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Title: Cymrite  
Author: World Heritage Encyclopedia
Language: English
Subject: List of minerals C (complete)
Publisher: World Heritage Encyclopedia


Category Phyllosilicate
(repeating unit)
Crystal symmetry Monoclinic
Unit cell a = 5.32 Å, b = 36.6 Å, c = 7.66 Å; β = 90°; Z = 8
Formula mass 393.47
Color Brown, greenish, colorless
Crystal habit Micacious, sheet-like, pseudohexagonal, also fibrous
Crystal system Monoclinic
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 [1][2][3][4][5]

Cymrite is a silicate mineral with the chemical formula BaAl2Si2(O,OH)8·H2O.[4] The mineral is named for Cymru, which is the Welsh word for Wales.[3]

Cymrite, with perfect cleavage and a monoclinic crystalline system, falls in the silicate group.[4] Silicates are formed of Silicon and Oxygen bonding together to form tetrahedra.[5] 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.[4] Cymrite, being monoclinic is anisotropic with two optic axes.


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.[1] 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.


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