Zinnwaldite

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Zinnwaldite
Fluorapatite-Quartz-Topaz-gha5a.jpg
Fluorapatite with topaz on zinnwaldite and quartz
General
CategoryPhyllosilicate mineral
Formula
(repeating unit)
KLiFeAl(AlSi3)O10(OH,F)2
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Unit cella = 5.29, b = 9.14
c = 10.09 [Å]; β = 100.83°
Identification
ColorGray-brown, yellow-brown, pale violet, dark green, color zoning common
Crystal habitWell-formed short prismatic or tabular crystals, pseudohexagonal, in rosettes or fan-shaped groups; lamellar or scaly aggregates; disseminated.
TwinningOn composition plane {001}, twin axis [310]
CleavagePerfect basal {001}
FractureUneven
TenacityLaminae °exible, elastic
Mohs scale hardness3.5 - 4.0
LusterPearly to vitreous
StreakWhite
DiaphaneityTransparent to translucent
Specific gravity2.9 - 3.1
Optical propertiesBiaxial (-)
Refractive indexnα = 1.565 - 1.625 nβ = 1.605 - 1.675 nγ = 1.605 - 1.675
Birefringence0.040 - 0.050
PleochroismDistinct, X = colorless to yellow-brown; Y = gray-brown; Z = colorless to gray-brown
2V angle0 - 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]

Large polylithionite flakes on a sample found by MetAmpere near St_Austell, Cornwall, England

References[edit]