Difference Between Dioctahedral and Trioctahedral

The key difference between dioctahedral and trioctahedral is that dioctahedral refers to having two of the three available octahedrally coordinated positions occupied, whereas trioctahedral refers to having all three available octahedrally coordinated positions occupied.

The terms dioctahedral and trioctahedral are adjectives that describe the number of occupied positions in an octahedral structure. We can find these terms described under phyllosilicates, where the structure of sheet silicates is studied.

CONTENTS

1. Overview and Key Difference
2. What is Dioctahedral 
3. What is Trioctahedral 
4. Side by Side Comparison – Dioctahedral vs Trioctahedral in Tabular Form
5. Summary

What is Dioctahedral?

Dioctahedral means having two of the three available octahedrally coordinated positions occupied. This structure is discussed under subtopic phyllosilicates, where there are sheet silicate structures. These sheet silicates are a specific group of minerals, including mica, chlorite, serpentine, talc, etc. These minerals form as a result of chemical weathering, and these are more abundant constituents of sedimentary rocks.

Concerning the basic structure of a sheet silicate mineral, it has interconnected six-membered rings of SiO₄^-4 tetrahedra. These tetrahedra tend to extend outward in infinite sheets. Out of these, four oxygen atoms are in the tetrahedra, and three oxygen atoms are shared with other tetrahedra, forming a network structure. This sharing of oxygen atoms leads to the structure Si₂O₅^-2.

When considering the formation of the octahedral structure, phyllosilicates usually contain a hydroxyl ion where the OH group occurs at the center of a six-membered ring. Therefore, the chemical formula of this hydroxyl containing group is Si₂O₅(OH)^-3.  When a cation bonds with this silicate sheet, it binds with the OH group and forms octahedral coordination. Therefore, a layer of cations can form (typically with ferrous ions, magnesium ions and aluminum ions), and the cations are in octahedral coordination with oxygen atoms and hydroxyl ions of the tetrahedral layer. If the metal cation bonded to this structure is magnesium or ferrous ion, then the octahedral structure is brucite, and if the metal ion is aluminum, then the structure is gibbsite. In brucite structure, all octahedral sites are occupied, and in gibbsite structure, the 3^rd cation site is unoccupied, which leads to the two structures trioctahedral and dioctahedral structure, respectively. In the dioctahedral structure, each oxygen atom or hydroxyl group is surrounded by 2 trivalent cations, which are usually aluminum cations.

What is Trioctahedral?

Trioctahedral means having all three available octahedrally coordinated positions occupied. This structure is observed mainly in brucite minerals, where sheet silicate structures contain each oxygen atom or hydroxyl ion surrounded by 3 divalent cations such as magnesium ion or ferrous ion.

The formation of an octahedral structure in phyllosilicates is described above under the subtopic dioctahedral.

What is the Difference Between Dioctahedral and Trioctahedral?

We can find the terms dioctahedral and trioctahedral described under phyllosilicates, where the structure of sheet silicates is studied. The key difference between dioctahedral and trioctahedral is that dioctahedral refers to having two of the three available octahedrally coordinated positions occupied whereas trioctahedral refers to having all three available octahedrally coordinated positions occupied.

Below is a summary of the difference between dioctahedral and trioctahedral in tabular form.

Summary – Dioctahedral vs Trioctahedral

The terms dioctahedral and trioctahedral are adjectives that describe the number of occupied positions in an octahedral structure. The key difference between dioctahedral and trioctahedral is that dioctahedral refers to having two of the three available octahedrally coordinated positions occupied, whereas trioctahedral refers to having all three available octahedrally coordinated positions occupied.

Reference:

1. “Trioctahedral Structure.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., Available here.

Image Courtesy:

1. “Gibbsite-fiu19d” By Rob Lavinsky, iRocks.com – (CC BY-SA 3.0) via Commons Wikimedia
2. “Brucite-231242” By Rob Lavinsky, iRocks.com – (CC BY-SA 3.0) via Commons Wikimedia