Cymrite
Cymrite (2.2 x 1.6 x 1.6 cm)
General
Category	Phyllosilicate
Formula (repeating unit)	BaAl2Si2(O,OH)8·(H2O)
IMA symbol	Cym[1]
Crystal system	Monoclinic
Crystal class	Domatic (m) (same H-M symbol)
Space group	Pm
Unit cell	a = 5.32 Å, b = 36.6 Å, c = 7.66 Å; β = 90°; Z = 2
Identification
Color	Brown, greenish, colorless
Crystal habit	Micacious, sheet-like, pseudohexagonal, also fibrous
Cleavage	Perfect on {001}, good on {110}
Fracture	Uneven
Tenacity	Brittle
Mohs scale hardness	2-3
Luster	Silky, Vitreous
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	3.49
Optical properties	Biaxial (-)
Refractive index	nα = 1.611 nβ = 1.619 nγ = 1.621
Birefringence	δ = 0.010
2V angle	0-5°
References	[2][3][4][5][6][7]

Cymrite is a silicate mineral with the chemical formula BaAl₂Si₂(O,OH)₈·H₂O.^[5] The mineral is named for Cymru, which is the Welsh word for Wales.^[4]

Cymrite, with perfect cleavage and a monoclinic crystalline system, falls in the silicate group.^[5] Silicates are formed of Silicon and Oxygen bonding together to form tetrahedra.^[6] The symmetry of Cymrite is classified as having a mirror plane. It has a moderate relief, meaning the contrast between the mineral and the epoxy of a thin section makes cymrite easily visible. The birefringence of the mineral is 0.01.^[5] Cymrite, being monoclinic is anisotropic with two optic axes.

Occurrence

Cymrite was discovered in Wales but is found in other areas throughout the United States, Africa, Greece and other parts of Europe. It occurs in generally high temperature-pressure areas such as the hydrothermal manganese silicate ore that makes up the Benalt Mine in Wales and in manganese rock that has undergone high-pressure metamorphism found in Greece.^[2] It is important to geologists because of its limited occurrence, when cymrite is present on a rock it indicates that the rock, at some point, must have experienced high pressure and temperature.

References