8. Compare and contrast the cleavage in the pyroxene and amphibole mineral group

Question

8. Compare and contrast the cleavage in the pyroxene and amphibole mineral groups, in detail. 9. Explain the role of Ca in controlling the symmetry of the single- and double-chain inosilicates 10. Al,O, is the second most common oxide in most magmas, after SiO,. Discuss the role of Your answer should include a discussion of the silicate chains in each group. aluminum in the silicate minerals, including the type of sites it can occupy and how it's existence in certain sites can affect the overall chemistry of a mineral.

Explanation / Answer

[3][Amphibole is important group of generally dark-colored, inosilicate minerals, forming prism or needlelike crystals,composed of double chain SiO
4tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Amphiboles can be green, black, colorless, white. Amphiboles crystallize into two crystal systems, monoclinic and orthorhombic. The chief differences from pyroxenes are that (i) amphiboles contain essential hydroxyl (OH) or halogen (F, Cl) and (ii) the basic structure is a double chain of tetrahedra.proxemes are single chain silicates but amphibole is

double chain silicate.

[2] at B site it define m4 geometry of silicates structures.

[3]Aluminum partitions preferentially into the unit with the smaller angle at low Al/(Al + Si) (less than ~0.3), and into the unit with the larger angle at higher concentrations.There is a decrease in their frequencies with increasing Al/(Al + Si) of the glasses and melts in the Al/(A1 + Si) range investigated (0–0.5).In phyllosulicates aluminium occur in octahedral coordination The dioctahedral phyllosilicates are based on the structure where the octahedral layers are similar to gibbsite, where Al+3 occupies the cation position.3 is substituted for every 4th Si+4 in the tetrahedral layer, this causes an excess -1 charge in each T-O-T layer. Al3+ substitutes for Si4+ in the tetrahedra, which gives the framework a net negative charge and so requires other cations in the structure to balance the charge. The Al – Si substitution is a very important phenomenon in minerals.

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