Slate, yay or nay?

[REW01]

I picked this up in the sugarloaf area of the snowy range. The French Slate is in the area but this is much lighter than the outcrops I’ve seen were much darker, kind of bluish in color. It’s pretty hard. Doesn’t seem to want to cleave but is foliated. 

 

So, slate? Phyllite? 

[REW01]

[ynot]

I got nothing on this one.

Trying to classify a rock from a picture is .....

[REW01]

Yeah I know it’s hard to ID a rock from photos. I saw no harm in posting it anyway. 

[Ludwigia]

Looks metamorphic to me.

[FranzBernhard]

2 hours ago, ynot said:

Trying to classify a rock from a picture is .....

ambiguous? .

 

Yes, its a hard one! I can see:

- foliation

- silky luster on foliation planes

- crenulation in two different directions (pic 3)

- streaks and layers of quartz and a brownish carbonate? (pic 4)

- pic 2 looks somewhat gneissic, but it I think it is not, this look is caused be quartz streaks

It could(!) be a quartz-rich phyllite or a phyllitic quartzite. But its just a possibility.

Franz Bernhard

[REW01]

1 hour ago, Ludwigia said:

Looks metamorphic to me.

Almost certainly. 

 

1 hour ago, FranzBernhard said:

ambiguous? .

 

Yes, its a hard one! I can see:

- foliation

- silky luster on foliation planes

- crenulation in two different directions (pic 3)

- streaks and layers of quartz and a brownish carbonate? (pic 4)

- pic 2 looks somewhat gneissic, but it I think it is not, this look is caused be quartz streaks

It could(!) be a quartz-rich phyllite or a phyllitic quartzite. But its just a possibility.

Franz Bernhard

It’s got me just a little stumped. I’m sure it’s not a gneiss. The crenulations and luster make me think phyllite as well.

 

But it’s a bit of a strange one. 

 

I also considered that I might be looking at faint glacial striae. Rocks with spectacular glacial polish and striation are present in the area. Unfortunately, because it was float, I don’t know what outcrop it may have come from. The closest exposure to where I picked it up are stromatolites.  Not helpful to ID this one. 

[Wrangellian]

I saw a lot of rock that looks like this in Eastern BC this past August where I encountered rocks of the Purcell Supergroup (Precambrian, 1.4by). It does have a metamorphic look but I figure the stuff I was dealing with couldn't have been terribly metamorphosed as there were stromatolites and ripple marks present. I believe a mildly metamorphosed shale would be referred to as slate, so that's what I'm going with! I don't know where the dividing line between slate and schist is...

[REW01]

9 hours ago, Wrangellian said:

I saw a lot of rock that looks like this in Eastern BC this past August where I encountered rocks of the Purcell Supergroup (Precambrian, 1.4by). It does have a metamorphic look but I figure the stuff I was dealing with couldn't have been terribly metamorphosed as there were stromatolites and ripple marks present. I believe a mildly metamorphosed shale would be referred to as slate, so that's what I'm going with! I don't know where the dividing line between slate and schist is...

Thanks for the input! This stuff is most likely from a precambrian formation. I'm leaning towards phyllite on this one.

 

The dividing line between the two is technically pressure and temperature. So the upper boundaries between low- and medium-grade metamorphism, I suppose? Let's say a shale is metamorphosed; you'd be increasing P/T conditions: Shale --> Slate --> Phyllite --> Schist --> Gneiss [protolith --> very low grade --> low-grade --> intermediate-grade --> high-grade]. In an ideal world, anyway. 

[Wrangellian]

Yes.. The question for me is, at which point do ripples, mud cracks and stromatolites disappear as opposed to being a little distorted? because the stuff I've handled definitely looks like it's been subjected to heat/pressure to varying degree but none of it has had enough to obliterate these things. I have always understood that slate might have some distorted fossils but none would survive beyond slate-level metamorphism. I could be wrong, but those stromatolites we collected looked to have no alteration whatsoever, while the surrounding slate looked low-grade metamorphosed.

I guess you piece looks like it's seen a bit more metamorphism than my stuff.

[REW01]

WOuldn't that depend on the rock? Different rocks have differing stable/unstable states and stress levels. Generally, yes, I agree most fossils cannot survive but the lowest grade metamorphism. I've seen slate with fossils before. In my case, here, I know my rock is most likely from a different formation than the stroms in the area.

 

If only slates and phyllites were always easier to distinguish....