Zinnwaldite

(repeating unit)KLiFeAl(AlSi₃)O₁₀(OH,F)₂IMA symbolZnw^[1]Crystal systemMonoclinicCrystal classPrismatic (2/m)
(same H-M symbol)Space groupCc (no. 9)Unit cella = 5.29, b = 9.14
c = 10.09 [Å]; β = 100.83°IdentificationColorGray-brown, yellow-brown, pale violet, dark green, color zoning commonCrystal 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}FractureUnevenTenacityLaminae °exible, elasticMohs scale hardness3.5 - 4.0LusterPearly to vitreousStreakWhiteDiaphaneityTransparent to translucentSpecific gravity2.9 - 3.1Optical propertiesBiaxial (-)Refractive indexnα = 1.565 - 1.625 nβ = 1.605 - 1.675 nγ = 1.605 - 1.675Birefringence0.040 - 0.050PleochroismDistinct, X = colorless to yellow-brown; Y = gray-brown; Z = colorless to gray-brown2V angle0 - 40°References^[2][3][4][5]

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.^[4]

Name and discovery

It was first described in 1845 in Zinnwald/Cinovec on the German-Czech Republic border.^[4]

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.^[2]

References