Phonolite with Nepheline
Nepheline Syenite var. Phonolite · Mineral Specimen and Volcanic Rock
Crystal System
Hexagonal system for Nepheline; the groundmass is typically aphanitic or microcrystalline.
Mineral Group
Silicates (Tectosilicates); specifically the Feldspathoid group.
Chemical Formula
(Na,K)AlSiO4 (for the white alkaline inclusion)
Formation Process
Igneous: Extrusive volcanic formation from silica-undersaturated magma, typically forming fine-grained groundmass with phenocrysts.
About This Crystal
An opaque, greenish-gray to brownish volcanic rock matrix containing a distinct cream-colored, circular phenocryst or amygdule. The texture is porphyritic with a dull to earthy luster.
Physical Characteristics
Grayish-green body color with a white streak. Lacks distinct cleavage in the matrix; conchoidal to uneven fracture. Specific gravity is approximately 2.5–2.6.
Optical Properties
Opaque in hand specimen. Under microscopy, constituents are uniaxial negative with low birefringence and a refractive index around 1.53.
Hardness & Durability
Origin Region
Found in alkaline volcanic provinces such as Mount Vesuvius (Italy), Transvaal (South Africa), and Arkansas (USA).
Hardness & Durability
Mohs hardness of 5.5 to 6. Brittle tenacity; stable under normal conditions but reactive to strong acids.
Care & Maintenance
Clean with warm soapy water and a soft brush. Avoid acidic cleaners which can etch the alkaline minerals; store away from harder minerals like quartz.
Rarity & Value
Common globally but of interest as a collector mineral or for geological study. Low commercial value unless containing rare earth elements.
Special Characteristics
Often exhibits weak orange to red fluorescence under short-wave UV light due to the presence of Nepheline or Sodalite group minerals.
Lore & History
Historically used as a 'clinkstone' because it rings when struck. In metaphysical traditions, it is associated with grounding and stabilizing the physical body.