HAÜYNE

Haüyne – (Na,Ca)4-8(Al6Si6(O,S)24)(SO4,Cl)1-2 – is a tectosilicate, a relatively rare feldspathoid, characteristic of silica-subsaturated sodic volcanic rocks. It is sought after as a collector mineral and can be used as a gemstone.

Haüyne is part of the Sodalite Group, along with lazurite, nosean, sodalite, tsaregorodtsevite and vladimirivanovite. It may contain some K. There is a white variety called “berzeline”. It may fluoresce in reddish orange to purple-pink under longwave UV light.

A variety of haüyne, rich in sulfur, is historically called “lazurite”. Lazurite is the main component of lapis lazuli, a rare metamorphic rock of intense blue color, very decorative and highly valued, with historic mines in Afghanistan.

1. Characteristics

Crystal system:  Cubic hexaoctahedral.         

Color: Blue, white, gray, yellow, green or pink.     

Habit: Typically dodecahedrons or pseudo-octahedrons. Rounded grains.       

Cleavage: {110} distinct.       

Tenacity: Brittle.        

Twinning: On {111}, common.       

Fracture: Irregular.       

Mohs Hardness: 5.5 – 6

Parting:  No.        

Streak: White to very pale blue.         

Lustre: Vitreous, resinous.          

Diaphaneity: Transparent.           

Density (g/cm³):  2.44 – 2.5

 

2. Geology and Deposits

Haüyne occurs in silica-subsaturated and Na-rich volcanic rocks, which are part of alkaline igneous complexes. These are rocks such as phonolites, trachytoids, trachyandesites, tephrites, basanite-foidites, foidites, foiditoids, tephritoids, tephrites, and phonolite-foidites, rocks containing leucite and nepheline.

It is found in some rare igneous rocks such as okaites (melilitolites) and melilitites.

Also in limestones that underwent contact metamorphism (skarns) and in rocks of regional metamorphism. It is the main component of lapis lazuli, in the form of the sulfur-rich variety known as lazurite.

 

3. Mineral Associations

Haüyne occurs with other feldspathoids (leucite, nepheline), feldspars (sanidine), pyroxenes (augite), micas (biotite, phlogopite), garnet (Ti-andradite), zeolites (natrolite, gonnardite), melilite, calcite, pyrite, titanite, pargasite, fluor-apatite and magnetite.

 

4. Transmitted Light Microscopy

Refraction indices:  n: 1.494 – 1.509

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless to blue, may occur in patches or irregular bands. More rarely green, red, yellow; does not have pleochroism.

Lazurite is the designation for the ultramarine, midnight blue, teal, and green colored varieties that make up lapis lazuli rock.

Relief:  Very low, negative.          

Cleavage:  {110} distinct. May be visible on a thin section.          

Habits: Equidimensional, granular, euhedral grains, which can have 6, 8 or 10 sides. More rarely octahedrons. There may be a zonation of inclusions; black margins (sulfides and iron oxides) are also frequent. Rounded vertices, corroded faces, and curves are common. If the grains are very small and occur as microliths in the matrix, they are easily overlooked.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Isotropic. A very low, anomalous birefringence is possible. In this case the blue color is still visible.           

Extinction: Isotropic.           

Elongation sign: Isotropic.            

Twins: No.         

Zoning: No information available.             

CONVERGENT LIGHT

Character: Isotropic.         

2V angle: Isotropic.         

Alterations: several types are possible.

Late-magmatic hydrothermalism alters haüyne to fibrous aggregates of zeolites (similar to serpentine).

Weathering leads to the formation of sericite.

Alteration forms kaolinite.

In the early stages of alteration, the rock may have a red color due to extremely fine-grained, disseminated limonite.          

May be confused with: several other minerals.

Sodalite, haüyne and nosean can be difficult to differentiate from each other on thin section.

Analcime is similar, but tends to occur only in cavities and shows irregular domains (zones) in CPL.

Leucite tends to show polysynthetic twins that vaguely resemble those of microcline, but are less sharp and the mineral is much darker.         

 

In PPL, two haüyne grains showing the characteristic blue color at two different intensities.
Haüyne 01: In PPL, haüyne grains with distinct blue color (right grain with zonation), characteristic geometric shapes, and black edges due to sulfides and iron oxides.
Haüyne 01: The same section as of the previous image, now in XPL. Anomalous birefringence and strong blue color mask the isotropy that the cubic mineral should have.
Haüyne 02: In PPL, altered brown hornblende phenocryst next to a haüyne phenocryst in very fine matrix volcanic rock. The haüyne is characterized by low relief, geometric shapes and black borders.
Haüyne 02: The same section as of the previous image, now in XPL. Haüyne shows anomalous birefringence.
Three small bluish-colored, black-edged haüyne grains in a volcanic rock with aligned feldspars in the matrix and altered brown hornblende and nosean phenocrysts. PPL.
Haüyne 03: In PPL, in a sanidine-nephelinite, sanidine phenocrystal (colorless), rock matrix (brown), natrolite (colorless horizontal band) and bluish haüyna crystal enclosed in the sanidine.
Haüyne 03: The same section as of the previous image, now in XPL. Haüyne is isotropic, sanidine slightly yellowish (thin section a little bit thicker than 30 microns), natrolites gray/white and rock matrix is dark.
Haüyne 04: In PPL, haüyne, variety lazurite. The blue color (no pleochroism!) of lazurite is very conspicuous and characteristic. In this case, haüyne does not form crystals, but is anhedral.
Haüyne 04: The same section as of the previous image, now in XPL. Lazurite is completely isotropic.
Haüyne 05: Haüyne, variety lazurite, in PPL. Blue colors of different intensities can be seen.
Haüyne 05: The same section as of the previous image, now in XPL. Lazurite shows anomalous anisotropy, with some isotropic (black) portions and other portions with the blue color still visible.

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical method for the identification of haüyne. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with haüyne.

Sample preparation: polishing haüyne is not difficult. It is similar to feldspar (plagioclase) polishing.       

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, indistinguishable from plagioclases.

Pleochroism: No.      

Reflectivity:  Very low (4%?)       

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Anisotropy was not observed.        

Internal reflections: Generalized, dark with a slight bluish tinge.

May be confused with:  recognition of haüyne in PPL is impossible. In XPL, the shape is characteristic and also a discreet tone of the internal reflections in blue. This blue color can be quite dark.

Haüyne 01: Image with Transmitted Light in PPL! Grain of haüyne in volcanic rock, with plagioclases around it (colorless laths), aegirine (greenish) and others. The same section in the next images.
Haüyne 01: The same section as of the previous image, now in Reflected Light in PPL. It is not possible to recognize haüyne in PPL in Reflected Light.
Haüyne 01: The same section as of the two previous images. The haüyne crystal can be recognized, but very dark, with a very discreet shade of blue.