Friday, September 4, 2020

ANAPAITE: NOTHING TO WRITE HOME TO MOMMA ABOUT

 

I was rummaging around in my drawers of minerals attempting to kill some time and trying not to feel sorry for being self-quarantined when I stumbled on an older looking perky box labeled anapaite.  A “long time” ago I had checked on this mineral after purchase and had written “phosphate” on a label. OK, so what was it?  The collecting locality was “Cerdanya, Catalonia Spain.”  That must be a nice place to visit came into my mind so I decided to check it out, not that I will ever make it to Spain.  I only knew a few insignificant “facts” about Catalonia: 1) Catalans are trying to win independence from Spain and speak the Catalan language; 2) Barcelona is the second largest city in Spain and is a major tourist destination on the Mediterranean; 3) Salvador Dali was a Catalan; 4) the northeastern part of Catalonia contains the Eastern Pyrenees, part of the Pyrenees Mountains, running east-west and forming the border between Spain and France—sort of.  The Principality of Andorra, a microstate of ~180 sq. miles, is stuffed in along the border as the official boundary splits to accommodate this tiny area.  As a kid I was a geography freak (still am) and studied intensely every map I could lay my hands on (not many in my small Kansas community).  Places like Andorra I could only imagine in my daydreams but did manage to nab some Andorran stamps with “pretty pictures.”  


Tectonic map of southern Europe, North Africa and the Middle

East, showing tectonic structures of the western Alpine 

Mountain Belt.  The Himalayan Mountain Belt continues to the

east off the map. Map Public Domain credited to Woudloper.

So, as I was learning about the mineral anapaite, I was also learning about the geology of the Pyrenees Mountains—who said I could not multitask?  The current configuration of the Pyrenees Mountains is related to the collision of the Iberian (Spain and Portugal) Plate and the Eurasian Plate in the later Cretaceous and early Tertiary.  The Iberian Plate was subducted by the overriding Eurasian Plate creating numerous faults and compressional structures as the once independent Iberian Plate was being squeezed between Africa and Europe.  In fact, the Pyrenees represent the western edge of the ~8000 miles long Himalayan-Alpine Belt that formed as a result of collisional dynamics between Arabia, India, Asia, and Europe—plates were on the move!

Sketch showing Eurasian Plate overriding Iberian Plate during the early Tertiary.

However, the Pyrenees’ collision dynamics were unlike the tectonic activity in the Himalayan and Alps mountains.   In the latter system, where continental plates with low densities collide, they do not subduct beneath each other, at least not much. But the crust does shorten, as the margins of the two colliding continents together with whatever was between them get scrunched together into huge folds and uplifts.  That collusion describes the Himalayan and Alps mountain zones (Gibson, 2014). However, the Pyrenees were not generated by a continent to continent collision, but merely reflect the confrontation of two distended but continuous continental domains during the Tertiary, leading to the incipient underthrusting of the Iberian crust under the European crust (Canerot, 2016).


Continent to Continent plate collision with crustal shortening causing rising elevation. Public Domain map with credit to U. S. Geological Survey.

In the later Tertiary (Miocene) several structural basins formed (probably due to extensional tectonics and faulting) within, and adjacent to, the Pyrenees including the Cerdanya Basin where several hundred feet of lacustrine (lake) sediment accumulated as organic ooze and diatomite (composed of fossilized diatoms which are microalgae). Within these lake rocks geologists located phosphate minerals, mostly anapaite and fairfieldite, in veins, nodules and spherulite beds.  These phosphatized sedimentary rocks are diagenetic in nature (alteration of existing minerals by chemical change) and very dependent on environmental conditions of deposition and are quite different from phosphate minerals described from marine systems (De las Heras and others, 1989). In marine units, phosphorus accumulation occurs from atmospheric precipitation, dust, glacial runoff, cosmic activity, underground hydrothermal volcanic activity, and deposition of organic material. The primary inflow of dissolved phosphorus is from continental weathering, brought out by rivers to the ocean (Delaney, 1998). 

Location of Cerdanya Basin along axis of Pyrenees Mountain.  Map from Lewis and others, 2000.

Almost all igneous rocks also contain a phosphate mineral and it is almost always an apatite group mineral that forms as an accessory mineral.  Again, the accumulation of igneous phosphatic mineral is much different than marine or lacustrine phosphates. When apatites occur as accessory minerals in igneous rocks they are of macroscopic size and are idiomorphic (identifying crystal form). These are generally fluorapatites which are relatively rich in CI and are characterized by high contents of trace elements. By contrast, apatites formed in surface environments are usually sub-microscopic and are barely visible. These latter apatites are usually carbonate fluorapatite in sediments, or hydroxyapatite in vertebrate skeletons (Lucas and others, 1997).   

Anapaite [Ca2Fe(PO4)2-4H2O] is an interesting mineral—for a number of reasons: 1) at the type locality in Anapa, Russia (Crimean Peninsula and formally in Ukraine), it is found in the stems of fossil trees and in phosphate bearing, oolitic iron ore; 2) the mineral occurs in two different crystalline forms—bladed/tabular, and spiky; 3) it is commonly found in rosettes, crust-like subparallel aggregates, in geode-like nodules, and fibers.  





Photomicrographs of green to gray-green anapaite in veins of matrix that may be an apatite group mineral. All crystals are sub millimeter in size.

Anapaite is often green in color but ranges to greenish white, gray-green and colorless.  It has a vitreous luster, a white streak, and is somewhat soft at ~3.5 (Mohs), and transparent. It is not a “write home to momma” mineral but is interesting due to its formation in a freshwater lake.

Early 1930s French postage stamp overprinted for use in Andorra.


REFERENCES CITED

Canérot, J., 2016, The Iberian Plate: myth or reality?: Boletín Geológico y Minero, vol. 127, No. 2/3.

Delaney, M.L., 1998, Phosphorus accumulation in marine sediments and oceanic phosphorus cycle: Biogeochemical Cycles, vol.12, no. 4.

De las Heras, X., J.O.Grimalt, J.Albaigés, R.Juliá, and P.Anadon, 1989, Origin and diagenesis of the organic matter in Miocene freshwater lacustrine phosphates (Cerdanya Basin, Eastern Pyrenees): Organic Geochemistry, vol. 14, issue 6.

Gibson, R., 2014, Alpine-Himalayan Orogeny http://historyoftheearthcalendar.blogspot.com

Lewis, C.J., S.J. Verge, and M. Marzo, 2000. High mountains in a zone of extended crust: insights into the Neogene – Quaternary topographic development of northeastern Iberia: Tectonics vol. 19, no. 1.

Lucas J., Prevot-Lucas L. 1997, On the genesis of sedimentary apatite and phosphate-rich sediments, In H. Paquet and N. Clauer: Soils and Sediments, Springer, Berlin, Heidelberg.

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