Vauxite facts for kids
Quick facts for kids Vauxite |
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Vauxite from the Siglio XX Mine, Llallagua, Bolivia. Specimen size 2 cm
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| General | |
| Category | Phosphate minerals |
| Formula (repeating unit) |
Fe2+Al2(PO4)2(OH)2.6H2O |
| Strunz classification | 8.DC.35 |
| Dana classification | 42.11.14.1 |
| Crystal symmetry | P1 |
| Identification | |
| Formula mass | 441.86 g/mol |
| Color | Blue, becoming greenish on exposure |
| Crystal habit | Minute tabular crystals, radial aggregates and nodules |
| Crystal system | Triclinic |
| Twinning | On {010}, twin and composition plane. |
| Cleavage | Fractured |
| Mohs scale hardness | 3.5 |
| Luster | Vitreous |
| Streak | White |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 2.39 to 2.40 |
| Optical properties | Biaxial (+) |
| Refractive index | Nx=1.551, Ny=1.555, Nz=1.562 |
| Birefringence | r>v |
| Pleochroism | (strong): X, Z colorless, Y blue |
| Other characteristics | Fluorescent. Not radioactive |
Vauxite is a beautiful blue mineral that belongs to the phosphate minerals group. It has a special chemical formula: Fe2+Al2(PO4)2(OH)2·6H2O. This means it contains iron, aluminium, phosphorus, oxygen, and hydrogen, plus water molecules.
Vauxite is often found with other similar minerals like paravauxite and metavauxite. Even though their names sound alike and they are found together, their internal structures are actually quite different! Vauxite was discovered in 1922 and named after George Vaux Junior, an American lawyer and mineral collector.
Contents
What is a Unit Cell?
Minerals like vauxite are made of tiny, repeating building blocks called unit cells. Imagine stacking identical LEGO bricks to build a wall; each brick is like a unit cell. For vauxite, its unit cell is a special shape called a rhomboid. This means its faces are parallelograms.
The unit cell has specific measurements:
- It has three side lengths, called a, b, and c.
- It also has three angles between these sides, called alpha (α), beta (β), and gamma (γ).
For vauxite, scientists have measured these values. They are very tiny, measured in Ångströms (Å), which is one ten-billionth of a meter!
- a is about 9.1 Å
- b is about 11.6 Å
- c is about 6 Å
- The angles are α = 98.3°, β = 92°, and γ = 108°.
Each unit cell of vauxite contains two complete sets of its chemical formula.
How Vauxite's Structure is Built
The inside of vauxite is like a complex, tiny city made of repeating patterns. Its structure is built from long, endless chains that run in one direction.
Chains of Octahedra
One type of chain is made of shapes called octahedra. An octahedron is like two pyramids joined at their bases. In vauxite, these octahedra have either an iron (Fe) or an aluminium (Al) atom in the middle. Six oxygen (O) atoms surround these central atoms. These octahedra link together by sharing edges, forming a chain.
Mixed Chains of Octahedra and Tetrahedra
Another type of chain is a mix of octahedra and tetrahedra. A tetrahedron is a shape with four faces, like a small pyramid. In these chains, the tetrahedra have a phosphorus (P) atom in the middle, surrounded by four oxygen atoms. The octahedra in these chains have an aluminium atom in the middle. These shapes link by sharing corners.
Putting the Chains Together
These different chains connect to each other, forming even bigger, more complex structures. Imagine three chains running side-by-side, linked together. Then, other phosphorus tetrahedra connect these triple chains. Water molecules (H2O) and other chemical groups are also part of this amazing structure. All these parts fit together perfectly to give vauxite its unique chemical formula and properties.
Vauxite's Crystal Shape
Vauxite crystals are usually very small. They often look flat, like tiny tablets. They can also form groups that spread out like rays or small rounded lumps called nodules. The crystals are often longer in one direction, following the length of the chemical chains inside them.
How Light Interacts with Vauxite
Vauxite is a triclinic mineral. This means its internal structure is not perfectly symmetrical. Because of this, light behaves differently when it passes through vauxite depending on the direction it travels.
Refractive Index
The refractive index tells us how much light slows down when it goes through a material. For vauxite, this value changes depending on the direction of the light. It ranges from 1.551 to 1.562. This is measured using a specific yellow light, like the light from a sodium lamp.
Optic Axes and Biaxial Minerals
Some crystals have special directions called optic axes. When light travels along an optic axis, it behaves in a simple way. Vauxite has two optic axes, which means it is called a biaxial mineral.
Optic Sign
When light travels through vauxite (not along an optic axis), it splits into two different rays. These rays travel at different speeds. Scientists use an "optic sign" to describe this. Vauxite is called "biaxial (+)", which tells us how these two light rays behave inside the crystal.
Dispersion of Optic Axes
The color of light can affect how it travels through vauxite. The positions of the optic axes and the angle between them can change slightly depending on the color of the light. For vauxite, the angle between its optic axes is greater for red light than for violet light.
Pleochroism: Changing Colors
Vauxite can appear to change color when you rotate it under a special kind of light called plane polarized light. This is called pleochroism. It happens because the crystal absorbs different colors of light depending on the direction the light is vibrating. Vauxite shows strong pleochroism: it looks colorless in some directions and blue in others.
Where Vauxite is Found
Vauxite is a "secondary mineral." This means it forms when other minerals, like apatite, change over time. It's found in veins that contain tin.
The only place in the world where vauxite has been found is the Siglo Veinte Mine (also known as Siglo XX Mine) in Llallagua, Bolivia. This makes it a very rare and special mineral!
See also
In Spanish: Vauxita para niños
