rhyolite porphyry - pinkish tan teaching student specimens - Unit of 5 specimens

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If making multiple purchases, please read the note following th+e red text below! Teaching student specimens of a pinkish tan rhyolite porphyry with small phenocrysts – Unit of 5 specimens Rhyolite porphyry is a silica-rich igneous rock with a composition similar to that of granite. Porphyries are igneous rocks that have had two stages of cooling. Typically they have phenocrysts, larger crystals (pheno = visible, in Greek), of feldspar surrounded by smaller crystals that may be so small that they are not visible without a hand lens. With a long period of underground cooling the phenocrysts become large. When deep underground cooling is only a relatively short period, the phenocrysts are small as in these specimens, since they did not have enough time to grow. In this porphyry, the feldspar phenocrysts have been converted to kaolin, so they are pseudomorphs of kaolin after orthoclase. They retain the shape but, unlike orthoclase, can be scratched by a knife. The composition of the phenocrysts was recently identified by x-ray reflection spectroscopy. Since feldspar alters to clay, this shouldn’t have been a surprise.An earth science student should recognize this as a rhyolite, the extrusive equivalent of a granite, with a similar silica-rich composition. I would give additional credit to a student who identified this as a porphyry, since the phenocrysts are small. The surrounding mass of microscopic crystals is typical of rhyolites – the texture of rhyolite is normally aphanitic, from the Greek words, a = not and phaneros = visible, referring to the crystals.Granite can also be porphyritic when feldspar phenocrysts form in a deep magma that was later brought closer to the surface where it finally cooled and crystallized. We really don’t like the tiny finger nail-sized specimens offered to students, as we feel these are too small to see rock textures and too small for examining the identifying characteristics of a mineral. We want the “Oh, Wow!” effect when students see the set of specimens they are going to work with. We collect these specimens so that five specimens, or possibly six, will fit in a small flat rate box, and we keep the price low, as we know these will be used in a classroom. For teachers we count like a baker and a dozen is more than 12, since you will always need a few spares. If you are a teacher, let us know. You are buying a unit of five student specimens. The photo is representative of what we ship. Making multiple purchases?Just add items to your cart. When finished shopping, click on “request total” at upper right in the Shopping Cart window. Do not proceed to checkout. We will send an eBay invoice with the correct combined shipping. The combined shipping will often be less than eBay shows. Please wait for that invoice before paying. There is no obligation to purchase. You can decide after seeing what shipping will cost.Shipping: Ships in a Priority Mail small flat rate box unless combined with other purchases. We always combine shipping to get your purchase to you at the lowest cost. We do not charge for handling. We refund any overpayments on shipping beyond our actual cost. International customers: We sell and ship internationally. If eBay’s page does not let you enter your country name to determine the shipping cost for something you are interested in, please contact us. We can tell you what the shipping would cost and how to make the purchase. You can then decide if you wish to complete the transaction. Use the Contact Seller link in the Seller Information box on the right side of this page. Pay securely with your debit or credit card through PayPal! You do not need a Pay Pal account to do this. Yes, we do combine shipping!

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