MONTEBRASITE

Montebrasite – LiAl(PO4)(OH) – is a relatively rare phosphate, typical of granitic pegmatites. It has been used as a Li ore and is sometimes cut as a gemstone.

It is classified in the Amblygonite Group, in the Amblygonite-Montebrasite Series. Montebrasite may contain Na. It sometimes exhibits fluorescence in orange colors under long-wave ultraviolet light.

Identification of montebrasite (more OH than F) and amblygonite (more F than OH) under a microscope, both under Transmitted Light and Reflected Light, is difficult due to the lack of information about it. Traditional Mineralogy textbooks simply ignore montebrasite. There is a test for recognizing montebrasite, which is the flame test. Pulverized montebrasite placed in a flame produces a bright red flame, confirming the presence of Li.

It is possible to confuse montebrasite (and amblygonite!) with several other minerals that occur in the same paragenesis. For example, with transparent, colorless to milky minerals with vitreous or sub-vitreous luster such as quartz, orthoclase, microcline, petalite, pollucite, spodumene, and beryl. These minerals in hand samples can also be confused, at least theoretically, with massive white rocks such as marbles, limestones, carbonatites, and anorthosite, if they occur in massive or fine-grained forms.

1. Characteristics

Crystal system: Triclinic pinacoidal.          

Color: Colorless, white, grayish-white, very pale brown, pale pink, pale yellow.

Habit: Equidimensional, short to long prismatic, up to 1.5 m. Cleavable masses, columnar, compact.

Cleavage: {100} perfect, {110} good, {0-11} distinct, {001} imperfect.

Tenacity:  Brittle.       

Twinning: Common, various types.       

Fracture: Irregular.       

Mohs Hardness: 5.5 – 6

Parting: No.

Streak:  White.

Lustre: Resinous, sub-vitreous, pearly in cleavages.

Diaphaneity: Transparent.           

Density (g/cm³): 2.98 – 3.04.

           

2. Geology and Deposits

Montebrasite occurs only as a late primary mineral in complex, zoned granitic pegmatites rich in Li and phosphates.

 

3. Mineral Associations

Montebrasite occurs associated with typical minerals of granitic pegmatites: quartz, feldspars (albite), micas (biotite, muscovite), fluorapatite, hydroxyapatite, Sn minerals (cassiterite, stannite), pollucite, topaz, tourmaline (elbaite), and columbite-tantalite.

Also with other Li minerals, such as lepidolite, lithiophyllite, triphyllite, petalite, and spodumene.

With other phosphates such as scorzalite, zanazziite, brazilianite, amblygonite, beryllonite, lacroixite, crandallite, and kosnarite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.594 – 1.615     nβ: 1.608 – 1.624      nγ: 1.616 – 1.645

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless.

Relief: Moderate.           

Cleavage: {100} perfect, {110} good, {0-11} distinct, {001} imperfect.
None of the angles formed by the cleavages is 90º. 

Habits: Massive, equidimensional, short to long prismatic. Crystals can be very large, reaching up to 1.5 meters in length.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Maximum birefringence of 0.026, corresponding to strong colors up to the middle of the 2nd order: yellow, orange, red, purple, blue and blue-green.

Extinction: oblique, because it is a triclinic mineral.

Elongation sign: No information available, it probably doesn´t apply.

Twins: Twins are common, with several possible types, including polysynthetic ones.

Zoning: No information available.             

CONVERGENT LIGHT

Character: B(+) or B(-). 

2V angle: : 66 – 101º 

Alterations: It can alter to mixtures of smectite, kaolinite, and micas, often as rims around still unaltered material. It also alters to turquoise, wavellite, wardite, and morinite.          

May be confused with: No information available.         

Caption.

5. Reflected Light Microscopy

Reflected light microscopy is clearly not the recommended analytical method for identifying montebrasite. The large size of the pegmatite crystals makes the use of reflected light impractical for identifying montebrasite and comparing it with other minerals from the paragenesis, many of which are identical to montebrasite under reflected light.

Sample preparation:        

PLANE POLARIZED LIGHT – PPL

Reflection color:        

Pleochroism:       

Reflectivity:         

Bireflectance:        

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:         

Internal reflections:       

May be confused with:        

General Characteristics: 

Caption.