Melilite with compositions dominated by the endmembers akermanite and gehlenite is widely distributed but uncommon. It occurs in
metamorphic and
igneous rocks and in
meteorites. Typical metamorphic occurrences are in high-temperature metamorphosed impure
limestones. For instance, melilite occurs in some high-temperature
skarns. Melilite also occurs in unusual
silica-undersaturated igneous rocks. Some of these rocks appear to have formed by reaction of
magmas with limestone. Other igneous rocks containing melilite crystallize from magma derived from the
Earth's mantle and apparently uncontaminated by the
Earth's crust. The presence of melilite is an essential constituent in some rare igneous rocks, such as olivine melilitite. Extremely rare igneous rocks contain as much as 70% melilite, together with minerals such as
pyroxene and
perovskite. Melilite is a constituent of some
calcium–aluminium-rich inclusions in
chondritic meteorites.
Isotope ratios of
magnesium and some other elements in these inclusions are of great importance in deducing processes that formed the
Solar System.
Melilitite is a volcanic rock composed of over 90% melilite, with small amounts of
olivine,
clinopyroxene, and
perovskite. The
intrusive equivalent is
melilitolite. When
pyroxene is absent from the rock, and the
accessory minerals are olivine,
magnetite,
leucite,
kalsilite,
nepheline, and perovskite, the rock is sometimes called
katungite. However, more modern classification schemes avoid the term
katungite and describe the rock instead as (for example) kalsilite-leucite-olivine melilitite, depending on the most abundant accessory minerals.
Alnöite is a melilitite devoid of glass or feldspathoids with melilite in its
groundmass. ==See also==