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Systematic Micaceous Minerals

Saturday, 29 December 2012 09:00 PM

Not all micaceous looking minerals fall in the MICA group of phyllosilicate minerals. Many of the other silicates appear micaceous but are not. We'll have a look at some of these minerals and hopefully explain the difference. Take a look at our article on Mica and Other Micaceous Minerals.

SOLD
Vermiculite - PD34687
Vermiculite - PD34687

Vermiculite

#PD34687
Locality
Iron mine, Kovdor Massif, Kola Peninsula, Russia 
Formula
Mg0.7(Mg,Fe3+,Al)6(Si,Al)8O20(OH)4·8H2
Availability
Sold
Size
8 x 6 x 1 cm - Sm Cabinet 

Phyllosilicate - Monoclinic - Not a Mica group member. But is micaceous. A member of the Montmorillonite-Vrmiculate group with 2:1 layering, meaning there are two (T) tetrahedral layers with an (M) octahedral layer in between. There is no (I) interlayer and the 2:1 layers are instead held together by weak Van der Waals bonds.

Requires Biotite, Chlorite, Hornbelnd and Serpentines to form. Vermiculite contains so much water it will exfoliate when burned.

This is a nice cleavage plate.

More on rare mica minerals


Vermiculite

Formula
Mg0.7(Mg,Fe3+,Al)6(Si,Al)8O20(OH)4·8H2
Crystal System
Monoclinic 
Crystal Habit
Lamellar, Scaly 
Cleavage
Perfect, None, None 
Luster
Vitreous - Dull 
Color
colorless, green, gray white, yellow brown 
Streak
greenish white 
Class
Monoclinic - Prismatic 
Fracture
Uneven 
Hardness
1.5-2 
Collection
Phillips Collection

Zinnwaldite

transitional member - #PD31933
Locality
Zinnwald, Erzgebirge, Bohemia, Czech Republic 
Formula
KLiFe2+Al(AlSi3)O10(F,OH)2  
Availability
Sold
Size
4 x 2 x 0.6 cm - Miniature 

Phyllosilicate - Monoclinic - Mica group member. Ordinary (True or Common) Mica - Trioctahderal. A transitional member on the Siderophyllite-Polylitionite join.

Just a small specimen.

More on rare mica minerals


Zinnwaldite

Formula
KLiFe++Al(AlSi3)O10(F,OH)2 
Crystal System
Monoclinic 
Crystal Habit
Pseudo Hexagonal, Micaceous, Platy 
Cleavage
Good, None, None 
Luster
Vitreous - Pearly 
Color
light brown, silvery white, gray, yellowish white, greenish white 
Streak
white 
Class
Monoclinic - Prismatic 
Fracture
Uneven 
Hardness
3.5-4 
Collection
Phillips Collection

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