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Pegmatites

Monday, 28 January 2013 08:00 AM

Since we live in the Black Hills, we enjoy our time visiting some of the old pegmatite mines in the Black Hills. But there are pegmatites of all ages, sizes, and shapes all over the world. They also contain some of the largest crystals in the world. Some are famous for their gem quality minerals and some are famous for their minute rare minerals. Join us for a neat review of pegmatite habits, features and of course minerals.

Albite

var. cleavelandite - #PD34923
Locality
Himalaya mine, San Diego Co., California, United States 
Formula
NaAlSi3O8 
Availability
Sold
Size
7 x 4.5 x 4 cm - Sm Cabinet 
Tagged
pegmatite  

Cleavelandite is a variety Albite, the sodic (Na) rich end member of the Albite-Anorthite Series. It is a common variety in most pegmatites and is usually found in white to slightly bluish compact platy masses. In the pegmatites of San Diego county, California, cleaveland forms large cauliflower-like groups with platy crystals in open pockets. This specimen is an excellent example of this habit. As a bonus a small 1.5cm doubly terminated Quartz group occurs on the Albite as well as a small 2mm light purple Fluorapatite crystal.


Albite

Formula
NaAlSi3O8 
Crystal System
Triclinic 
Crystal Habit
Blocky, Striated, Granular 
Cleavage
Perfect, Good, None 
Luster
Vitreous (Glassy) 
Color
white, gray, greenish gray, bluish green, gray 
Streak
white 
Class
Triclinic - Pinacoidal 
Fracture
Uneven 
Hardness
Collection
Phillips Collection
On Dakota Matrix
Previously Sold Albite
On Dakota Matrix
Other Available Albite
WebMineral
View Albite
Mindat
View Albite

Amblygonite

- Montebrasite - #PD34926
Locality
Dunton Gem quarry, Newry, Oxford Co., Maine, United States 
Formula
LiAlPO4
Availability
Sold
Size
12 x 8 x 6 cm - Cabinet 
Tagged
phosphate , pegmatite  

This is an excellent well-formed crystal of the primary phosphate, labeled as "amblygonite." All amblygonites should be labeled as within the series of amblygonite-montebrasite. Montebrasite is the OH analogue whereas amblygonite is the F end member. Ii is doubtful that in hand specimen the two can be discerned. Moore (MR May/June 1973) states that triphylite will crystallize before amblygonite-montebrasite. Crystals are known to be very large up to a meter across in several cases. Perfect crystals are usually not the case since they are typically found as rounded nodules probably a result of metasomatic reactions with the remaining fluid in the pegmatite system so this specimen is a nice example. The outer portion of the specimen is probably altered to kaolinite. Amblygonite can sometimes be hard to tell apart from ordinary feldspars but a simple hardness test can differentiate the two. However, be careful since sometimes amblygonite will have the kaolinite rim.


Amblygonite

Formula
LiAlPO4
Crystal System
Triclinic 
Crystal Habit
Columnar, Euhedral Crystals, Prismatic 
Cleavage
Perfect, Good, Distinct 
Luster
Vitreous - Pearly 
Color
white, yellow, gray, bluish gray, greenish gray 
Streak
white 
Class
Triclinic - Pinacoidal 
Fracture
Uneven 
Hardness
5.5-6 
Collection
Phillips Collection

Baratovite

in Albite - #PD34934
Locality
Darai-Pioz Glacier, Alayskiy Range, Tien Shan Mts, Tajikistan 
Formula
KLi3Ca7Ti2(SiO3)12F2 
Availability
Sold
Size
11 x 7 x 4 cm - Cabinet 
Tagged
pegmatite  
Special Info
Fluorescent 
Special Info
Type Locality 

This occurrence may or may not be classified as a pegmatite, although it certainly pegmatitic - it is igneous and coarse grained. There is more to the definition of a pegmatite, but not here. This specimen is rich in Baratovite, which occurs as coarse cleavable crystal grains to over 1cm. The grains are a bright blue un shortwave UV, However all the pink does not fluoresce, which is puzzling. This is a large specimen with associated sphene and albite.

This locality, according to Mindat, has produced no less than 130 minerals and 30 type locality minerals. The area must be vast as part of the glaciated massif. More on Baratovite can be found in our Mineralpedia.


Baratovite

Formula
KLi3Ca7Ti2(SiO3)12F2 
Crystal System
Monoclinic 
Crystal Habit
Platy, Aggregates 
Cleavage
Perfect, None, None 
Luster
Vitreous - Pearly 
Color
pearl white 
Streak
white 
Class
Monoclinic - Prismatic 
Hardness
3.5 
Collection
Phillips Collection
Special Info
Fluorescent
Special Info
Type Locality

Beryl

with Cassiterite - #PD34910
Locality
Orchard quarry, Buckfield, Oxford Co., Maine, United States 
Formula
Be3Al2Si6O18 
Availability
Sold
Size
9 x 5.5 x 6.5 cm - Sm Cabinet 
Tagged
beryllium , pegmatite  

A typical example of pegmatite showing green beryl crystals to 3cm long suspended in albite matrix with cassiterite crystal grains to 6mm.


Beryl

Formula
Be3Al2Si6O18 
Crystal System
Hexagonal 
Crystal Habit
Crystalline - Coarse, Prismatic, Columnar 
Cleavage
Imperfect, None, None 
Luster
Vitreous - Resinous 
Color
green, blue, yellow, colorless, pink 
Streak
white 
Class
Hexagonal - Dihexagonal Dipyramidal 
Fracture
Brittle - Conchoidal 
Hardness
7.5-8 
Collection
Phillips Collection
Mineralpedia
View Beryl
On Dakota Matrix
Previously Sold Beryl
On Dakota Matrix
Other Available Beryl
WebMineral
View Beryl
Mindat
View Beryl

Beryl

#34930
Locality
Zimmerman pegmatite, Copper Mtn dist., Fremont Co., Wyoming, United States 
Formula
Be3Al2Si6O18 
Availability
Sold
Size
11 x 10 x 7 cm - Cabinet 
Tagged
pegmatite  

This specimen exemplifies the characteristics of beryl from a pegmatite. As is typical of outer zone beryl crystals, this specimen shows a tapering in the long dimension probably a result of inward growth in the direction in which the pegmatite forms. I don't usually sell broken specimens like this, but it is a marvelous teaching example showing the crystal habit and properties such as the basal (c-axis) cleavage.

At one time pegmatites were the major source of beryllium. These days beryllium is mined from non-pegmatitic sources namely the bertrandite epithermal deposits of Spor Mountain and Gold Hill deposits of Utah and Seward Peninsula in Alaska.

Beryllium is used in computer and telecommunications products, in aerospace and defense applications, appliances, and electronics.


Beryl

Formula
Be3Al2Si6O18 
Crystal System
Hexagonal 
Crystal Habit
Crystalline - Coarse, Prismatic, Columnar 
Cleavage
Imperfect, None, None 
Luster
Vitreous - Resinous 
Color
green, blue, yellow, colorless, pink 
Streak
white 
Class
Hexagonal - Dihexagonal Dipyramidal 
Fracture
Brittle - Conchoidal 
Hardness
7.5-8 
Mineralpedia
View Beryl
On Dakota Matrix
Previously Sold Beryl
On Dakota Matrix
Other Available Beryl
WebMineral
View Beryl
Mindat
View Beryl
SOLD
Beryl - PD35036
Beryl - PD35036

Beryl

#PD35036
Locality
Harding mine, Taos Co., New Mexico, United States 
Formula
Be3Al2Si6O18 
Availability
Sold
Size
11 x 7 x 7 cm - Cabinet 
Tagged
beryllium , pegmatite  

Large mass of light pink Beryl.


Beryl

Formula
Be3Al2Si6O18 
Crystal System
Hexagonal 
Crystal Habit
Crystalline - Coarse, Prismatic, Columnar 
Cleavage
Imperfect, None, None 
Luster
Vitreous - Resinous 
Color
green, blue, yellow, colorless, pink 
Streak
white 
Class
Hexagonal - Dihexagonal Dipyramidal 
Fracture
Brittle - Conchoidal 
Hardness
7.5-8 
Collection
Phillips Collection
Mineralpedia
View Beryl
On Dakota Matrix
Previously Sold Beryl
On Dakota Matrix
Other Available Beryl
WebMineral
View Beryl
Mindat
View Beryl

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