OLIVINE MELILITITE LAPILLI TUFF
This is one of the strangest & visually
distinctive volcanic rocks I've come across. This is an olivine
melilitite lapillistone. This means it is a clastic-textured,
extrusive igneous rock dominated by grains larger than ash (>2 mm in
size). Grains of this size are called lapilli. A
lapillistone (lapilli tuff) is a deposit of lapilli that's been lithified - in
this case, the grains have been cemented together by calcite. The
lapilli in this sample range in shape from subrounded to angular. There's
quite a few <2 mm grains in the rock as well.
The lapilli grains themselves are composed of an
unusual igneous rock called porphyritic olivine melilitite, which consists of a
mix of olivine, diopside pyroxene, and melilite (calcium sodium magnesium
aluminosilicate - (Ca,Na)2(Al,Mg)(Si,Al)2O7).
Locality & Geology: Howenegg, Hegau Volcanic Province, Rhine Graben,
southwestern Germany.
Age:
early Late Miocene, 10 m.y.
Olivine melilitite lapillistone (above & below; wet, cut surfaces; above:
5.3 cm across; below: 1.8 cm across) with grains of olivine melilitite
(gray & brown) cemented together by calcite (yellow).
The lapilli grains (see the gray & brown chunks in
the closeup photo below) are composed of porphyritic olivine melilitite.
