MELILITE

Melilite – (Ca,Na)2(Mg,Al)(Si,Al)2O7 – is a rarer sorosilicate, typical of alkaline volcanic rocks. It has no economic importance.

“Melilite” is not a mineral, but merely a generic term referring to minerals of the Melilite Group, a solid solution composed of nine members: akermanite, alumoakermanite, bennesherite, ferri-gehlenite, gehlenite, gugiaite, hardystonite, hydroxygugiaite, and okayamalite. The most important members, most common in Petrology, are those in the series between gehlenite – Ca2Al(AlSiO7) and akermanite – Ca2Mg(Si2O7).. There are many synthetic minerals classified in this group, multiferroic and with very interesting properties.

1. Characteristics

These characteristics are of gehlenite.

Crystal system: Tetragonal scalenohedral.

Color: Yellowish-brown, colorless, greenish-gray.

Habit: Short prismatic shape, resembling cubes modified by the octahedron. Granular, massive. 

Cleavage:  {001} good, {110} bad.      

Tenacity: Brittle.        

Twinning: Simple in {100} and {001}. Lamellar by {001}. 

Fracture: Without information, it’s probably irregular.

Mohs Hardness: 5 – 6

Parting:  No.        

Streak: White, gray-white.         

Lustre: Vitreous, resinous.          

Diaphaneity: Transparent.           

Density (g/cm³): 3.03

 

2. Geology and Deposits

Melilite is a mafic mineral that occurs only in calcium-rich ultramafic volcanic rocks and is characteristic of melilitites. It is found in rocks of the sanidinite facies and rarely in alnoite dikes.

It can be found in the contact metamorphic aureole of granites, in skarns developed in high-grade impure marbles.

It may contain typical parallel rod inclusions such as zeolite fibers parallel to the Z-axis. It may contain glass as inclusions.

 

3. Mineral Associations

In volcanic rocks, it is associated with feldspathoids (nepheline, leucite), augite, olivine, diopside, and apatite.

In skarns, it occurs with carbonates (calcite, dolomite), garnets (pyrope, grossular, andradite), vesuvianite, wollastonite, spinel, phlogopite, rankinite, bicchulite, spurrite, fassaite, clinozoisite, and pleonast.

 

4. Transmitted Light Microscopy

Refraction indices:  nω: 1.670      nε: 1,660  (gehlenite)

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Akermanite is colorless; gehlenite is colorless, rarely pale honey-yellow to brown with weak pleochroism. 

Relief: Moderate.           

Cleavage: Akermanite: {001} distinct; Gehlenite {001} and {110} distinct.           

Habits: Tabular on (001), prismatic (hk0). Square and elongated basal sections. Akermanite forms thin lamellae according to {001}; gehlenite short columns parallel to {100} and thick plates along {001}.  

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence of 0.008 in akermanite and 0.011 in gehlenite: 1st order colors: various shades of gray to at most straw yellow. Melilite with birefringence = 0.000 is common in lavas, showing anomalous light brown or “Berlin blue” interference colors due to its strong scattering.

Extinction: Usually paralell.           

Elongation sign: ES(+) or ES(-), always the opposite of the optical character. 

Twins: Rare interpenetration twins.         

Zoning: It may exhibit zonation with acicular inclusions.             

CONVERGENT LIGHT

Character: U(+) or U(-)          

2V angle: No.         

Alterations: It is easily altered by hydrothermal action, forming zeolites: the growth of nearly isotropic fibers of mordenite, nucleating along the basal planes of the crystals, gives rise to the “rod-like” texture.          

May be confused with: Paragenesis, birefringence, and frequent “Berlin blue” colors are diagnostic.

Vesuvianite has a higher relief and no cleavage.

Apatite forms hexagonal prismatic crystals and never exhibits anomalous interference colors.

Volcanic glass is isotropic, and zoisite is B(+).         

Melilite 01: In PPL, large lamellar melilite crystal, diagonally across the image. Image obtained with medium magnification and zoom, as the crystals are small.
Melilite 01: The same section as of the previous image, now in XPL. Melilite shows blue anomalous interference colors.
Melilite 02: In PPL, volcanic rock with a large quantity of tabular melilite crystals. These melilite crystals are mixed with volcanic glass, are opaque, and contain small pyroxenes. Their tabular shape and occurrence (matrix of volcanic rocks) are very similar to plagioclase, but melilites never exhibit polysynthetic twinning.
Melilite 02: The same section as of the previous image, now in XPL. Melilite interference colors are usually anomalous blues, and extinction is parallel.
Melilite 03: In PPL, tabular melilite crystals in the matrix of a volcanic rock.
Melilite 03: The same section as of the previous image, now in XPL. Melilite shows bluish interference colors.

5. Reflected Light Microscopy

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

Sample preparation: Polishing melilite is not difficult and is similar to polishing the pyroxenes and olivines that are associated with it in volcanic rocks.       

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, lighter than feldspars and approximately the same color as pyroxenes and olivines.

Pleochroism: No.      

Reflectivity:  Low (<<10%)       

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: No anisotropy is observed.        

Internal reflections: Widespread, dark.     

May be confused with: Pyroxenes, which accompany it in paragenesis, are present in volcanic rocks. Only the lamellar habit is diagnostic. As a rule, it is not possible to identify melilite by reflected light. 

Melilite 01: In PPL, melilite-basalt. In very dark gray, feldspars. In lighter gray, tabular or prismatic, respectively melilite and pyroxenes, indistinguishable (center of the image). In the same lighter gray color, large grains of olivine. In pinkish-brown (on the right), magnetite. In yellow, chalcopyrite.
Melilite 01: The same section as of the previous image, now in XPL. Chalcopyrite is very dark, magnetite is isotropic. Melilite cannot be identified by reflected light.