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Felsite? Rhyolite? Felsitic Rhyolite?

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  • Felsite? Rhyolite? Felsitic Rhyolite?

    Can anyone explain these to me and tell me what's what? Is it rhyolite? Is it felsite? Example pics would be awesome too.

    How many different types of this rock were utilized in New England? I know of Attleboro Red, Blue Hills, Marblehead... I know there are others but those are the only three I feel I can identify.

    Thanks in advance.

  • #2
    Some info here
    Rhyolites in Carolina Slate Belt There are hundreds of different varieties of Rhyolite in the state, but the term Rhyolite is also used as a generic catch-all term

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    • #3
      Thanks Looks2Much, but I was hoping for New England specific info...

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      • #4
        I see. Wish I could help more.

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          Still, thanks

      • #5

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        • #6
          I keep hearing Felsite & Rhyolite used for the same rocks. I'm as confused as you are over this. I wish I could have a sit down with an expert and have him go through the various materials I have found. Making a frame of just different lithics would be an interesting display.

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            Nice to see I'm not the only one.

        • #7
          Matt, in the unpublished revised edition of his New England typology, the late Jeff Boudreau listed the following lithics for New England. He did not list quartz, so these are mostly the metavolcanics:

          Blue Hill River Rhyolite(Ma)
          Braintree Hornfels(Ma)
          Mattapan Rhyolite(Ma)
          Sally Rock Rhyolite(Ma)
          Pine Tree Brook Rhyolite(Ma)
          Hingham Rhyolite(Ma)
          Nantasket Argillite(Ma)
          Saugus ("jasper") Rhyolite(Ma)
          Marblehead Rhyolite(Ma)
          Wakefield "Salt and Pepper" Rhyolite(Ma)
          "Attleboro Red" Rhyolite(Ma)
          Melrose Green Rhyolite(Ma)
          Mylonite(Ma)
          Barrington Argillite(RI)
          Chicopee Indurated Shale(Ma)
          Kineo Rhyolite(ME)
          Munsungun Chert(ME)
          Vinalhaven Rhyolite(ME)
          Mt. Jasper/Israel River/Jefferson Rhyolite(NH)
          Normanskill Chert(NY)
          Onondaga Chert(NY)
          Jasper(PA and RI)
          Lockatong Argillite(NJ and PA)

          Jeff included descriptions and photos of all these. Now, I always referred to Attleboro Red as felsite for instance. I asked my friend Bill about this, he edited the guide, and of course I could not ask Jeff. Bill told me felsite could actually be considered a variety(?) of rhyolite. I'll ask him again, because I believe Paul Frey would disagree. At the same time, I'll always go by how Jeff described them at this point, since nobody knew our regional lithics better then he did. Obviously, we all hope Jeff 's updated and expanded typology will be published one day.



          Rhode Island

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          • #8
            Here are some photos.

            Photo 1: Saugus ("jasper") Rhyolite. Most collectors call this Saugus Jasper, but it's actually rhyolite
            Photo 2: Saugus ("jasper") Rhyolite
            Photo 3: Braintree Hornfels. Very popular for Jack's Reef points.
            Photo 4: Hingham Rhyolite
            Photo 5: Chicopee Indurated Shale
            Photo 6: Lockatong Argillite. Always extremely weathered, regardless of age.
            Photo 7: Lockatong Argillite
            photo 8: Marblehead Rhyolite. There is a variety of black Marblehead that shows little to zero patina, even Early Archaic pieces. These two pieces are that variety.



            Rhode Island

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            • #9
              Exactly what I was looking for, awesome, thank you Charlie.

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              • #10
                Some basic geology for you in relation to the family series to which all of these rocks belong:


                Rhyolites


                Rhyolite is not a single rock, so as for most rocks 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). 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” (the term means that they are igneous and "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.

                Dacites

                Dacites 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.

                Andesites

                “Andesites” (again, that’s a group of rocks, not a single rock type) are still silica-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 they're not very suitable for knapping (as opposed to chipping and pecking). 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.

                Felsites

                There is no such rock as felsite! 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 with a microscope (sometimes supported by chemical analysis) to tell whether it’s a rhyolite or an andesite. So, such rocks are given a “non-commital” field 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).



                If this kind of information helps your understanding, I can post a little more about the effects of metamorphism of these rocks which then takes us into metarhyolites and other metavolcanics.
                I keep six honest serving-men (they taught me all I knew); Their names are What and Why and When and How and Where and Who.

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                  Excellent info. I welcome any info you'd like to add.
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