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.