Menilite

Opaline concretions found in marls, gypsums and shales. Sometimes considered a variety of opal but probably impure and really just young Flint nodules which haven’t recrystallised to quartz yet; thus really a rock rather than a mineral.

Originally reported from Ménilmontant, Paris, Ile-de-France, France as nodules in the shale (Menilite formation?) (de Saussure, 1795). The analysis gives 10% water/organic matter.

Note: The name is also used informally for a bituminous shale with chert beds from the Menilite Formation (Eocene/Oligocene).

Jameson's System of Mineralogy from 1816 says that Menilite was divided by Hoffmann into two sub-species - "Brown Menilite" and "Grey Menilite":

Brown Menilite = Leberopal (liver opal)

Color: Chestnut-brown, sometimes inclines to liver brown. On the surface it has sometimes a bluish colour.
Always occurs tuberose, seldom larger than a fist, often smaller. External surface rough and dull.

The Spanish "menilitas", which are by far the most abundant ones in collections, occur in a diatomaceous earth deposit. The nodules consist of light toffee-brown to pale grey massive opal, with abundant inclusions of diatoms (themselves composed of opal) from the host rock, with a white chalky exterior of powdery opal, so three "varieties" of opal in a single nodule. The interiors are hard and compact and take a good polish, so sometimes used for opaque lapidary work.


Grey Menilite = Grauer Menilite (of Hoffmann)

Yellowish-grey sometimes inclining to wood-brown.
Tuberose but more compressed than the brown subspecies, and the external surface is smoother.

Occurs at Argenteuil near Paris embedded in clayey marl and gypsum.