Felsic describes igneous silicate minerals, magmas, and rocks whose composition is dominated by silica- and alkali-bearing phases (principally feldspar and quartz) rather than iron‑ and magnesium‑rich minerals. Chemically, felsic systems are enriched in the lighter major elements—silicon, oxygen, aluminium, sodium, and potassium—which determine their characteristic mineralogy and melt chemistry. Typical felsic assemblages include quartz, muscovite, potassium feldspar (orthoclase) and sodium‑rich plagioclase (albite‑dominant), and the most ubiquitous felsic rock is granite. These compositional traits impart generally light coloration and relatively low bulk densities (specific gravities < 3). Felsic magmas are correspondingly more viscous and have lower crystallization and melting temperatures than mafic magmas. In contrast, mafic compositions are enriched in iron and magnesium, producing darker, denser, hotter‑melting, and lower‑viscosity igneous rocks.
Acid rock
In contemporary petrography, “acid rock” denotes volcanic rocks whose bulk composition is silica‑rich, conventionally defined as containing more than ~63 wt.% SiO2; rhyolite exemplifies this compositional class. This usage is a quantitative, compositional shorthand and does not imply that the solid phases of such rocks contain a protonated silicic acid species.
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The historical pairing of “acid” versus “basic” rocks arose in nineteenth‑century geologic nomenclature and rested on a chemical model that envisaged “silicic acid” (expressed as H4SiO4 or Si(OH)4) as the dominant form of silicon in siliceous rocks. That conception supported a contrast with mafic or “basic” rocks richer in MgO and FeO, but it is now regarded as archaic. In reality, silicate minerals are built from a three‑dimensional framework of polymerized SiO4^4– tetrahedra rather than from monomeric protonated species. Nevertheless, silicate weathering and dissolution can release silicon into aqueous solution as dissolved silicic acid (H4SiO4/Si(OH)4), which accounts for the partial empirical validity of the old acid/basic intuition in water–rock interactions. Consequently, modern use of “acid” is properly understood as a bulk SiO2‑based classification label, not a statement about molecular speciation in the solid state.
Etymology
The term “felsic” is a technical petrological coinage formed from elements of the words feldspar (“fel-“) and silica (“-si-“), and is applied to igneous rocks and magmas dominated by silica and feldspar minerals. Its use therefore conveys a mineralogical composition marked by abundant quartz and alkali (potassium) feldspars and the light-coloured mineral assemblages typical of granitoid and rhyolitic lithologies. The superficial similarity between “felsic” and the German adjective “felsig” (“rocky”) is accidental: “felsig” stems from German Fels (“rock”) and is etymologically independent of the feldspar–silica construction. The root feldspar itself is borrowed from German Feldspat (Feld “field” + spat[h] “mineral”), which explains the Germanic source of the element “fel-” while preserving the term’s technical reference to the feldspar group. In classification contexts “felsic” thus functions as a descriptive category contrasting silica- and feldspar‑rich compositions with more magnesium‑ and iron‑rich (mafic) counterparts.
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Classification of felsic rocks
Felsic rocks are defined by a predominance of silica‑rich, light‑coloured minerals: when quartz combined with orthoclase and plagioclase exceeds about 75% of the rock, the lithology is classed as felsic. Specimens with especially high light‑mineral abundance (commonly >90%) are often described as leucocratic, a term that emphasizes their “white” dominance. Petrographic study of thin sections—for example, a felsic volcanic lithic fragment observed under a microscope with a millimetre‑calibrated scale box—provides the microscopic detail needed to quantify these mineral proportions and thereby support classification decisions that cannot be made reliably in hand sample alone.
Field and laboratory nomenclature differ according to texture, grain size, and origin. In the field, very fine‑grained, light‑coloured volcanic rocks are tentatively termed felsite; this is a preliminary, textural field name that may be revised after thin‑section petrography or chemical analysis. For extrusive felsic rocks the Le Maitre TAS chemical scheme (1975) is used to assign formal volcanic names. Phaneritic (coarse‑grained) felsic plutonic rocks are classified on the QAPF diagram to give granite‑type names; because “granite” already implies abundant feldspar plus quartz, plutonic names commonly append the species of any conspicuous mafic minerals present (e.g., hornblende‑bearing granite, pyroxene tonalite, augite‑megacrystic monzonite).
Texture also governs primary naming: many felsic volcanic rocks contain conspicuous mafic phenocrysts (hornblende, pyroxene or mafic feldspar), and such phenocryst phases are incorporated into informal names (for example, “hornblende‑bearing felsite” or “pyroxene rhyolite”). Rocks that have undergone metamorphism or for which the protolith is ambiguous should be named on metamorphic grounds when appropriate (e.g., “felsic schist”); highly sheared granites can mimic rhyolitic textures and thus require consideration of protolith, fabric, and mineralogy to avoid misclassification.
A concise summary of common texture–name relationships is as follows:
– pegmatitic → granite pegmatite
– coarse‑grained (phaneritic) → granite
– coarse‑grained and porphyritic → porphyritic granite
– fine‑grained (aphanitic) → rhyolite
– fine‑grained and porphyritic → porphyritic rhyolite
– pyroclastic → rhyolitic tuff or breccia
– vesicular → pumice
– amygdaloidal → no standard felsic name
– vitreous/glassy → obsidian or porcellanite
These criteria—mineral proportion, texture, protolith, and chemical classification—are applied together to produce a consistent and informative felsic rock name.