LOAFING AROUND WITH ALBITE, SCHORL, AND MONAZITE

 

Public Domain: artist unknown.

The other morning I was just sort of loafing around, enjoying my coffee, thinking about the day, when I spotted a rock over in the corner mostly hidden under some junk.  Yea, I kind of remember that piece—came with a “box of rocks” and lacked a label.  I was always going to look at it “someday” and try to make an ID.  Well, this lazy morning, somewhere around 300 in the pandemic self-quarantine, would be a good opportunity to take a peek.  Besides the lack of a label, I had tossed it aside since the softball size of the specimen was much larger than I usually tackle since it exceeds the operating distance of my binocular scope. But I grabbed my 10 power loupe and wow, this was an interesting rock, but it still lacked a locality label.  However, it had a very distinct look, almost like a poorly cemented conglomerate with large albite and schorl crystals and something orange and small---that should help with the “picture ID” on MinDat.org. Time to refill the coffee mug and get started.

After spending time on the internet, and perusing mineral books and articles, I have concluded my rock came from Portugal No. 2 quarry, Malpartida e Vale de Coelha, Almeida, Guarda, Portugal. Now I know very little (not much at all) about the geology of Portugal and the best I could find is from MinDat.org: Granite quarry with several pegmatite layers rich in pockets. Adjoining pockets may show quite a different mineralization. there are other granite quarries in the vicinity, presumably with similar minerals. 19 valid minerals known.

The mystery rock with a mass of albite crystals while the black crystals are schorl.  Width of specimen ~13 cm.

 Partial schorl crystal; length ~2.5 cm. 

White, pearly luster, albite (sodium-rich feldspar) with scattered, terminated schorl crystals.

Note the tabular, stacked parallel crystals of albite at <-----.

Not all schorl crystals are large--note the tiny needle-like crystal.  

A base geologic map indicates that northern Portugal is a granitic terrain that evidently is associated with the Variscan Orogeny, a Late Paleozoic continental collision between Euramerica (Laurussia) and Gondwana to form the supercontinent of Pangaea.  There appears to be tens of granite quarries scattered across the area and this granite terrain contains the Central Iberian Pegmatite Belt. Ages of the various pegmatites are close to those that characterize the granites, ~300 Ma (evolution of older granites) and ~290 Ma (later granites). The parental plutonites are syn-tectonic two-mica granites and late to post-tectonic biotite granites (Dias and others, 2013).

And that is about my sum knowledge of Portuguese geology except that the Douro River flows through the granite and the associated soils produce the famous port grapes for the wines!

In looking closely at the rock, I discovered a mass of albite (plagioclase feldspar; usually white to gray) with a variety of different size schorl crystals (most common member of the Tourmaline Group; black in color).  But the most interesting crystal half concealed in the albite mass was a partial, large (~2.1 cm) crystal of what I believe is monazite, a reddish-brown phosphate mineral that contains rare-earth elements (REE/Ce/La/Nd/Sm/Gd)(PO₄). According to MinDat.org the Portugal #2 Quarry does not list a dominant monazite REE so identifies it as monazite ?

A large (width ~2.1 cm) monazite crystal stuck in the mass of white albite crystal.  Note the smaller, transparent, golden, tabular crystal of monazite "on top of" the larger crystal.

The albite mass also contains a number of small (1-2 mm) orange to orange-brown crystals that I first thought were spessartine garnets since they were common in photos supplied by MinDat.org.  However, something was wrong since the crystals seemed not to resemble typical garnet crystals of any kind.  These crystals were flattened, thin, and wedge shaped.  In addition, there are consolidated masses of these colorful crystals.  So, readers can examine my photos and perhaps come to their own conclusions—did I pick the correct collecting locality?  Are these crystals correctly identified as monazite ?  At any rate, I had an interesting time with the mysterious rock!

The above photomicrographs are ~1.0 cm. width FOV and show the brown to orange brown small crystals on monazite ?  encased in the albite mass.

I remain uncertain about the "crud" often covering schorl.

I believe the white material at the end of the pointer is ?aragonite.

REFERENCES CITED 

 Dias, P.A., B. Pereira, J. Azevedo, J. Oliveira, Leal Gomes, and J. Carvalho, 2013, Pegmatite Productive Terrains in the Variscan Granite Hosts From Northern and Central Portugal: 23rd International Mining Congress & Exhibition of Turkey • 16-19 April 2013 ANTALYA [https://repositorium.sdum.uminho.pt/bitstream/1822/30028/1/IMCET2013_Dias_et%20al.pdf]

 

 Give me a mystery, just a plain and simple one, a mystery which is diffidence and silence, a slim little bare-foot mystery: give me a mystery, just one.              Yevgeny Yevtushenko