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A new material for me.

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  • ajbhawkowl
    replied
    Nice one 

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  • Butch Wilson
    replied
      Hope you got all that Rock Queen, I knew it was coming.......whew!   Thanks Painshill, I think?

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  • painshill
    replied
    Re Butch’s comment, I suspect it’s somewhere in the territory of rhyolite too and perhaps what many would call “felsite”. Apologies in advance for being the “Geology Police” here Butch, but this may put a little flesh on the bones.

    Rhyolite
    Rhyolite is not a single rock, so we should talk of “rhyolites” as a group. They are igneous volcanic rocks - extruded from the ground as molten but viscous magma (lava) which may on occasion have vesicles (bubbles). They definitely have no relationship to siltstones, whether silicified or not. Volcanic ash may accumulate as sedimentary silt-sized particles, but it never goes on to form rhyolites. Since the extrusion processes can be violently explosive, volcanic ash and fallout debris can however get mixed into the molten lava and be present as fragments in the solidified rock.
    Rhyolites are “felsic” (ie silica-rich) and always have a texture that ranges from glassy through to fine granular, which is very suitable for knapping. The crystals in the matrix are too small to be seen with the naked eye (known as aphanitic texture). That’s a function of the rapidity with which the magma has cooled, but the same magma – if it cools very rapidly – will form obsidian rather than rhyolite.
    If the appearance is coarser, then that’s due to the presence of phenocrysts (discrete crystal inclusions) of feldspar, quartz, biotite and occasionally hornblende which are not part of the base matrix (known as porphyritic texture). Mostly, those phenocrysts will be pale in colour since feldspar and quartz are common. The colours of the matrix are also typically light, ranging from white to pale grey, tan and pale pink/red/purple.

    Dacite
    Dacite belongs in this family too, sitting as an intermediary between rhyolites and the next group (andesites, below). But they have a special origin and a particular composition arising from magma that came from below an oceanic crust.

    Andesite
    “Andesites” (again, that’s a family, not a single rock type) are still quartz-rich but typically a little lower in quartz than rhyolites. That, in combination with a slower cooling rate still gives an aphanitic matrix with crystals smaller than the eye can see but the texture is less fine and rarely glassy. Normally it’s not very suitable for knapping (as opposed to chipping). The spectrum extends further to basalts.
    Andesites come in many colours, including those found in rhyolites plus green, but in general are darker. Dark grey is common and they can be as dark as basalts. The way to distinguish them from basalts is by holding a freshly broken edge up to a bright light. It will be translucent – unlike basalts which are opaque. Those colours apply to the matrix. Phenocrysts are more likely to be present in a greater variety of possible minerals – commonly striated feldspar with one or more dark minerals (such as hornblende, pyroxene, biotite, magnetite and even garnet), but never quartz. You only get quartz phenocrysts in Rhyolites.

    Felsite
    There is no such rock! On the spectrum that transitions from rhyolites through to andesites, sometimes the rock is borderline texture between rhyolites and andesites but has no phenocrysts to assist in identification. It’s impossible for anyone except a petrologist (sometimes supported by chemical analysis) to tell whether it’s a rhyolite or an andesite. So, such rocks are given a “non-commital” name: felsites. It’s a name of convenience for indeterminate rocks within the spectrum (which may or may not be suitable for knapping).
    Typically, rocks in this category are pale in colour - white, light to medium grey, light pink (occasionally dark red), pale yellow, pale brown, pale purple, or light green. They can occasionally be as dark as basalts (although never black), but if you put a thin edge up to a bright light, it will be semi-translucent and appear almost white. Basalts will be dark and opaque, even on the thinnest of edges.

    All of the matrix colours above refer to unweathered rocks. With the exception of obsidian, they all weather relatively quickly, acquiring a thick rind that may extend all the way to the centre of flakes or points. Pale tan colours are normally the end result of weathering.

    Obsidian, rhyolites and andesites are frequently found together since they form in the same way. The loose generic term for the range of non-granitic, dark-coloured igneous rocks beyond andesites is “trap rock” (and that term includes the basalts at the far end of the spectrum).

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  • Hoss
    replied
    is that a river find? Pocelllanite has a color like that but look grainier to me
    Here is a link from an agate basin point found in Montana

    http://forums.arrowheads.com/forum/g...te-agate-basin

    If your point was a river or creek find it may have just been smoothed by the flowing water and sand

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  • kayakaddict
    replied
    what a find!  I wish I could see it even more closely..look really closely under good light...are there sparkles?  Are they everywhere or spaced apart?  Quartzite that is tight grained might have that sparkle all over but it also might not be real bright.... In my experience, but don't just listen to me.  I m not an expert...yet!

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  • chase
    replied
    This looks to me as a really high grade Quartzsite.

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  • Butch Wilson
    replied
    Nice point and material, now this is gonna get me in trouble with the geology folks. It looks like a type of material we in NC call "Rhyolite",
    A Silicified siltstone, whew that one is hard to say. Interesting discussion at

    http://forums.arrowheads.com/forum/g...olina-rhyolite

    Good luck !

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  • G10+
    replied
    I cant help with that but that is a cool blade u got there

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  • Rock queen
    started a topic A new material for me.

    A new material for me.

    Any idea on the type of material? Not the norm of what we find.

    Click image for larger version

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