The term 'porphyry' describes a specific texture found in igneous rocks, where they contain larger crystals of certain minerals (especially feldspar or quartz), these being surrounded by a composite of minerals that have a much smaller sized crystal. The larger crystals are called 'phenocrysts' or 'sprinkle grains'. Porphyritic structures are characteristic of lavas and the igneous rocks found in dykes and sills. The phenocrysts are separated by the slower cooling of magma at depth, while the smaller-sized matrix is caused by a more rapid cooling after the magma is squeezed into a crack in the Earth's crust and/or it has reached the Earth's surface.
Porphyry deposits are formed when a column of rising magma is cooled in two stages. In the first, the magma is cooled slowly deep in the crust, creating the large crystal grains with a diameter of 2 mm or more. In the second and final stage, the magma is cooled rapidly at relatively shallow depth or as it erupts from a volcano, creating small grains that are usually invisible to the unaided eye.
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MINERALS
Minerals, naturally occurring elements and chemical compounds having characteristic physical properties such, crystal shape, color, line color, gloss, hardness, cleavage, fracture, density and optical conditions.
At GZ you will in this stone find especially the minerals feldspar or quartz.
Feldspar
Plagioclase is a member of the feldspar group (like orthoclase) and is a framework silicate. Plagioclase consists of a solid solution between the albite and anorthite end-members, and together with quartz is the most common of the rock forming minerals.
Orthoclase is a member of the feldspar group (like plagioclase) and is a framework silicate. Orthoclase, also known as alkali feldspar or K-feldspar, is one end-member of a solid solution between orthoclase and albite.
Orthoclase is found in silica-rich igneous rocks such as granite, and in high grade metamorphic rocks.
Quartz
Quartz is a complex silicate in which all the oxygen atoms of the SiO4 tetrahedra are shared between two tetrahedra, leading to complex 3-dimensional frameworks. For this reason, quartz is referred to as a framework silicate.
Quartz is among the most common of all rock forming minerals and is found in many metamorphic rocks, sedimentary rocks, and those igneous rocks that are high in silica content such as granites and rhyolites. It is a common vein mineral and is often associated with mineral deposits.
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Questions:
1. Answer the questions under by visiting the Coordinates.
A.Using the terms in the cache page to help you, describe the crystals, color, texture and the minerals that you see in the stone?
B. What mineral is it that makes up what you have answered in question A?
C. What average size are the crystals?
D. Has this stone cooled in one stage or more than one? Please explain how you reached your conclusions.
2. (It’s voluntary to post a photo in your online log of your visit)
