{"product_id":"dacite-large-hand-display-specimen-of-gray-porphyritic-dacite-a-rock-common-in-the-archean-continents-copy","title":"dacite - large hand\/display specimen of gray porphyritic dacite, a rock common in the Archean continents","description":"\u003cp\u003e\u003cstrong\u003e\u003cspan style=\"color: #660000;\"\u003e\u003c\/span\u003e\u003c\/strong\u003e\u003cspan style=\"color: #990000;\"\u003e\u003cstrong\u003eDacite was a common rock in the Archean continents, forming when hot young oceanic crust subducted beneath thick continental plates. As the hot oceanic plate subducted into the mantle, it partially melted, dewatered and became hydothermally altered, with the melt gaining quartz and sodium and forming a magma that differentiated and became even more sodium and silica rich as it rose. As it cooled near the surface, quartz (sodium silicate), plagioclase (sodium feldspar) and hornblende crystallized. \u003cbr\u003e\u003cbr\u003e\u003cstrong\u003eToday, subducting oceanic crust is colder and does not melt before dewatering, inhibiting the formation of dacite. Dacite is the volcanic equivalent of granodiorite and can be thought of as a quartz rich andesite. This dacite was intruded during the Tertiary. The phenocrysts are plagioclase and quartz, with smaller biotite. \u003c\/strong\u003e\u003cbr\u003e\u003cbr\u003e\u003cstrong\u003eSecondary students should be able to identify this as a porphyry because of the phenocrysts - the larger crystals of quartz and feldspar that are surrounded by a finer gray groundmass - and because of the color, they should see that it is intermediate between oceanic basalts (dark and dense) and granitic continental rocks (lighter and less dense).\u003c\/strong\u003e\u003c\/strong\u003e\u003c\/span\u003e\u003c\/p\u003e\n\u003cp\u003e\u003cspan style=\"color: #990000;\"\u003e\u003cstrong\u003e\u003cstrong\u003eThis dacite was collected on Mt. General, near Hinkley in San Bernardino County, California\u003c\/strong\u003e\u003c\/strong\u003e.\u003c\/span\u003e\u003c\/p\u003e\n\u003cp\u003e\u003cspan style=\"color: rgb(0, 0, 0);\"\u003e\u003cstrong\u003eNotice, in the QAPF diagrams, how dacite overlaps granodiorite and tonalite in composition. Select one diagram, click to enlarge, and hover over it to get left and right image arrows. Toggle back and forth to compare intrusive and extrusive.\u003c\/strong\u003e\u003c\/span\u003e\u003c\/p\u003e\n\u003cp class=\"p1\"\u003e \u003c\/p\u003e\n\u003cp\u003e \u003c\/p\u003e\n\u003cp\u003e \u003c\/p\u003e\n\u003cp class=\"p1\"\u003e\u003cspan class=\"s1\" style=\"color: #ff2a00;\"\u003e\u003cb\u003eSelect a specimen:\u003c\/b\u003e\u003c\/span\u003e\u003cb\u003e When more than one specimen is shown, you can select a particular specimen by telling us what is in the photo with it, a blue or black and silver pen, a black mechanical pencil or one of those plus some number of coins, or you can let us make the selection.\u003c\/b\u003e\u003c\/p\u003e\n\u003cp class=\"p1\"\u003e\u003cspan class=\"s1\" style=\"color: #ff2a00;\"\u003e\u003cb\u003eShipping:\u003c\/b\u003e\u003c\/span\u003e\u003cb\u003e By Priority Mail or USPS Ground Advantage, whichever is cheaper, unless we find a better rate.\u003c\/b\u003e\u003cb\u003e Click \u0026gt; \u003c\/b\u003e\u003ca href=\"https:\/\/geologicalspecimensupply.com\/pages\/about-shipping\"\u003e\u003cspan style=\"color: #2b00ff;\" class=\"s2\"\u003e\u003cb\u003ehere\u003c\/b\u003e\u003c\/span\u003e\u003c\/a\u003e\u003cb\u003e \u0026lt; for shipping rates. See comment at \u003c\/b\u003e\u003cspan style=\"color: #ff2a00;\" class=\"s1\"\u003e\u003cb\u003eNote!\u003c\/b\u003e\u003c\/span\u003e\u003cb\u003e Use back button to return to this page.\u003c\/b\u003e\u003c\/p\u003e\n\u003cp\u003e\u003cspan style=\"color: #000000;\"\u003e\u003cstrong\u003e \u003c\/strong\u003e\u003c\/span\u003e\u003cstrong\u003e\u003cspan style=\"color: #ff0000;\"\u003eMaking multiple purchases? \u003cspan style=\"color: #000000;\"\u003eClick\u003c\/span\u003e\u003c\/span\u003e on the \"combine shipping\" button in the shopping cart. We'll send an invoice with combined shipping. A link in that invoice will bring you back to checkout, no hassle.\u003c\/strong\u003e\u003c\/p\u003e","brand":"Geological Specimen Supply","offers":[{"title":"Default Title","offer_id":42712660901975,"sku":null,"price":12.5,"currency_code":"USD","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0960\/7650\/files\/dacitelgH_D12.5pcl2coins.jpg?v=1781036690","url":"https:\/\/geologicalspecimensupply.com\/products\/dacite-large-hand-display-specimen-of-gray-porphyritic-dacite-a-rock-common-in-the-archean-continents-copy","provider":"Geological Specimen Supply","version":"1.0","type":"link"}