Formation Mechanisms for Mineral Replacement

For hetero-oriented replacement, dissolution–precipitation is metasomatic mechanism. Two indispensable conditions are required: (1) a mineral prone to be replaced on one side; (2) a same or similar mineral as replacive one on the other side, acting as nucleation center for its growth. An impurity in the rock could serve as a crystal nucleus only when replacement should occur while identical or similar mineral is absent aside. As to co-orientated replacement, for sheet alumosilicate minerals, ion exchange may be the metasomatic mechanism. For frame-alumosilicate and non-silicate minerals, the metasomatic mechanism has not been conclusively identified so far. Micropores developed in replacive mineral may be an important function causing the co-oriented replacement. Hetero-oriented albitization of K-feldspar is quite distinct from co-oriented albitization of K-feldspar. Both of them occurred separately without transition. Their formation conditions must differ from each other.