SERPENTINE AND HYDROMAGNESITE

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.

 

Cut surface of a serpentinite.  Unknown location.  Width of photo ~6.5 cm.

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).  

 

Serpentinite (green) with incrusting hydromagnesite.  Width of photo ~4.5 cm.

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   

GOETHITE FROM GRAVES MTN, GEORGIA

I am always on the lookout for “strange” minerals or at least something rather unfamiliar to my limited mineralogical knowledge.  Therefore, while attending the recent Rocky Mountain Federation show in Sandy, Utah, I nabbed a single and isolated specimen labeled “turgite” that was in the “sale bin”.  After all, what is a frugal collector to do when something interesting and “cheap” pops out in front of you?

 

Graves Mountain Goethite var. Turgite.  Note bladed and mammillary goethite covered by hematite.  Golden-colored material in upper right is pyrophyllite.  Length of specimen ~12 cm.

After returning home I begin to inquire about turgite in my reference books and on various web sites (esp. www.mindat.org).  It turns out that turgite is not recognized as a true mineral since it is a mixture of both hematite and goethite.  Some rockhounds continue to use the term as a variety of hematite [Fe₂O₃] and/or goethite [Fe⁺³O(OH)] as in “goethite var. turgite”.

Whatever the case, turgite usually appears as mammillary, botryoidal, stalactitic or bladed goethite that has its surface covered by hematite.  Or perhaps the goethite has even partially altered to hematite.  What makes turgite “famous” and quite collectable is the fact that the hematite often displays a beautiful spectrum of colored iridescence and resembles a fire agate (at least to me).  Good specimens often are quoted for hundreds of dollars.  Now my specimen cost me a couple of bucks and only has a few spots of golden iridescence—until closely examined under a binocular microscope!

As a bonus, my turgite specimen has some nice crystals/exposures of golden-colored pyrophyllite, an aluminum silicate [Al₂(Si₄O₁₀)(OH)₂].  There are also “spots” where tiny crystals of quartz are very loosely cemented and contain microcrystals of pyrite and rutile (both almost impossible to photograph). 

 

Very soft blades of pyrophyllite.  Width ~3.5 cm.

Graves Mountain, where my specimen was collected, is perhaps Georgia’s best known collecting locality and is especially noted for specimen minerals (besides turgite and pyrophyllite) of kyanite [Al₂(SiO₄)O], pyrite [FeS₂], rutile ([TiO₂] world-class), lazulite [(MgFe⁺²)Al₂(PO₄)₂(OH)₂], and numerous others.  In fact, the Mountain was originally mined for the aluminum-rich, refractory kyanite used in such items as spark plug insulators and later as insulated space shuttle tiles (T. Hanley, 2005).

It appears, from my limited knowledge, that the geology of Graves Mountain is quite complex!  It lies in the “foothills” of the Appalachian Mountains, and is thrown into the Piedmont Physiographic section of the mountains.  In introductory terms, the great Appalachian Belt resulted from the collision of parts of Gondwana (Africa and South America) with Laurentia (North America).  The tectonic events started in the Ordovician (Taconic Orogeny), hit another high point in the Devonian (Acadian orogeny) and culminated in the Alleghenian Orogeny (Mississippian through Permian).  These events were compressional in nature and produced the: 1) gently folded late Paleozoic rocks of the inner region, the Appalachian Plateau; 2) more steeply folded and faulted early to middle Paleozoic rocks of the Valley and Ridge (sandstone producing ridges; shale producing valleys); 3) the badly deformed metamorphic terrane of Precambrian and earliest Paleozoic rocks of the Blue Ridge; and 4) the complicated and varied terrane of faults, metamorphic rocks and igneous intrusions known as the Piedmont.  

     

One section of the greater Piedmont is the Carolina Slate Belt, essentially an old volcanic terrane (rocks ~560 Ma; right around the Precambrian-Cambrian boundary) that has been subjected to later metamorphism.  These rocks originated in an island arc system that was part of the proto-Atlantic Ocean. As proto-North America banged into Gondwana the island arc was “squeezed” and accreted (stuck onto) to North America (Allard, 1999).  Perhaps this island arc appeared something like the modern Philippine Islands in the Sea of Japan.

Graves Mountain itself has a unique geology in that the principle rocks are pyritiferous kyanite granofels and sericite (type of muscovite) schist. Allard (1999) believes these rocks are the result of metamorphism of a hydrothermal alteration system---a subsurface hot water vent! I presume the surficial goethite (stable Fe+3) formed from the oxidation of pyrite (and perhaps lazulite) with the unstable Fe+2.

So, if you have not seen an iridescent goethite/hematite from Graves Mountain, take a look on the web.  They can be beautiful specimens for your cabinet.  

REFERENCES CITED

Allard, G. O., 1999, Graves Mountain, Slate Belt, Georgia Geology in Graves Mountain and Magruder Mine, Wilkes and Lincoln Counties Georgia, M. V. Hurst and C. Winkler III. Eds.: Southeastern Geological Society Guidebook Number 38. http://segs.org/wp/wp-content/uploads/2010/01/SEGS-Guidebook-38.pdf

Hanley, T., 2005, The New Georgia Encyclopedia: www.georgiaencyclopedia.org.