Zinnwaldite

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Zinnwaldite
Zinnwaldite2.jpg
General
Category Phyllosilicate mineral
Formula
(repeating unit)
KLiFeAl(AlSi3)O10(OH,F)2
Crystal symmetry 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.
Crystal system Monoclinic - Prismatic
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 [1][2][3]

Zinnwaldite, KLiFeAl(AlSi3)O10(OH,F)2, potassium lithium iron aluminium silicate hydroxide fluoride is a silicate mineral in the mica group. The IMA status is as a series between siderophyllite (KFe2Al(Al2Si2)O10(F,OH)2) and polylithionite (KLi2AlSi4O10(F,OH)2) and not considered a valid mineral species.[3]

Name and discovery[edit]

It was first described in 1845 in Zinnwald/Cinovec on the German-Czech Republic border.[3]

Occurrence[edit]

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.[1]

References[edit]