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.
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Cut surface of a serpentinite.
Unknown location. Width of photo
~6.5 cm.
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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).
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Serpentinite (green) with incrusting hydromagnesite. Width of photo ~4.5 cm.
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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!
REFERENCES CITED
Benimoff, A. L., 2012, The Geology of Staten Island: www.library.csi.cuny.edu/dept/as/geo/sigeo.htm