Zinnwaldite - Reference.org

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Category Phyllosilicate mineral

Formula(repeating unit) KLiFeAl(AlSi3)O10(OH,F)2

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 ; β = 100.83°

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

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°

Additional information

Zinnwaldite

Phyllosilicate mineral series in the mica group

Zinnwaldite, K Li Fe Al(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.