TrachyteTrachyte is an extrusive rock, belonging to the alkali series of intermediate volcanic rocks. The major mineral component of trachyte is alkali feldspar (e.g. orthoclase), and it generally contains no quartz. Trachyte often displays trachytic texture, where acicular to tabular feldspar phenocrysts align in one direction, suggestive of flow prior to cooling. Trachyte lava is of moderate viscosity and forms thick lava flows and domes. Two famous trachyte volcanoes are Mt Kilimanjaro (the highest mountain on the African continent), and Mt Erebus in Antarctica. Trachyte is the volcanic equivalent of syenite.
Other specimens - Click the thumbnails to enlarge
Colour - variable but often light coloured, generally light coloured phenocrysts.
Texture - generally porphyritic (can be trachytic), sometimes aphanitic.
Mineral content - orthoclase phenocrysts in a groundmass of orthoclase with minor plagioclase, biotite, hornblende, augite etc..
Silica (SiO 2) content - 60%-65%.
Uses - building and paving stone; dimension stone for building facings, foyers etc.
New Zealand occurrences - Banks Peninsula, Dunedin Volcano.