Primary & Secondary Minerals

Primary minerals 

Primary minerals crystallize at high temperatures when magma cools to form igneous rocks like granite or basalt.

Secondary minerals form as primary minerals from these rocks decompose or are altered, for example, by chemical weathering near the Earth's surface. Soil contains a mixture of primary and secondary minerals.

Primary minerals differ in their resistance to weathering. Which minerals are the most (and which are least) stable in soil environments?

The least stable minerals are those that crystallized first from the magma, i.e., those which crystallized at the highest temperatures. 

Quartz versus the rest

The least weatherable primary minerals in soils are the same as those in beach sand.

Before being washed up on a beach, sand grains have survived weathering, erosion, transport, and deposition without decomposing.

The dominant mineral in most beaches is quartz. Quartz, and a few less abundant minerals like rutile and zircon, are so persistent that, for our purposes, they can be regarded as non-weatherable.

Quartz is also the most common mineral in the sand and silt fractions of soil.

Clay Minerals

Clay minerals are the most abundant secondary minerals in soils. They have small crystal sizes and are found in the clay particle size fraction (<0.002 mm). Clay minerals containing silicon are called silicate clay minerals. Non-silicate clay minerals include iron and aluminum oxide minerals (called sesquioxides).

Because clay particles are small, their surface area is large. Many soil chemical reactions take place at clay surfaces. In contrast, the quartz-dominated sand and silt fractions are chemically quite inert.

Clay mineral crystals are stacks of plate-like crystal units called layers.