Introduction
Mafic describes silicate minerals and igneous rocks whose chemistry is dominated by magnesium and iron rather than silica. This ferromagnesian composition commonly yields darker-colored rocks and minerals. Typical mafic mineral assemblages include olivine, pyroxene, amphibole, and biotite, which together control the textures and physical behavior of mafic igneous rocks. Common rock types—basalt (volcanic), diabase/dolerite (subvolcanic), and gabbro (plutonic)—exhibit similar mafic chemistry but differ in grain size and fabric according to their cooling histories. Feldspar in these rocks is usually the calcium-rich endmember of the plagioclase series, reflecting the Na–Ca balance in mafic magmas. The term ferromagnesian is frequently used interchangeably with mafic to emphasize the combined presence of iron and magnesium in the mineralogy.
In 1912 Charles W. Cross, J. P. Iddings, L. V. Pirsson and H. S. Washington introduced the term “mafic” (a blend of magnesium and ferric) to denote minerals and rocks dominated by ferromagnesian (Fe–Mg) composition. Their work grew out of an earlier scheme that divided igneous rock–forming minerals into two broad chemical‑petrological groups: “salic” (e.g., quartz, feldspars, feldspathoids) and “femic” (e.g., olivine, pyroxene). That initial dichotomy, however, proved imperfect: aluminous hydrous silicates such as micas and Al‑rich amphiboles were deliberately excluded from the femic group despite their significance in many assemblages, while certain calcium‑bearing phases with low Fe–Mg contents (notably wollastonite and apatite) were nevertheless placed with the femic minerals.
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To correct these inconsistencies the authors established a separate “alferric” category to capture aluminium‑rich minerals distinct from both salic and ferromagnesian types. They then consolidated the ferromagnesian constituents under the label “mafic,” rejecting the alternative “femag” (proposed by A. Johannsen in 1911) on grounds of clarity and euphony. This sequence of refinements produced a more consistent nomenclature for Fe–Mg‑rich igneous minerals and rocks that has endured in petrological practice.
Mafic minerals constitute the ferromagnesian component of igneous assemblages and are characterized by elevated iron (Fe) and magnesium (Mg) contents that strongly influence the chemistry and bulk appearance of mafic rocks. Historically the term has been associated with dark-colored minerals, but contemporary petrological practice defines the mafic fraction on mineralogical and compositional grounds rather than by color alone.
Modern classification schemes, notably the IUGS system for igneous rocks, therefore treat the mafic mineral fraction as a compositional category that may include minerals with light outward appearance when their chemistry is ferromagnesian; melilite is an explicit example of a light-colored phase that is counted as mafic under the IUGS framework. For high-precision classification, the mafic tally can be extended to include minor or accessory phases—such as zircon and apatite—so that the compositional inventory more accurately reflects the rock’s ferromagnesian budget and refines rock-type assignment.
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Rocks — Mafic
The term mafic is a field designation for dark-colored igneous rocks rather than a formal IUGS rock-class name; it is applied where ferromagnesian minerals—primarily pyroxene and olivine—predominate. By a mineralogical criterion, mafic rocks have a color index (the volumetric proportion of dark mafic minerals) of roughly 50–90%, indicating a dominant abundance of these minerals.
Chemically, mafic compositions are defined by relatively low silica (SiO2) contents, typically about 45–55 wt%, a range that corresponds to basalts in the TAS (Total Alkali–Silica) scheme for volcanic rocks. Mafic rocks are enriched in iron (Fe), magnesium (Mg) and calcium (Ca), which impart dark coloration and higher bulk densities compared with silica‑ and aluminium‑rich felsic rocks. Felsic suites, by contrast, are enriched in Al and Si and in alkali metals (K, Na), accounting for their lighter color, different mineral assemblages, and lower densities.
In volcanological and petrological practice most mafic volcanic products are classified as basalts, reflecting their typical mineralogy and silica range. Mafic magmas and lavas exhibit lower viscosity than felsic counterparts because reduced silica polymerization lessens melt connectivity; consequently volatiles escape more readily and eruptions of mafic lavas are generally less explosively violent. The mafic designation corresponds roughly to the older petrological category “basic rock,” linking historical terminology to modern compositional and mineralogical classifications.