Replacement reactions (reaction rim/ corona)

In metamorphic petrology the reactions generally involves the replacement of minerals by their polymorphs and often the reactions may not complete.

In many reactions, a pseudomorph may develop in which the new mineral retain the older minerals shape. “Some reactions produce intimate, typically wormy-looking intergrowths of two or more minerals called symplectite texture. Pseudomorphs may thus by monomineralic or a symplectitic intergrowth.

• In ultramafic rocks the olivine are typically replaced by serpentine while retaining the central part unaltered creating a vein like mesh patter. If orthopyroxene present it is usually completely replaced by pseudomorphs of fibrous serpentine called ‘bastite’.

REACTION RIMS/ CORONA

• Reaction rim, as the name suggests it involves reaction between the two minerals where reaction is not completed and leads to either replacement of one or both minerals.
• Thus reaction rims are frozen records of metamorphic reactions.
• The reaction rim reflects retrograde alteration, polymorphic transformation, diffusion along grain boundaries, solid-state reactions.

Let’s see in the image below that there are two minerals A and B reacts to form either one mineral C (monomineralic reaction rim) or two minerals C and D (polymineralic reaction rim). If the diffusion is limited the reaction products are limited to a zone.

If the reaction is enough to create a complete rim around a mineral it is called ‘corona’. Once the corona or rim is developed the reaction can only continue through the diffusion Sometimes a monomineralic coronas are also called as moats. Again if it is polymineralic intergrowth are called as ‘symplectite corona’.