They are characterised by a concentration of crystals, visible under the petrographic microscope. They can be subdivided into the following types:
Typic. Composed of "normal"
crystals, without evidence of pseudomorphism or biological origin.
Pseudomorphic. The crystals have shapes that have been imposed by other previously existing materials. They come from the mineralization of organic residues or they inherit the shape of other minerals.
Bio. They have a biological origin, but without conserving traces of biological structures. They are the result of biological activity, but not biological remains.
Impregnative. Compound feature, formed of the crystalline material that impregnates other soil material, for example the soil matrix.
The terminology proposed by Friedman (1965) is used to describe their internal fabric.
Equigranular. Where the constituent crystals are the same size. Depending on their shape, the following subtypes can be distinguished:
Xenotopic. Formed by anhedral crystals (crystals without a defined external shape)
Idiotopic. Formed by euhedral crystals (crystals with a defined external shape)
Hypidiotopic. Formed by subhedral crystals
Inequigranular. Where the constituent
crystals vary in size. Depending on the relations between the different
grain sizes, the following subtypes can be distinguished:
Porphyrotopic. Formed by large crystals embedded in a mass of smaller crystals
Xylotopic. Formed by large
crystals that include other smaller ones
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