HOLMQUISTITE

Holmquistite – {Li2}{Mg3Al2}(Si8O22)(OH)2 – is a very rare amphibole that occurs associated with lithium-bearing pegmatites. It does not form ore, but it serves as a guide in the identification of lithium-bearing pegmatites (see London, D. 1986, Holmquistite as a Guide to Pegmatitic Rare Metal Deposits. Economic Geology, v. 81, p. 704-712).

The Holmquistite Group brings together orthorhombic amphiboles in the Lithium-bearing Amphibole Subgroup. Holmquistite may contain Ca, Na, Ti, Mn, K, and F and is dimorphic with clinoholmquistite. It forms a series with ferroholmquistite, which is extremely rare (only one occurrence, in the Greenbushes pegmatite, Australia).

Macroscopically, holmquistite can be confused with glaucophane.

1. Characteristics

Crystal system: Orthorhombic bipyramidal. 

Color: Black, dark violet, light sky blue. 

Habit: Prismatic, acicular, fibrous, columnar in bundles, may be massive. Crystals up to 10 cm.

Cleavage: {210} perfect, like all amphiboles. Striations in {210}.       

Tenacity: Brittle.        

Twinning: No information available.      

Fracture: No information available.       

Mohs Hardness: 5.5

Parting: In {001}, {112} and {113};        

Streak: White with a sky-blue hue.         

Lustre: Vitreous.          

Diaphaneity: Transparent.           

Density (g/cm³):  2.95 – 3.13

          

2. Geology and Deposits

Holmquistite occurs exclusively as an alteration product (metasomatic replacement) in the marginal portions (aureoles) of lithium-bearing granitic pegmatites, near the contact with basic and ultrabasic country rocks such as amphibolites. Therefore, the formation of holmquistite depends on mafic protoliths.

The lithium-bearing pegmatites in question are highly fractionated pegmatites, rich in Li-Be-Sn-Ta, where spodumene formed early. Holmquistite never occurs around pegmatites where spodumene is restricted to the interior zone. It can occur in pegmatites like petalite. Holmquistite aureoles can range from one to 20 meters in thickness, containing coarse-grained holmquistite, epidote, pale biotite, chlorite, albite, titanite, calcite, Fe-Ti oxides, and quartz, a typical assemblage of the greenschist facies.

Holmquistite generally forms from hornblende, always through Li metasomatism. It can also form from pyroxene or biotite.

 

3. Mineral Associations

It is associated with quartz, feldspars (plagioclase (albite), microcline), micas (biotite, muscovite, lithium-bearing micas of the siderophyllite series – polylithionite (“zinnwaldite”)), spodumene, tourmaline, magnetite, clinozoisite, clinoholmquistite and fairfieldite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.613 – 1.625 nβ: 1.634 – 1.645 nγ: 1.646 – 1.666

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Pale yellow to violet, with strong pleochroism:
X = colorless, yellow,
Y = colorless lavender
Z = blue, lilac.
Strong pleochroism in shades of violet when using strong colors.

Relief: Moderate.

Cleavage: Two good cleavages intersecting, in the basal sections, at 54 and 126º. 

Habits: Prismatic, acicular, fibrous, can be columnar to massive.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Maximum birefringence of 0.033 to 0.041, corresponding to strong, vivid, colorful colors up to the end of the 2nd order.           

Extinction: Probably paralell.           

Elongation sign: No information available.    

Twins: No information available.          

Zoning: No information available.              

CONVERGENT LIGHT

Character: B(-)         

2V angle: 49º         

Alterations: No information available.          

May be confused with: Few other minerals are similar, as the habit and cleavages characterize it as an amphibole, and the pleochroism is very typical, extremely diagnostic.

Glaucophane is similar, but the paragenesis is different and it exhibits pleochroism in different colors.         

Caption.

5. Reflected Light Microscopy

Reflected light microscopy is clearly not the recommended analytical method for identifying holmquistite. However, it is important to prepare a polished slide or section to identify opaque minerals that occur associated with holmquistite, such as magnetite.

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.