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What Are Mafic Silicate Crystals?

Mafic silicates are a general classification of minerals found in igneous rocks. The term "mafic" refers to the minerals' high content of the elements magnesium and iron, combined with oxides of silicon and other elements. Mafic minerals are typically dark and dense. This differentiates them from the minerals geologists call "felsic," which have low concentrations of the two elements. The most common mafic minerals include olivine, pyroxenes and amphiboles; the plagioclase feldspars are included in the classification.

Olivine
- Olivine represents a group of closely-related minerals, the fayalite-forsterite series, that are classified based on the ratio of iron to magnesium in the crystals. Unlike most other mafic silicates, olivine does not contain aluminum in its crystal lattice. Olivine crystals range in color from yellowish-green to greenish-black, with rare reddish-brown specimens. The mineral hardness is 6 1/2 to 7. A gem form of olivine, called peridot, is the August birthstone.
Pyroxene Group
- The pryoxenes are more chemically complex than olivine, including a wide range of trace elements as well as magnesium, iron and some aluminum. Pyroxene group members have a hardness of 6 1/2 to 7. Pyroxenes form in a wide range of colors that often depend on the trace elements included in the crystals. Unlike most dark mafic minerals, crystals of the lithium-bearing pyroxene spodumene (also called kunzite) may be pink or lavender.
Amphibole Group
- The amphiboles are a family of mafic silicate minerals that, in addition to iron and magnesium, contain varying amounts of aluminum, calcium and water. They are the only hydrous, or water-bearing, mafic mineral. Many amphiboles also contain trace amounts of other elements such as lithium, sodium or manganese. Amphiboles typically grow to form long, thin crystals shaped like needles or prisms and are almost always dark green or black. The most common amphibole is the mineral hornblende.
Plagioclase Feldspars
- Feldspars are among the most common minerals among igneous rocks. There are two types of feldspar: a potassium-rich feldspar named orthoclase that is a felsic mineral, and a mafic variety named plagioclase. Plagioclase feldspar crystals are typically prismatic, blocky or stubby and gray to dirty white in color. The mineral ranges from a sodium-rich version called albite to a calcium-rich version called anorthosite.
Occurrence
- The mafic minerals occur in dark igneous rocks such as basalt (fine-grained) and gabbro (coarse-grained). Scattered crystals, particularly of horneblende (an amphibole) may be found with more felsic minerals in lighter-colored igneous rocks such as granite and rhyolite. There are rare occurrences of rock entirely composed of olivine, called dunite; or of pyroxene, called pyroxenite.

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