SERANDITE

Serandite – NaMn2+2Si3O8(OH) – is a very rare inosilicate that occurs in extremely specific parageneses. It is not an ore mineral, but well-formed specimens command high prices in the collector’s mineral market.

It belongs to the Wollastonite Group, within the Pectolite-Serandite series, forming a series with pectolite (NaCa₂Si₃O₈(OH)). It is the Na-analogue of tanohataite and the Mn-analogue of pectolite. It may contain Fe, Mg, Al, and H₂O as impurities and generally contains some Ca. There is a variety with a higher calcium content.

1. Characteristics

Crystal system: Triclinic pinacoidal.          

Color: Colors ranging from pink and orange to red, in various intensities. It can be brown or colorless.

Habit: Prismatic, tabular, acicular, massive. Can be fibrous or radial.
Crystals up to 20 cm.

Cleavage: {001} perfect, {100} perfect.       

Tenacity: Brittle.        

Twinning: It exhibits occasional {110} contact twins, and more commonly [010] twins on the {110} plane.

Fracture: Irregular, splintery.       

Mohs Hardness: 5 – 5.5

Parting: No.         

Streak:  White.        

Lustre: Vitreous to resinous.          

Diaphaneity: Transparent.           

Density (g/cm³):  3.34

 

2. Geology and Deposits

Serandite can occur in alkaline intrusive complexes composed of gabbros and nepheline syenites, where pegmatites cut across syenites and nepheline syenites. Sodalite also appears in xenoliths within this paragenetic association. Mount Saint Hilaire (Canada) is an example of this type of occurrence.

It can occur in cavities within phonolites, such as at Point of Rocks (New Mexico, USA).

It has also been found in volcanogenic, manganese-bearing terrigenous deposits that have undergone contact metamorphism, such as the Tumannoe deposit (Russia).

 

3. Mineral Associations

In a type locality (Island of Rouma, arquipelago of Los, Guinea) it occurs with nepheline, arfvedsonite, eudyalite, sodalite, astrophyllite, villiaumite, analcime, fluorite, aegirine, and leucophanite.

It also occurs with albite, microcline, manganoneptunite, rhodocrosite, vuonnemite, aegirine, natrolite, polylithionite, and ussingite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1,668    nβ: 1,671     nγ: 1,703

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless; never exhibits pleochroism.

Relief: High.           

Cleavage: Larger crystals likely exhibit well-developed cleavage.

Habits: Massive, tabular, prismatic; it can be acicular and even fibrous.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Maximum birefringence of 0.035, corresponding to colors up to the end of the second order: intense, colorful, ranging from yellow, purple, and blue to orange.

Extinction: No information abvailable, probably oblique.

Elongation sign: No information abvailable.

Twins: May show twins.         

Zoning: No information abvailable.             

CONVERGENT LIGHT

Character: B(+)          

2V angle: 39º         

Alterations: No information abvailable.          

May be confused with: No information is available, but the paragenesis is highly diagnostic.

Caption.

5. Reflected Light Microscopy

Reflected-light microscopy is evidently not the recommended analytical method for identifying serandite. However, preparing a polished section is important for identifying the opaque minerals associated with serandite, such as oxides (magnetite, hematite, ilmenite), manganese-bearing minerals (todorokite, ranciéite, etc.), and others.

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.