous” localities are those in the European Alps, Australia, England, Quebec Canada, and California and Maine, USA. Serpentine is the State Rock of California.
Hydromagnesite is a hydrated magnesium carbonate (CO3)4(OH)2-4H2O. It forms in many different environments (such as caves) but germane to this discussion is a type where magnesium in “serpentines” or serpentinites begins weathering to incrusting hydromagnesite. I have a piece of serpentine (unknown exact mineral) with incrusting white hydromagnesite. Under a microscope the tiny lathlike crystals of hydromagnesite (monoclinic-prismatic [pseudo-orthorhombic]) are quite “nice”, white in color, mostly transparent, individuals are vitreous while masses are earthy, and have a hardness of ~3.5 (Mohs).
The serpentine-hydromagnesite specimen came from Staten Island, New York, an area with abundant outcrops of serpentinite. The geology of the island is sort of fascinating (Benimoff, 2012): With high points in the Ordovician (Taconic Orogeny), the Devonian (Acadian Orogeny), and the Mississippian-Permian (Alleghenian Orogeny), ancient North America (known as Laurentia) was colliding with Gondwana (ancient Europe and Africa). The proto-Atlantic ocean that separated the two continental masses continually closed until the final suture in the Permian with the formation of the great Appalachian Belt and the supercontinent Pangaea. Sometime during the early stages of this mountain building event, a piece of ocean crust from the proto-Atlantic ocean broke off and became incorporated into the collision zone. The altered remnant of this broken off piece now forms the oldest bedrock unit of Staten Island, the serpentinite. This rock unit is Silurian (~ 430 Ma) in age, and consists predominately of the serpentine minerals, antigorite, chrysotile, and lizardite.
I would like to better understand the actual formation of hydromagnesite so perhaps a reader could enlighten me! About all I know is that carbon dioxide reacts with the magnesium in the serpentine and forms the solid magnesium carbonate.
My philosophy is the same as my favorite private eye (Guy Noir): one man is still trying to find the answers to life’s persistent questions!
Benimoff, A. L., 2012, The Geology of Staten Island: www.library.csi.cuny.edu/dept/as/geo/sigeo.htm