# Minerals/Aluminides

The aluminides are those naturally occurring minerals with a high atomic % aluminum.
In the image on the right of a flake of native aluminum, the scale bar = 1 mm.
"Aluminium is the third most abundant element (after oxygen and silicon) in the Earth's crust, and the most abundant metal there. It makes up about 8% by mass of the crust, though it is less common in the mantle below."

## Native aluminums

The image at the top of this resource is one of two images exhibiting native aluminum.
This flake was discovered, "During a field trip to the NW Rila Mountain in the early 1960s, one of us (V.A.) investigated the desilicated pegmatite apophysis and, from the phlogopite zone (Fig. 1c), collected a rock specimen with a protruding metallic flake visible to the naked eye (Fig. 2) [from which the above image was cropped]."
The designation for native aluminum is Al$^0$ as indicated in, "Here we present data for a unique Al$^0$ flake protruding from the phlogopite matrix of a rock specimen collected from a desilicated pegmatite vein."
The second image of native aluminum is shown on the right of this section. The sample is from a mud volcano in the Caspian Sea near Baku, Azerbaidzhan.
The type locality for native aluminum is the Tolbachik volcano, Kamchatka, Russia.
Native aluminium metal is extremely rare and can only be found as a minor phase in low oxygen fugacity environments, such as the interiors of certain volcanoes. Native aluminium has been reported in cold seeps in the northeastern continental slope of the South China Sea, where these deposits may have resulted from bacterial reduction of tetrahydroxoaluminate Al(OH)$_4$$^−$.
Overall, the Earth is about 1.59% aluminium by mass (seventh in abundance by mass). Aluminium occurs in greater proportion in the Earth's crust than in the Universe at large, because aluminium easily forms the oxide and becomes bound into rocks and stays in the Earth's crust, while less reactive metals sink to the core. In the Earth's crust, aluminium is the most abundant metallic element (8.23% by mass) and the third most abundant of all elements (after oxygen and silicon). A large number of silicates in the Earth's crust contain aluminium. In contrast, the Earth's mantle is only 2.38% aluminium by mass. Aluminium also occurs in seawater at a concentration of 2 μg/kg.
Aluminum occurs as aluminosilicates in feldspars, the most common group of minerals in the Earth's crust, in the minerals beryl, cryolite, garnet, spinel, and turquoise. Impurities in Al$_2$O$_3$, such as chromium and iron, yield the gemstones ruby and sapphire, respectively.

## Bayerites

Bayerite is a polymorph of gibbsite and has the same chemical formula. Part of the challenge of determining the unique structure of bayerite versus gibbsite is that bayerite appears to transform to gibbsite under certain circumstances. The structures may also interleave. The other two polymorphs: nordstrandite and doyleite, are also variations with possible interleaving.
Early structural determinations using powder diffraction studies appeared contradicting.
The first structural study (1942) found bayerite to be hexagonal with two formula units (Z) per unit cell. The lattice parameters were a=5.01 Å and c=4.76 Å.
A second study (1951) found bayerite to be monoclinic.
A third study (1958) found bayerite to be hexagonal with a=5.047 Å and c=4.730 Å with Z=2.
Bayerite has a monoclinic structure and lattice parameters of a=5.062(1) Å, b=8.671(2) Å, c=4.713(1) Å, β =90.27(3)° with space group P2$_1$/a.
As of 2014, bayerite has the monoclinic (P2$_1$/m) (or brucite) structure.
In the diagram on the right, an idealized "structure of gibbsite is projected on (001), showing geometric relations of the cells of gibbsite (solid lines), bayerite (dashed lines), doyleite (dotted lines) and nordstrandite. [Subscripts are] g for gibbsite, d for doyleite and n for nordstrandite. The oxygen atoms are at heights of 0.11 (shaded large circles) and -0.11 (unshaded)."

## Corundums

Corundum is α-Al$_2$O$_3$. It has Z = 6 formula units per hexagonal unit cell.
Corundum is 40 at % aluminum.

## Doyleites

Doyleite "the mineral from Mont St. Hilaire [is] triclinic, space group P1̅ from morphology, a 5.002(1), b 5.175(1), c 4.980(2) , α 97.50(1), β 118.60(1), γ 104.74(1)°, Z = 2."

## Gibbsites

Gibbsite, Al(OH) $_3$, is one of the mineral forms of aluminium hydroxide, often designated as γ-Al(OH) $_3$ (but sometimes as α-Al(OH)$_3$.). It is also sometimes called hydrargillite (or hydrargyllite). Doyleite and nordstrandite are triclinic forms.
The structure of gibbsite is analogous to the basic structure of the micas. The basic structure forms stacked sheets of linked octahedra. Each octahedron is composed of an aluminium ion bonded to six hydroxide groups, and each hydroxide group is shared by two aluminium octahedra.
Gibbsite is often found as a part of the structure of other minerals. The neutral aluminium hydroxide sheets are found sandwiched between silicate sheets in important clay groups: the illite, kaolinite, and montmorillonite/smectite groups. The individual aluminium hydroxide layers are identical to the individual layers of gibbsite and are referred to as the gibbsite layers.
Gibbsites have the chemical formula Al(OH)$_3$ with eight formula units per monoclinic unit cell.
Gibbsites have only about 14.3 at % aluminum.

## Nordstrandites

"The primitive P1̅ unit cell of nordstrandite was confirmed to contain four formula units, unlike doyleite (Z = 2). The layered structures of nordstrandite and doyleite were shown to be closely related to that of bayerite, differing from one another by the interlayer shift vectors only."

## Hypotheses

* In the case of native elements, proof of concept often consists of an actual specimen found in a natural setting with a composition that is similar to its setting rather than man-made artifacts of the same element.
* Native minerals can result from subsequent environmental modifications of mining tailings.