How to identify Andesite

To correctly identify Andesite (Andesite), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Typically gray to dark gray, sometimes reddish-brown or black. The color can vary depending on the proportion of mafic minerals and the oxidation state of iron.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy in the groundmass; phenocrysts (especially plagioclase) can exhibit a vitreous luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Aphanitic (fine-grained) to porphyritic. Porphyritic textures are very common, with phenocrysts of plagioclase, pyroxene, and/or hornblende set in a fine-grained groundmass. Vesicular textures can also occur.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Phenocrysts often show euhedral to subhedral crystal forms. Plagioclase phenocrysts are typically tabular, pyroxenes prismatic, and hornblende acicular or prismatic.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable to the rock as a whole, but constituent minerals like plagioclase, pyroxene, and hornblende exhibit characteristic cleavage planes (e.g., plagioclase has two cleavages at nearly 90 degrees, pyroxene at nearly 90 degrees, hornblende at 56 and 124 degrees).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Volcanic arcs associated with subduction zones (e.g., Andes Mountains, Cascade Range, circum-Pacific 'Ring of Fire'). It forms lava flows, dikes, sills, and pyroclastic deposits from stratovolcanoes.

Key Facts

Hardness

6-7 (Mohs scale, for constituent minerals like plagioclase and pyroxene; the rock itself is tough but can be scratched by harder minerals).

Specific Gravity

2.5-2.8 g/cm³.

Crystal System

Not applicable to the rock as a whole; constituent minerals have their own crystal systems (e.g., plagioclase is triclinic, pyroxene is monoclinic or orthorhombic).

Color

Gray, dark gray, reddish-brown, black.

Luster

Dull to earthy (groundmass), vitreous (phenocrysts).

Transparency

Opaque (rock), translucent to opaque (phenocrysts).

Fracture

Conchoidal to uneven.

Cleavage

Absent in the rock; present in constituent minerals.

Composition

Intermediate igneous rock, 52-63 wt% SiO2. Dominant minerals are plagioclase feldspar (andesine or labradorite), pyroxenes (augite, hypersthene), and hornblende. Biotite and quartz may be minor constituents. Alkali feldspar is typically absent or very minor.

Physical characteristics

• Crystal Habit: Aphanitic groundmass; porphyritic texture with euhedral to subhedral phenocrysts of plagioclase (tabular), pyroxene (prismatic), and hornblende (prismatic/acicular).
• Cleavage Type: Not applicable to the rock; constituent minerals exhibit good to perfect cleavage (e.g., plagioclase: two directions at nearly 90°; pyroxene: two directions at nearly 90°; hornblende: two directions at 56° and 124°).
• Fracture Type: Conchoidal to uneven, depending on grain size and presence of phenocrysts.
• Tenacity: Brittle.
• Luster Type: Dull to earthy for the fine-grained groundmass; vitreous for individual phenocrysts.

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