Feldspathoid - Encyclopedia

    Class : Silicates
    Subclass : Tectosilicates
    Crystal System : Variable
    Chemistry : Variable
    Rarity : Very common

Feldspathoids are a group similar in structure to feldspars (hence their name). They differ in their lower SiO2 content : they contain around 1/3 less silica and are therefore essentially formed from magmas rich in alkalis (K, Na) and poor in SiO2. They are essential minerals of magmatic rocks undersaturated in silica (nepheline syenites, phonolites, trachytes...). Their structure frequently includes fairly large structural gaps, which results in a drop in density (2.15 to 2.5 for feldspathoids against 2.54 to 2.75 for feldspars), and integration into these gaps of large ions (K, Na, Cl) or anionic groups (CO3, SO4...). The most common minerals in the group are nepheline and leucite. There are also less common species, such as sodalite, which has the particularity of containing chlorine, cancrinite and noseane, which incorporate CO3 and SO4 respectively, and lazurite which retains SO4, sulfur and chlorine in its gaps. structural. Finally, let us note that analcime is classified in the feldspathoids by certain authors and in the zeolites by others.

13.00 ct sodalite cabochons
Nepheline & schorlomite from Jebel Bou-Agrao, Morocco
Analcime from Port d'Agrès, Aveyron, France
Lazurite from Afghanistan

Hardness : Variable
Density : Variable
Fracture : Uneven to conchoidal

Streak : White

TP : Translucent to transparent
IR : Variable
Birefringence : Variable
Optical character : Variable
Pleochroism : Variable
Fluorescence : Variable

Solubility : Hydrofluoric acid

Magnetism : None
Radioactivity : None


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