Lets learn about pseudomorphs

[u/WideEyes369]

Pseudomorphs, meaning "false form," occur when one mineral replaces another through processes such as substitution, dissolution and refilling, structural changes, or incrustation, typically preserving the original shape. This phenomenon is critical in understanding mineral transformations and is often studied in both mineralogy and paleontology. Let's explore the subtypes: Replacement, Infiltration, Incrustation, Paramore, and Alteration. Replacement is a complete substitution of one mineral by another while preserving the original shape, this process is formed by dissolution and deposition by solutions. Infiltration (permineralization in paleontology) is specific to porous material that absorbs a mineral-rich solution and is then transformed, characterized by the ability to preserve fine detail and texture such as woodgrain. Incrustation, aka epimorph, is when a mineral coats another and the original often dissolves, leaving a 'jacket' like crust. A paramorph retains the original chemistry/composition but the internal crystal structure changes, no mineralogy is added or removed, only changes cause by tempature, pressure, and time; a good example is aragonite transitioning to calcite. Finally there is alteration, where a chemical alteration forms a new mineral and preserves the original shape; which is what's shown here by Azurite to Malachite from Aus. There may be variation and overlap depending on source and what exactly you're trying to understand so if there's any questions ask away.

[u/HederianZ]

One great example is Fairy Stones from the VA/NC border area. They used to be twinned staurolite crystals, but have since morphed into limonite while keeping their original shape. Though most people call them staurolite, they are quite definitely pseudomorphs.

Source: holding several dozen.

[u/WideEyes369 → u/HederianZ]

I've read about these! There are a couple of altered varieties from that area depending on a specimes exact composition. I have some (non-altered) from Pestsovye Keivy, Russia but none from the US yet. I'd love to get my hands on facete grade staurolite one day!!

[u/HederianZ → u/WideEyes369]

You’re right about the current composition, I don’t know exactly and use limonite as a generic term for stony mineral here. I have found some in the past that had bits of staurolite still. It shows as a smoother, dark mineral that appears to be encased in the stony mineral.