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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.

Polylithionite

with Tainiolite - #PD34808
Locality
Lepkhe-Nel'm Mt, Seidozero Lake, Lovozero Massif, Kola Peninsula, Russia 
Formula
KLi2AlSi4O10F2 
Availability
Sold
Size
3 x 2.5 x 2.5 cm - Thumbnail 

Phyllosilicate - Monoclinic - Mica group member. End member. Ordinary (True or Common) Mica - Trioctahderal with lithium and aluminum in the (M) positions, Si and boron in the (T) positions, F>OH in the (A) position and K as the Interlayer cation.

Occurs as tabular crystals with hexagonal outline, also scaly, fine grained, white, gray, pale green, pale pink and violet. Can be fluorescent green. Typically found in alkaline syenites and their pegmatites including MSH and Ilimaussaq.

A nice specimen with colorless plates of Tainiolite.


Polylithionite

Formula
KLi2AlSi4O10F2 
Crystal System
Monoclinic 
Crystal Habit
Pseudo Hexagonal, Tabular, Scaly 
Cleavage
Perfect, None, None 
Luster
Pearly 
Color
bluish, colorless, violet, greenish, pearl white 
Streak
white 
Class
Monoclinic - Prismatic 
Fracture
Uneven 
Hardness
2-3 
Collection
Phillips Collection

Pyrophyllite

#GJ34805
Locality
Indian Gulch, Mariposa Co., California, United States 
Formula
Al2Si4O10(OH)2 
Availability
Sold
Size
9 x 7 x 4 cm - Sm Cabinet 

Phyllosilicate - Monoclinic - not a Mica group member but is a member of the Talc series. Talc is found as tabular crystals {001}. foliated and stellate masses, inelastic laminae, less abundant than Talc. Forms from the alteration of Feldspars, in Al-rich rocks, under acidic conditions >300 d C...below which Kaolinite forms.


Pyrophyllite

Formula
Al2Si4O10(OH)2 
Crystal System
Triclinic 
Crystal Habit
Earthy 
Cleavage
Perfect, None, None 
Luster
Pearly 
Color
brown green, brownish yellow, greenish, gray green, gray white 
Streak
white 
Class
Triclinic - Pinacoidal 
Fracture
Flexible 
Hardness
1.5-2 
Collection
Jungles Collection
SOLD
Tetraferriphlogopite - PD34681
Tetraferriphlogopite - PD34681

Tetraferriphlogopite

with Forsterite - #PD34681
Locality
Iron mine, Kovdor Massif, Kola Peninsula, Russia 
Formula
KMg3(Fe3+Si3O10)(OH,F)2 
Availability
Sold
Size
4 x 2.5 x 2 cm - Miniature 

Phyllosilicate - Monoclinic - Mica group member. Ordinary (True or Common) Mica - Trioctahderal with Mg, some substitutions for of Fe3+ for silicon at the (T) positions and OH>F at (A) otherwise would be Fluorotetraferriphlogopite.

Found in ultrabasic alkaline massif of Kovdor.

Nice 3-4mm crystals in matrix.


Tetraferriphlogopite

Formula
KMg3(Si3Fe3+)O10(OH)2 
Crystal System
Monoclinic 
Class
Monoclinic - Prismatic 
Hardness
Collection
Phillips Collection

Tetraferriphlogopite

with Forsterite - #PD34682
Locality
Iron mine, Kovdor Massif, Kola Peninsula, Russia 
Formula
KMg3(Si3Fe3+)O10(OH)2 
Availability
Sold
Size
5.5 x 5.5 x 3 cm - Sm Cabinet 

Phyllosilicate - Monoclinic - Mica group member. Ordinary (True or Common) Mica - Trioctahderal with Mg, some substitutions for of Fe3+ for silicon at the (T) positions and OH>F at (A) otherwise would be Fluorotetraferriphlogopite.

Found in ultrabasic alkaline massif of Kovdor.


Tetraferriphlogopite

Formula
KMg3(Si3Fe3+)O10(OH)2 
Crystal System
Monoclinic 
Class
Monoclinic - Prismatic 
Hardness
Collection
Phillips Collection
SOLD
Tobelite - PD34812
Tobelite - PD34812

Tobelite

#PD34812
Locality
Ohgidani deposit, Tobe, Shikoku Island, Japan 
Formula
(NH4)Al2(Si3Al)O10(OH)2 
Availability
Sold
Size
Chip < 1cm 

Phyllosilicate - Monoclinic - Mica group member. Ordinary (True or Common) Mica - Dioctahderal with Al3+,, OH at the (A) and dominant NH4>K at the interlayer position.

In Bernard & Hrysl "Minerals and Their Localities" describe this mineral as white to yellow to green clayed aggregates composed of tiny flakes. Mindat list five worlwide localties, one in the USA as Five MIle Pass, Utah. Maybe our friend Joe Marty will collect some fo this stuff for us.

More on rare mica minerals


Tobelite

Formula
(NH4)Al2(Si3Al)O10(OH)2 
Crystal System
Monoclinic 
Crystal Habit
Massive - Fibrous, Scaly 
Cleavage
Perfect, None, None 
Luster
Vitreous - Silky 
Color
yellowish green, pearly white 
Streak
white 
Class
Monoclinic - Prismatic 
Hardness
Collection
Phillips Collection
Mineralpedia
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SOLD
Vermiculite - PD34691
Vermiculite - PD34691

Vermiculite

#PD34691
Locality
Palabora mine, Phalaborwa, Limpopo, South Africa 
Formula
Mg0.7(Mg,Fe3+,Al)6(Si,Al)8O20(OH)4·8H2
Availability
Sold
Size
11 x 8 x 0.4 cm - 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.

Larger cleavage plate.


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

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