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Montebrasite
Current inventory:  0 gems
 

Chemistry:  LiAl(PO₄)(OH,F)  
[Lithium Sodium Aluminum Phosphate Hydroxide Fluoride]

Discovered in 1872;   IMA status: Valid (pre-IMA; Grandfathered).
Montebrasite is named after Montebras, France where it was first found.

Classification

Mineral Classification:

Phosphates

Strunz 8th Ed. ID:

7/B.02-40

Nickel-Strunz 10th Ed. ID:

8.BB.05

8 : PHOSPHATES, ARSENATES, VANADATES
B : Phosphates, etc., with additional anions, without H₂O
B : With only medium-sized cations, (OH, etc.):RO₄ about 1:1

Related to:

Amblygonite Group: Griphite, Tancoite, Amblygonite, Montebrasite, Natromontebrasite, Tavorite. Amblygonite-Montebrasite Series. The Hydroxyl analogue of Amblygonite.

Crystal Data

Crystallography:

Triclinic - Pinacoidal

Crystal Habit:

Crystals typically equant, may be short to long prismatic, to 10 cm; as coarse cleavable or blocky to rounded nodular masses.

Twinning:

Common on [111], producing tabular composite crystals; microscopic polysynthetic twinning on [111].

Physical Properties

Cleavage:

[100] Perfect, [110] Good, [011] Distinct

Fracture:

Irregular/Uneven to Sub-Conchoidal

Tenacity:

Brittle

Hardness (Mohs):

5.5 - 6.0

Density:

2.98 - 3.04 (g/cm³)

Luminescence:

None

Radioactivity:

Not Radioactive

Optical Properties

Color:

Bluish, Colorless, Greenish, Greenish Gray, Grayish White, pale Brown, pale Yellow

Transparency:

Translucent to Transparent

Luster:

Vitreous to Greasy, Pearly on cleavages

Refractive Index:

1.594 - 1.646  Biaxial ( +/- )

Birefringence:

0.020 - 0.030

Dispersion:

r < v

Pleochroism:

None

Occurances

Geological Setting:

A late primary and secondary mineral in zoned granite pegmatites; may be an ore of lithium.

Common Associations:

Albite, Crandallite, Hydroxylapatite, Lacroixite, Lepidolite, Lithiophilite, Muscovite, Petalite, Pollucite, Spodumene, Tourmaline, Triplite

Type Locality:

Montebras Mine, Montebras-en-Soumans, Boussac, Creuse, Limousin, France

Year Discovered:

1872

View mineral photos:

Montebrasite Mineral Photos and Locations

More Information

Mindat.org
Webmineral.com

Montebrasite is an uncommon phosphate mineral and a relatively rare gem. It is very closely related to Amblygonite and varies only by Amblygonite being richer in hydroxide instead of fluorine. The structures of the two minerals are the same and there are no discernable physical differences between the two. It may be that if many Amblygonite gems where retested, they may actually be Montebrasite. In general, it is thought that most yellow gems on the market are Amblygonite, and most colorless gems are Montebrasite, but this does not always hold true.

A few localities for gemmy material include Montebras, Creuse, France (the type locality); Minas Gerais, Southeast Region, Brazil; Newry, Oxford County, Maine, USA.
 

  
Montebrasite gems for sale:

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