Tinguaite
Fine-grained nepheline-bearing phonolitic dike rock
A fine-grained green phonolitic dike rock rich in nepheline and aegirine, the hypabyssal equivalent of phonolite.
- Mohs hardness
- 5.5-6
- Color
- Greenish-gray to gray
- Type
- igneous
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Overview
Tinguaite is a fine-grained, dike-forming (hypabyssal) igneous rock equivalent to phonolite, composed of alkali feldspar, nepheline, and the green sodic pyroxene aegirine. The aegirine commonly forms slender needles producing a distinctive felted or 'tinguaitic' texture, and gives the rock a greenish tint.
The rock is named after the Tinguá massif (Serra do Tinguá) in Rio de Janeiro State, Brazil. It belongs to the silica-undersaturated, feldspathoid-bearing alkaline rock family.
Tinguaite is mainly of scientific and collector interest as a classic phonolitic dike rock from alkaline intrusive complexes.
Formation & geology
Tinguaite forms when silica-undersaturated, alkali-rich phonolitic magma intrudes as dikes and sheets at shallow to moderate depth. Moderately rapid cooling yields a fine grain size and the characteristic interlocking aegirine needles that define the tinguaitic texture.
It is typically associated with nepheline-syenite and phonolite complexes in alkaline igneous provinces. The type area is the Tinguá massif near Rio de Janeiro, Brazil, and similar tinguaitic dikes occur in alkaline complexes worldwide, including parts of Europe, Africa, and North America, wherever feldspathoid-bearing melts are injected into fractures.
How to identify it
Tinguaite is a fine-grained, greenish-gray to gray rock, its color reflecting abundant green aegirine, often visible under a lens as tiny needles woven through pale nepheline and feldspar. It is moderately heavy and lacks quartz.
Hardness is about 5.5-6. The greenish hue and felted aegirine texture help distinguish it from bostonite (which is feldspar-rich and dark-mineral-poor) and from ordinary trachyte. Like other feldspathoid rocks, confirming nepheline often requires thin-section work, but the green, silica-undersaturated, dike-rock character is a strong field guide.
Uses & significance
Tinguaite has little commercial use as a building or ornamental stone, though its greenish color can make appealing specimens. Its significance is mainly scientific, illustrating the dike-rock (hypabyssal) expression of phonolitic, feldspathoid-bearing magmatism in alkaline complexes.
For collectors, tinguaite is a desirable representative of the nepheline-bearing alkaline rock family and the tinguaitic texture. It has no established metaphysical tradition and appeals chiefly to petrologists, students, and mineral and rock collectors.
Frequently asked questions
What is tinguaite?
It is a fine-grained phonolitic dike rock made of alkali feldspar, nepheline, and green aegirine, representing the hypabyssal equivalent of phonolite.
Why is tinguaite green?
Its greenish color comes from abundant aegirine, a sodium-iron pyroxene that often forms slender needles throughout the rock.
Where is tinguaite found?
Its type locality is the Tinguá massif near Rio de Janeiro, Brazil, and it occurs in alkaline igneous complexes around the world.
Does tinguaite contain quartz?
No. It is silica-undersaturated and contains the feldspathoid nepheline instead of quartz.
Tinguaite guides
In-depth guides for identifying, valuing, and understanding Tinguaite.
