Andesite is an aphanitic (fine-grained) to porphyritic (coarse-grained) igneous rock that is intermediate in its content of silica and low in alkali metals. It has less than 20% quartz and 10% feldspathoid by volume, with at least 65% of the feldspar in the rock consisting of plagioclase.

Andesite is a fine-grained rock that forms when the magma erupts onto the surface and crystallizes quickly. Andesite and diorite have a composition that is intermediate between basalt and granite. This is because their parent magmas formed from the …

Extrusive, or volcanic, igneous rock form when molten rock (lava) cools on the surface. Rockman's igneous rock specimens are hand broken as opposed to being crushed which helps keep cleavage and fracture characteristics intact and provides freshly broken areas for better examination of the specimens.

Hornblende andesite from Mt. Shasta, CA. Wards Collection of North American Rocks. Smith College sample WS-29.

The collection consists of 40 slides from the Plymouth teaching collection, including igneous, metamorphic and sedimentary rocks. UNDER CONSTRUCTION! This Collection is currently under development, so you may notice reduced functionality as new samples are brought online.

A group of iCRAG members (UCC, TCD, NUIG and UCD) in partnership with The Open University have created a new collection of Irish rocks and associated learning materials for the Virtual Microscope of Earth Sciences.

Andesite is an extrusive volcanic rock of intermediate composition. In a general sense, it is the intermediate type between basalt and rhyolite. It is fine-grained to porphyritic in texture, and is composed predominantly of sodium-rich plagioclase plus …

Sample Number: 001 Source: Krantz Location: Germany Description: Andesite with small (1-3 mm) phenocrysts of plagioclase, augite, hornblende, biotite, and magnetite in a fine-grained (<0.5mm) groundmass of the same minerals.

Raw Andesite Specimen Set of 12 Samples, 1" Andesite features abundant phenocrysts in a fine grained matrix, and offers a great demonstration of igneous porphyry. Ideal for identification & classification exercises. Andesite is an excellent example of porphyry in igneous rocks.

He curated a collection of 42,000 rock specimens, as well as numerous thin sections, from the 1880s up to his death in 1939: these specimens form the core of what is now known as the Harker Collection.