The Setters Schist, part two
The other day I mentioned the Setters Schist.
Here's a couple of cobbles of the same formation, but lower stratigraphically than the stuff we saw on the University of Maryland petrology trip. The basal Setters has beautiful metamorphic tourmalines lying willy-nilly within the plane of foliation:
According to Mindat.org, "the general formula for this group may be written:
Here's a couple of cobbles of the same formation, but lower stratigraphically than the stuff we saw on the University of Maryland petrology trip. The basal Setters has beautiful metamorphic tourmalines lying willy-nilly within the plane of foliation:
According to Mindat.org, "the general formula for this group may be written:
AD3G6(BO3)3[T6O18]Y3Z, where:
- A = Ca, Na, K, or is vacant (large cations);
- D = Al, Fe2+, Fe3+, Li+1, Mg2+, Mn2+ (intermediate to small cations - in valence balancing combinations when the A site is vacant);
- G = Al3+, Cr3+, Fe3+, V3+ (small cations);
- T = Si (and sometimes minor Al3+, B3+);
- Y = O and/or OH; and
- Z = F, O and/or OH."
Note the constant there: boron! ...A lot of boron! Three boron atoms per unit cell... These metamorphic rocks have a sedimentary protolith. Where did the pre-metamorphic sediments get all that boron from?
Any ideas?
Labels: geology, maryland, metamorphism, minerals, piedmont
4 Comments:
Are you sure those are tourmalines and not some kind of amphibole? (I swear I've seen a rock that looked like that and contained lots of amphiboles...)
That's what I thought at first too -- I mean, tourmalines? Come on? Right?
But that's what they are...
Here's a source referencing them.
And another.
And another.
High boron concentrations are often associated with volcanic rocks weathering into enclosed basins- for example, Death Valley and Alvord Desert, Oregon. Does this unit predate Triassic rifting?
Well, duh. I realized a couple of minutes after I posted the last comment that there couldn't have possibly been a metamorphic event of the magnitude necessary after the Triassic rifting. Oh well.
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