SCAPOLITE: A SORT OF FORGOTTEN MINERAL

Why should things be easy to understand?

Thomas Pynchon

Scapolite is one of those minerals that sort of rings a bell somewhere in the recesses of your mind; however, you cannot quite pinpoint the location! About the only thing that finally surfaced in my mind came from basic mineralogy and pointed out that scapolite is usually an alteration product of feldspar (which one?), and is a metamorphic mineral (which facies?)!  I sort of left it at that point until a few years ago when I saw some beautiful faceted gemstones labeled “scapolite.”  Perhaps my mineralogy factoids were a figment of my imagination for those faceted gems looked nothing like some less-than-spectacular specimens I remembered from class.  

Perhaps I could forgive my mind since I was a third-year college student trying to reconcile memorizing mineral crystal systems with understanding the bombing of the 16^th Street Baptist Church in Birmingham and the assassination of President Kennedy in Dallas.  In fact, the assassination of Kennedy is one of those moments in history that persons of my age have imprinted on their minds--- I was heading to Mineralogy class!  Why did the crystal systems matter when young girls and presidents were being murdered?  I guess the short answer is that I did not want to return to my home town and work in my father’s gasoline station.  And then, there were rumors about “goings-on” in southeast Asia with the military draft picking up and men of my age learning a new trade.  So, back to learning about Monoclinic and Hexagonal minerals (and I never really understood the Systems and became a paleontologist).  And, scapolite became lost!

Age is an issue of mind over matter.  If you don’t mind, it doesn’t matter.

Mark Twain 

Scapolite reappeared in my mind back in 2012 when I was working on a post describing idocrase/vesuvianite.  The latter mineral was named by the famous German mineralogist Abraham Gottlob Werner and an informal variety of scapolite is called werernite.  Long story---read the November 18, 2012 Blog posting.  At any rate, I then took scapolite from the back recesses and shoved it toward the front of my mind and four years later am finally getting around to describing some specimens that I picked up along the way!

Scapolite is a silicate but is not really an individual mineral!  It is a solid solution series between end members marialite (sodium chloride rich) and meionite (calcium carbonate rich): Na₄Al₃Si₉O₂₄Cl to Ca₄A_l6Si₆O₂₄CO₃.  The sodium and calcium are interchangeable with each other as are the chlorine and the carbonate radical, therefore leaving an infinite number of chemical compositions. In addition, the calcium may include some strontium while the sodium may include potassium. And SO₄ may substitute for some CO₃ (Evans and others, 1969).  It appears that “pure” end members never occur in nature so intermediate compositions are the norm; however, these intermediate members vary considerably in chemical composition and remain unnamed.  Members of the solid solution series are essentially indistinguishable (visual) from each other and therefore scapolite is simply used for all. 

Scapolite comes in a variety of spectral colors ranging from colorless to white and yellow, purple, blue, red, green, pink, brown, gray, orange and various mixed compositions.  However, all varieties have a white streak. The transparency ranges from completely opaque to translucent to completely transparent while the luster ranges from vitreous to dull and pearly.  As scapolite weathers to “mica” the luster becomes dull and the diaphaneity becomes opaque. The hardness of ~5.5-6.0 (Mohs) makes gemmy varieties more suitable for pendants rather than rings.  Scapolite crystals are Tetragonal and generally come in two distinct forms: short and fat, or long and prismatic.  Gemmy varieties are usually prismatic and commonly striated.  A couple of my specimens show masses of non-gemmy and opaque crystals.  Many times, crystals fluoresce under both short and long wave UV.

Scapolite is one of the few minerals that have a “square” cross-section that helps in identification.  Compare photos below of a weathered crystal from Monmouth Township, Ontario, Canada, with a crystal diagram from the Goldschmidt atlas and found on www.mindat.org and courtesy of www.smorf.nl.

Cross-sectional view of scapolite crystal, non-terminated, collected from Grenville Terrane near Bancroft, Ontario.  Note square shape of crystal and compare with sketch below.  Width of crystal ~1.6 cm; length ~2.3 cm. 

Crystal diagram of scapolite from the Goldschmidt atlas and found on www.mindat.org and courtesy of www.smorf.nl. Note square shape.

I thought scapolite was perhaps a mineral indicative of a specific metamorphic facies.  However, I have learned the “mineral” occurs in a variety of metamorphic conditions ranging from regionally metamorphosed schists and gneisses to higher temperature and pressure amphibolites and granulites (usually as an alteration of feldspar minerals and producing non-gemmy crystals).  In addition, scapolite, at times gemmy, is found in marble produced by contact metamorphism. At other times scapolite in these calc-silicate rocks contain inclusions of clinopyroxene, quartz, titanite and calcite (Ocean Drilling Program).   It is also found, at times, in pegmatites associated with contact metamorphism, and basalt ejected from volcanos.  I certainly am far from a mineralogist/petrologist but have spent numerous hours reading “lots of articles” concerning scapolite, and trying better to understand the chemistry and genesis.  I have somewhat failed in my understanding and concluded that it is a very complex mineral found in several different environments and is quite difficult to identify as to a specific mineral.

Just because we don't understand doesn't mean that the explanation doesn't exist.       Madeleine L’Engle

My collection includes two specimens composed of a non-gemmy mass of opaque crystals collected from around Bancroft, Ontario, Canada.  Also from near Bancroft is a single, squat weathered crystal. 

Above two photos are masses of opaque, non-gemmy scapolite crystals.  Note nice terminations on crystals with T pointer and nice square shape with SQ pointer..  Width FOV top ~4.0 cm, bottom ~4.3 cm.  Both specimens have tiny crystals of an amphibole (katnophorite/hornblende??) and ferroan phlogophite on reverse.

  

Map of proto-North America showing addition of crust (yellow) to continent in late Precambrian by plate collision tectonics (Grenville Orogen).  High temperature and pressure accompanies these collision events and creates large expanses of metamorphic rocks and allows for the formation of minerals like scapolite. Map from Karlstrom and others (1999).

The Bancroft area of Ontario, part of the Grenville Province, is thought to have been the margin of North America during the Proterozoic part of the Precambrian.  The rocks are composed of two tectonic elements: 1) high-grade gneisses that were part of the 1.7-1.4 Ga continental margin; and 2) a package of volcanic, plutonic, and sedimentary rocks that are thought to be a collage of arc components accreted at ca. 1.17 Ga (island arc material stuck onto the early continent by plate collision) (Keck Geology Consortium, 2011).

From the Dara-i-Pech pegmatite field, Chapa Dara District, Konar Province, Afghanistan, I have several small gemmy crystals lavender in color.  The crystals are prismatic in nature and have at least one terminated end.  The location of the crystal mine is in the northeastern part of the country where lower Paleozoic rocks are intruded by Cretaceous-Tertiary granite and granodiorite intrusions (creating contact metamorphism--cooking the limestone).   Due to political instability in Afghanistan, specifics about gemstone localities are difficult to ascertain.  

Nice gemmy scapolite crystals.  length of longest crystal is ~1.1 cm.

I also have a partial violet crystal from the Marble Occurrence, Morogoro Region, Uluguru Mountains, Tanzania.  As best that I can determine, the area is the site of plate collisions in the latest Precambrian.  Metamorphism and thrust faulting left small patches of marble on older rocks (Fritz and others 2009).  If you have the inclination to read about some really complex geology, check out the Fritz article!

Partial crystal of gemmy scapolite with undetermined inclusions.  maximum width of crystal ~1.1 cm.

And finally, I have a beautiful, free form cab of crystal-clear, gemmy scapolite collected from Espirito Santo, Brazil (along with a second specimen, a nice gemmy, prismatic crystal).  Espirito Santo is a coastal Brazilian state north of Rio de Janeiro and east of the famous mineral-producing state of Minas Gerais.  It was difficult to acquire much information about the area except that really gem quality aquamarines are mined from the Mimoso do Sul Mine.  The gem bearing rocks are latest Precambrian in age (100 Ga to 54 Ga) and seem related to the Aracuai Orogeny and include a wide variety of metamorphic rocks and igneous intrusions.  The Aracuai Orogeny added crustal rock to the local Brazilian Craton. I presume, but remain uncertain, that the gem scapolite came from some of the marble units.

Prismatic, gemmy, clear with yellow tint, scapolite crystal. Length ~3.0 cm.

Gemmy, clear with yellow tint, free-form cab of scapolite. The X is beneath the cab to show the transparent nature of the crystal (thickness 6 mm.).  Length ~2.3 cm.

So, when it comes to scapolite:  I don't think I'm old enough or experienced enough to give anyone any guidance. All I would like say is that as long as you're having fun, I think you're doing the right thing.                Sania Mirza

REFERENCES CITED

Evans, B.W., D.M. Shaw, and D.R. Haughton, 1969, Scapolite stoichiometry: Contributions to Mineralogy and Petrology, v. 24, issue 4.

Fritz, H., V. Tenczer, C. Hauzenberger, E. Wallbrecher and S. Muhongo, 2009, Hot granulite nappes—Tectonic styles and thermal evolution of the Proterozoic belts in East Africa: Tectonophysics, v. 477.

Karlstrom, K.E., S.S. Harlan, M.L. Williams, J. McLelland, J.W. Geissman, Karl-Inge Åhäll, 1999, Refining Rodinia: Geologic Evidence for the Australia–Western U.S. connection in the Proterozoic:  GSA Today, v. 9, No. 10.

Keck Geology Consortium, 2011, Anatomy of a mid-crustal suture: Geology of the Central Metasedimentary Belt boundary thrust zone, Grenville Province, Ontario:  http://www.keckgeology.org/2011-ontario-canada.

Ocean Drilling Program, Unknown date, Macroscopic description of calc-silicate rocks:  http://www.odp.tamu.edu/publications/161_SR/chap_18/c18_3.htm