Zinnwaldite

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]

Zinnwaldite
Fluorapatite with topaz on zinnwaldite and quartz
General
Category	Phyllosilicate mineral
Formula (repeating unit)	KLiFeAl(AlSi3)O10(OH,F)2
IMA symbol	Znw[1]
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	Cc (no. 9)
Unit cell	a = 5.29, b = 9.14 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	[2][3][4][5]

Name and discovery

It was first described in 1845 in Zinnwald/Cinvald (today Cínovec) 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

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. Handbook of Mineralogy
3. Webmineral data
4. 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.