What happens to feldspar during chemical weathering?

Hydrolysis. Feldspar is formed in underground geological temperature and pressure regimes. When this happens, it is chemically weathered by hydrolysis. This is the reaction between a water molecule and an ion in the feldspar that releases a hydrogen molecule, which becomes attached to a separate product.

Does plagioclase feldspar have twinning?

In addition to simple twins, plagioclase feldspar often shows repeated or multiple twinning (Figures 57c and 59a) usually on a microscopic scale, which gives crystals a striped appearance (Figure 59c). This is often referred to as lamellar twinning and is a characteristic feature of plagioclase feldspars.

What is the chemical formula for plagioclase feldspar?

What happens when water is added to feldspar?

An example of hydrolysis is when water reacts with potassium feldspar to produce clay minerals and ions. On the recently broken unweathered surface (Figure 8.11, left) feldspar is visible as bright white crystals. On a weathered surface (right) the feldspar has been altered to the chalky-looking clay mineral kaolinite.

Does plagioclase react to acid?

For example, calcite (CaCO3) is soluble in acidic solutions. This reaction shows calcium plagioclase feldspar, but similar reactions could also be written for sodium or potassium feldspars. Those ions can eventually combine (probably in the ocean) to form the mineral calcite.

When feldspar is broken down via chemical weathering what are the resulting weathering products?

There are two main types of chemical weathering. On the one hand, some minerals become altered to other minerals. For example, feldspar is altered — by hydrolysis — to clay minerals.

Does plagioclase have twinning?

Plagioclase commonly shows this type of twinning, called the Albite Twin Law, with {010} as the twin plane. Such twinning is one of the most diagnostic features of plagioclase.

How and why the plagioclase feldspar in basaltic rocks differs from that in granitic rocks?

All feldspar minerals have two directions of perfect cleavage. These properties make plagioclase feldspars relatively easy to identify with a hand lens in coarse-grained igneous and metamorphic rocks. Plagioclase in granitic rocks is normally white, pink, or red in color. In basaltic rocks it is normally gray to black.

Does plagioclase feldspar react to acid?

Does K feldspar react to acid?

In the breakdown of K-feldspar, acidity is consumed. We can see that H+ is used up and K+ is released to solution, a kind of “ion exchange” reaction. Also released to solution is dissolved silica or silicic acid, written H4SiO4.

What are the two categories of plagioclase feldspars?

divided into 2 broad categories: plagioclase, which contains calcium and sodium; and orthoclase, which contains potassium. The plagioclase feldspars represent the “continuous branch” of Bowen’s Reaction Series, and form a complete series between anorthite (the pure calcium

What are the striations on a feldspar mineral?

microscopic) parallel lines cut into the face of a mineral fragment. All feldspars which have striations are plagioclase, but not all plagioclase has striations. Only orthoclase can be pink or blue, and only plagioclase can be dark gray

What do the striations on plagioclase look like?

Striations are often visible on plagioclase, and resemble very fine (almost microscopic) parallel lines cut into the face of a mineral fragment. All feldspars which have striations are plagioclase, but not all plagioclase has striations. Only orthoclase can be pink or blue, and only plagioclase can be dark gray to black.

What kind of plagioclase is found in maficand intermediate rocks?

In general terms, maficand intermediate rocks contain plagioclase, with the more calcium present, the more mafic the resulting rock. Orthoclase occurs only in the felsicigneous rocks. So if we’re faced with the need to name an igneous rock, it is important to be able to distinguish between plagioclase and orthoclase.