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Volcanoes and Their Products
Published in Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough, Earth Materials, 2019
Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough
Although sometimes absent, when present, phenocrysts help distinguish different kinds of volcanic rocks. Figure 7.47 shows the common phenocrysts in rocks ranging from felsic to ultramafic. These phenocryst minerals are the same minerals that may be present as microscopic crystals in the groundmass. From felsic to ultramafic, the volcanic rocks are rhyolite, dacite, andesite, basalt, and komatiite. Quartz and K-feldspar are generally restricted to relatively felsic rocks, olivine and pyroxene to relatively mafic rocks, and the other minerals to intermediate rocks. Plagioclase is a solid-solution mineral in volcanic rocks of many sorts and varies from being Na-rich for felsic rocks to being Ca-rich for mafic and ultramafic rocks. Figure 7.47 is only an approximation, because the minerals present as phenocrysts depend on many different things.
Recognising the different types of building stone
Published in John A. Hudson†, John W. Cosgrove, Understanding Building Stones and Stone Buildings, 2019
John A. Hudson†, John W. Cosgrove
The Bodmin granite shown in Figure 3.6 is dominated by plagioclase, i.e., white feldspars. The tabular shape of these crystals is apparent and, as noted above, reflects the fact that these were the first minerals to crystallise from the magma. When conspicuous crystals are larger than the grains of the rock groundmass, they are termed phenocrysts. Minerals such as quartz that crystallise out later, have to crystallise in the gaps within the crystal mix of feldspars and, as a result, the quartz crystals are smaller and less well formed. The Kemnay granite shown in Figure 3.6 is also dominated by plagioclase feldspars. It is finer grained than the Bodmin Moor granite, indicating that it cooled more rapidly. The Rapakivi granite which is also shown in Figure 3.6 has an easily recognisable texture: the feldspars form large, ovoid phenocrysts 10 to 50 mm in diameter set in a finer groundmass. The feldspars are composite, i.e., made up of two types: the centres are pink or brown orthoclase which is entirely mantled by grey plagioclase. The word ‘Rapakivi’ in Finnish means ‘weathered rock’.
Optical mineralogy
Published in W.S. MacKenzie, A.E. Adams, K.H. Brodie, Rocks and Minerals in Thin Section, 2017
W.S. MacKenzie, A.E. Adams, K.H. Brodie
In an igneous rock, large crystals in a matrix or groundmass of much smaller crystals are described as phenocrysts (Figure 3). In a metamorphic rock similar large crystals which have grown in a mass of smaller crystals are termed porphyroblasts (Figure 1, Figure 4). If the porphyroblast contains abundant small inclusions, it is termed a poikil- oblast. In some rocks it is not certain whether the large crystals grew from an igneous magma or in a later stage metamorphic event. In these cases it is perhaps better to describe the crystals as megacrysts. In some deformed metamorphic rocks, large relict grains are preserved in a finer grained matrix and these are termed porphyroclasts.
Petrogenetic links between the Dunedin Volcano and peripheral volcanics of the Karitane Suite
Published in New Zealand Journal of Geology and Geophysics, 2018
Oliver E. McLeod, James D. L. White
The Karitane Suite inclusions share many textural features with the inclusions described above; including melanocratic appearance, cuspate to ellipsoidal margins, sizes of 5–15 cm, phenocrysts (plagioclase and hornblende), microlitic groundmass, and reaction rims. One obvious contrast is that the inclusions are non-vesicular, which is rare for fluidal mafic inclusions elsewhere (Bacon 1983). Overall however, the inclusions are considered to provide robust evidence for mingling of basaltic and benmoreitic magma. The possibility that the inclusions are crustal xenoliths is discounted because they are ubiquitously rounded as opposed to angular (e.g. Eichelberger 1980). The inclusions are mineralogically and texturally distinct from the local mafic lavas (hawaiite and basanite) because they have abundant hornblende, do not contain olivine, and have equant microcrystalline textures. Based on their textures, it is inferred that at least three distinct periods of crystallisation have occurred, and these relate to changes in magmatic conditions: (1) initial slow cooling produced intergrowths of plagioclase, augite and hornblende phenocrysts; (2) hornblende nucleation increased and began to form dense arrangements of microlites and (3) hornblende was resorbed and pseudomorphed by magnetite.