Zinnwaldite

Zinnwaldite
General
Category	Phyllosilicate mineral
Formula (repeating unit)	KLiFeAl(AlSi3)O10(OH,F)2
Crystal system	Monoclinic - Prismatic
Space group	Monoclinic Point Group: 2/m
Unit cell	a = 5.29 Å, b = 9.14 Å, c = 10.09 Å: β = 100.83°
Identification
Color	Gray-brown, yellow-brown, pale violet, dark green, color zoning common
Crystal habit	Well-formed short prismatic or tabular crystals, pseudohexagonal, in rosettes or fan-shaped groups; lamellar or scaly aggregates; disseminated.
Twinning	On composition plane {001}, twin axis [310]
Cleavage	Perfect basal {001}
Fracture	Uneven
Tenacity	Laminae °exible, elastic
Mohs scale hardness	3.5 - 4.0
Luster	Pearly to vitreous
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.9 - 3.1
Optical properties	Biaxial (-)
Refractive index	nα = 1.565 - 1.625 nβ = 1.605 - 1.675 nγ = 1.605 - 1.675
Birefringence	0.040 - 0.050
Pleochroism	Distinct, X = colorless to yellow-brown; Y = gray-brown; Z = colorless to gray-brown
2V angle	0 - 40°
References

Zinnwaldite, KLiFeAl(AlSi₃)O₁₀(OH,F)₂, is a potassium lithium iron aluminium silicate hydroxide fluoride silicate mineral in the mica group.

Name and discovery

It was first described in 1845 in Zinnwald/Cinovec on the German-Czech Republic border.

Occurrence

It occurs in greisens, pegmatite, and quartz veins often associated with tin ore deposits. It is commonly associated with topaz, cassiterite, wolframite, lepidolite, spodumene, beryl, tourmaline, and fluorite.