How Does Dacite and Andesite Differ?

Dacite and andesite are both fine-grained volcanic and extrusive igneous rocks. Volcanic or extrusive igneous rocks form when magma cools fast on the Earth’s surface. The quick cooling gives little time for large grains to grow, so they have a fine-grained texture.

Besides the fine-grained texture, these rocks can have other textures. For instance, they have vesicular, porphyritic, and ophitic forms. Despite the similarities they share, dacite and andesite have differences. Let us briefly examine each of these rocks before discussing their differences.

About dacite

Dacite is a fine-grained, light-colored volcanic rock with felsic composition. It is usually off-white or maybe blue-gray or has various shades of gray. Less often, colors are black, pale brown, reddish, or yellow.

This rock is high in silica (>63%) and has mainly plagioclase feldspar, quartz/tridymite, and minor mafic minerals like biotite, pyroxene, hornblende, olivine, and sometimes glass. It may also have considerable alkali feldspar.

Accessory minerals are Fe-Ti oxides, sphene,  apatite, zircon, and, less often, garnet.

Lastly, the plagioclase feldspars in dacite rocks are oligoclase, labradorite, or sanidine.

About andesite

Andesite is a fine-grained, intermediate volcanic rock. These rocks are light to medium gray. However, some have shades of greenish-gray or black, orange, blue or black.

Andesite has intermediate silica content (57-63 wt.%) and composition. It has primarily sodium-rich plagioclase, pyroxene and less amounts of hornblende. Also, it may have minor quartz or feldspathoids and, less often, biotite or muscovite.

Accessory minerals are Fe-Ti oxides, apatite, zircon, and garnets.

Lastly, the sodic plagioclase is usually andesine but can range from anorthite to oligoclase. 

How do dacite and andesite differ?

Here is how dacite and andesite differ:

1. Composition

Dacite is an acidic rock with 63-69 wt.% silica with up to 7-8 wt.% alkalis, while andesite is an intermediate rock in which silica is 57-63 wt.% and alkalis 6-7%

Furthermore, dacite has less mafic or minerals and more felsic or lighter minerals.

Bottom line: Dacite has more silica and felsic minerals, while andesite has less and more mafic minerals.

2. QAPF classification

Using the QAPF classification of volcanic rock, plagioclase is at least 65% of the total feldspars of both rocks.
However, dacite is a volcanic rock in which quartz is 20-60% of the QAPF content by volume and has feldspathoids.

On the other hand, in andesite, quartz is less than 20% or felspathoids less than 10% of QAPF content by volume.

Bottom line: Dacite has more quartz and not feldspathoids while andesite has less quartz and may have feldspathoids. However, these two don’t co-exist.

3. Colors

Dacite is a light-colored or leucocractic rock since it has lesser mafic minerals. Its color index M is less than 20.

On the other hand, andesite is a mesocratic rock, i.e., it lies between lighter and dark-colored minerals. Its color index M is ≈20 to 35.

Bottom line: Since it is lower in mafic mineral content, when held side by side, dacite rocks are lighter than andesite. However, some dacite specimens may have darker colors than andesite. Therefore, color alone isn’t sufficient to distinguish these two. 

Conclusion

Dacite rock is lighter in color and has more silica, quartz, and felsic minerals than andesite. In contrast, andesite is darker and has less silica, felsic minerals, and more mafic minerals than dacite.

Also, andesite may have feldspathoids or quartz, while dacite only has quartz.

If you have porphyritic varieties, dacite often has quartz phenocrysts, while andesite usually doesn’t.