Porphyry ( POR-fə-ree) is any of various granites or igneous rocks with coarse-grained crystals such as feldspar or quartz dispersed in a fine-grained silicate-rich, generally aphanitic matrix or groundmass. In its non-geologic, traditional use, the term porphyry usually refers to the purple-red form of this stone, valued for its appearance, but other colours of decorative porphyry are also used such as "green", "black" and "grey". The term porphyry is from the Ancient Greek πορφύρα (porphyra), meaning "purple". Purple was the colour of royalty, and the Roman "imperial porphyry" was a deep purple igneous rock with large crystals of plagioclase. Some authors claimed the rock was the hardest known in antiquity. Thus porphyry was prized for monuments and building projects in Imperial Rome and thereafter. Subsequently, the name was given to any igneous rocks with large crystals. The adjective porphyritic now refers to a certain texture of igneous rock regardless of its chemical and mineralogical composition or its color. Its chief characteristic is a large difference in size between the tiny matrix crystals and the much larger phenocrysts. Porphyries may be aphanites or phanerites, that is, the groundmass may have microscopic crystals as in basalt, or crystals easily distinguishable with the eye, as in granite.
Formation Most igneous rocks have some degree of porphyritic texture. This is because most magma from which igneous rock solidifies is produced by partial melting of a mixture of different minerals. At first the mixed melt slowly cools deep in the crust. The magma begins crystallizing, the highest melting point minerals closest to the overall composition first, in a process called fractional crystallization. This forms phenocrysts, which usually have plenty of room for growth, and form large, well-shaped crystals with characteristic crystal faces (euhedral crystals). If they are different in density to the remaining melt, these phenocrysts usually settle out of solution, eventually creating cumulates; however if the partially crystallized magma is then erupted to the surface as a lava, the remainder of the melt is quickly cooled around the phenocrysts and crystallizes much more rapidly to form a very fine-grained or glassy matrix. Porphyry can also form even from magma that completely solidifies while still underground. The groundmass will be visibly crystalline, though not as large as the phenocrysts. The crystallization of the phenocrysts during fractional crystallization changes the composition of the remaining liquid magma, moving it closer to the eutectic point, with a mixed composition of minerals. As the temperature continues to decrease, this point is reached, and the rock is entirely solidified. The simultaneous crystallization of the remaining minerals produces the finer-grained matrix surrounding the phenocrysts, as they crowd each other out. The significance of porphyritic texture as an indication that magma forms through different stages of cooling was first recognized by the Canadian geologist, Norman L. Bowen, in 1928. Porphyritic texture is particularly common in andesite, with the most prominent phenocrysts typically composed of plagioclase feldspar. Plagioclase has almost the same density as basaltic magma, so plagioclase phenocrysts are likely to remain suspended in the magma rather than settling out.
Rhomb porphyry Rhomb porphyry is a volcanic rock with gray-white large porphyritic rhombus-shaped phenocrysts of feldspar (commonly anorthoclase) embedded in a very fine-grained red-brown matrix. The composition of rhomb porphyry places it in the trachyte–latite classification of the QAPF diagram. Rhomb porphyry is found in continental rift areas, including the East African Rift (including Mount Kilimanjaro), Mount Erebus near the Ross Sea in Antarctica, the Oslo graben in Norway, and south-central British Columbia.
Use in art and architecture
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