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
IMA symbolZnw[1]
Crystal systemMonoclinic
Crystal 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°
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[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]

Large polylithionite flakes on a sample found by British Lithium near St Austell, Cornwall, England
Large polylithionite flakes on a sample found by British Lithium near St Austell, Cornwall, England

References

  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ a b Handbook of Mineralogy
  3. ^ Webmineral data
  4. ^ a b c Mindat
  5. ^ Rieder, M.; Hybler, J.; Smrcok, L.; Weiss, Z. (1996). "Refinement of the crystal structure of zinnwaldite 2M_1". European Journal of Mineralogy. 8 (6): 1241–1248. doi:10.1127/ejm/8/6/1241.