Mineral Information and Data:

Montebrasite
LiAlPO4(OH) [CNMNC approved formula]

Named For the locality, Montebras, France, from which it was first described. The Hydroxyl analogue of Amblygonite.

IMA status : Valid species (Pre-IMA 1872) Grandfathered
CLASSIFICATION 
Dana
(8th edition) :
41.05.08.02  
  (41) Anhydrous Phosphates Containing Hydroxyl or Halogen
  (41.05) where (A B)2 (XO4) Zq
  (41.05.08) Amblygonite Group
  
Nickel-Strunz
(10th edition) :
08.BB.05
  (08) Phosphates, Arsenates, Vanadates
  (08.B) Phosphates, etc. with Additional Anions, without H2O
  (08.BB) With only medium-sized cations, (OH, etc.):RO4 about 1:1
  
Crystal system: Triclinic System
Point group (H-M): 1 — pinacoidal
Space group: C1
Unit cell: a = 6.713(1) Å, b = 7.708(1) Å, c = 7.0194(7) Å, α=91.31(1)°, β = 117.93(1)°, γ = 91.77(1)°, Z=4
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}
  
Color: Pale brown, pink, pale yellow, gray, white, colorless; colorless in transmitted light
Diaphaneity: Translucent to transparent
Luster: Vitreous, pearly on cleavages
Hardness (Mohs): 5.5 – 6
Measured Density: 2.98 – 3.04 g/cm3
Cleavage: Perfect on {100}; good on {110}; distinct on {011}
Tenacity: Brittle
Fracture: Uneven to subconchoidal
  
Geologic Setting: A late primary and secondary mineral in zoned granite pegmatites; may be an ore of lithium.
Mineral Association: Hydroxylapatite, spodumene, lepidolite, petalite, pollucite, tourmaline, triplite, lithiophilite, lacroixite, crandallite, muscovite, albite.
  
Other Properties: Gemstone Extremely Rare.
Comments: Species and name natromontebrasite are discredited because natromontebrasite is a mixture of OH-rich amblygonite with lacro ixite and subordinate wardite

 

Montebrasite
Origin: Newry, Oxford Co, Maine, U.S.A.
Sample size: Microscopic image
Photo and Owner: Lou Perloff

 

Montebrasite
Several tabular honey yellow crystals of montebrasite to 8 mm
throughout albite matrix.
Origin: Telirio mine, Linopolis, Minas Gerais, Brazil
Sample size: 3 x 3 x 3 cm
Photo: Tom Loomis

 

Montebrasite
(zoom-in of above)

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