Felsite
Fine-grained felsic volcanic rock (quartz + feldspar)
A general term for light-colored, fine-grained volcanic rocks rich in quartz and feldspar, like rhyolite.
- Mohs hardness
- 6-7 (rock aggregate)
- Color
- Light grey, pink, tan to reddish
- Type
- igneous
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Overview
Felsite is a general field term for any light-colored, fine-grained (aphanitic) extrusive igneous rock composed chiefly of quartz and feldspar, in which the individual crystals are too small to identify with the naked eye. It is essentially the textural catch-all for felsic volcanic rocks such as rhyolite and dacite when their precise composition is uncertain.
Felsite is typically pale grey, pink, tan, or reddish, dense, and may contain scattered phenocrysts (a porphyritic variety is called felsophyre). It often has a smooth, sometimes flinty appearance.
The term is useful in the field before laboratory analysis pins down the exact rock type.
Formation & geology
Felsite forms when silica-rich (felsic) magma erupts or intrudes at shallow depth and cools quickly, so that quartz and feldspar crystallize as a fine-grained, light-colored mass rather than coarse crystals. It is the extrusive counterpart of granite.
It occurs in lava flows, dikes, sills, and shallow intrusions associated with continental volcanism and granitic magmatism. Felsites are found in volcanic terranes worldwide and are common in ancient volcanic belts and dike swarms.
How to identify it
Look for a hard, light-colored (grey, pink, tan, or red) rock with a fine, even, sometimes flinty texture where individual minerals cannot be seen. It may contain a few visible phenocrysts of quartz or feldspar.
Its pale color indicates abundant quartz and feldspar; its fineness indicates rapid, near-surface cooling. Hardness near 6-7 and a lack of visible grains are key.
Look-alikes include rhyolite (often the same rock, may be flow-banded), chert (sedimentary, conchoidal fracture, no feldspar), and trachyte (alkali feldspar dominated). Felsite is a broad term that may overlap with these.
Uses & significance
Felsite is used as crushed stone for construction aggregate, road base, and concrete because it is hard and durable. Dense, attractively colored felsite has been used as dimension and ornamental stone and, historically, for stone tools because of its fine grain and toughness.
It has no significant gemstone value, though porphyritic felsite with attractive phenocrysts appeals to collectors. As a field term, its main significance is descriptive, marking felsic volcanic rocks before detailed classification.
Frequently asked questions
Is felsite the same as rhyolite?
Often, yes. Felsite is a general term for fine-grained felsic volcanic rocks; rhyolite is a specific felsite, but the term covers others too when composition is uncertain.
What does felsite mean?
Felsite refers to a light-colored, fine-grained igneous rock made mostly of quartz and feldspar, with crystals too small to see by eye.
What is the intrusive equivalent of felsite?
Granite is the coarse-grained intrusive rock with the same felsic, quartz-and-feldspar-rich composition.
How can I tell felsite from chert?
Felsite is an igneous rock containing feldspar and quartz, while chert is sedimentary, made of microcrystalline silica with a glassy conchoidal fracture and no feldspar.
Felsite guides
In-depth guides for identifying, valuing, and understanding Felsite.
