Meionite

Meionite is a tectosilicate belonging to the scapolite group with the formula Ca₄Al₆Si₆O₂₄CO₃.^[6] Some samples may also contain a sulfate group. It was first discovered in 1801 on Mt Somma, Vesuvius, Italy. It was named by Rene Just Haüy after μειωυ, the Greek word for less, in reference to the less acute pyramidal form as compared to vesuvianite.

Meionite
Meionite (scapolite) crystals (cross-shaped) in a quartz matrix, 7.2 × 4.0 × 3.0 cm. Worcester County, Massachusetts.
General
Category	Tectosilicate minerals, scapolite group
Formula	Ca4Al6Si6O24CO3
IMA symbol	Me[1]
Strunz classification	9.FB.15
Crystal system	Tetragonal
Crystal class	Dipyramidal (4/m) (same H-M symbol)
Space group	I4/m
Unit cell	a = 12.179(1) Å, c = 7.571(1) Å, Z = 2
Identification
Color	Colorless, white, grey, pink, violet, blue, yellow, orange-brown, brown
Cleavage	Distinct/good on {100}{110}
Fracture	Irregular/uneven, conchoidal
Tenacity	Brittle
Mohs scale hardness	5–6
Luster	Vitreous, resinous, pearly
Streak	White
Diaphaneity	Transparent, opaque
Specific gravity	2.74–2.78
Optical properties	Uniaxial (−)
Refractive index	1.556 to 1.600
Birefringence	0.024 to 0.037
References	[2][3][4][5]

As an end-member of the scapolite solid solution, meionite has the largest cell dimension ^[7] and very high thermal stability at high pressures. This indicates that meionite is one of the primary minerals in deep seated basic or intermediate magmatic processes. Meionite also breaks down to grossularite + kyanite + quartz + calcite at high pressure, similar to the upper pressure limits of anorthite. This sets its occurrence to the crustal rocks.^[8]