Albite: A Petrological Puzzle – Should it be Placed on the Alkali Feldspar or Plagioclase Side of QAPF Diagrams?

The QAPF (Quartz, Alkali feldspar, Plagioclase, and Feldspathoid) diagram, also known as the Steckeisen diagram, is a widely used classification system for igneous rocks. It is based on the mineralogy of the rocks and divides them into four major groups based on their mineral content. The diagram is a useful tool for petrologists, allowing them to quickly identify the type of rock and its characteristics. However, there is an ongoing debate as to whether albite, a common feldspar mineral, should be placed on the alkali feldspar or plagioclase side of the diagram. In this article we will explore this issue in detail.

Albite: A brief overview

Albite is a plagioclase feldspar mineral with the chemical formula NaAlSi3O8. It is a common mineral in igneous and metamorphic rocks and is commonly found in granites, syenites and diorites. Albite is a member of the plagioclase series of minerals, which includes a range of feldspars with varying amounts of sodium and calcium. Albite has a high sodium content, which distinguishes it from the other plagioclase minerals.

The debate: Alkali feldspar or plagioclase?

The placement of albite on the QAPF diagram has been a topic of debate among petrologists for many years. Traditionally, albite has been placed on the plagioclase side of the diagram with the other plagioclase minerals. This is because albite is a member of the plagioclase series and shares many of the same characteristics as the other plagioclase minerals, such as cleavage and twinning.

However, some petrologists argue that albite should be placed on the alkali feldspar side of the diagram. This is because albite is a sodium-rich feldspar, and the alkali feldspar minerals (orthoclase, microcline, and sanidine) are also sodium-rich. In addition, albite can form solid solutions with the other alkali feldspar minerals, making it more similar to them than to the other plagioclase minerals.

The in-between option

Another option for placing albite on the QAPF diagram is to place it in between the alkali feldspar and plagioclase sides. This would reflect the fact that albite shares characteristics with both groups of minerals. However, this option has not been widely adopted due to the difficulty of accurately placing albite on the diagram and the lack of consensus among petrologists.

Conclusion

In conclusion, the placement of albite on the QAPF diagram remains a topic of debate among petrologists. While traditionally placed on the plagioclase side, some argue that its similarities to the alkali feldspar minerals make it better suited for the alkali feldspar side. Others suggest an in-between position. Ultimately, the placement of albite on the diagram may depend on the specific rock type and the goals of the analysis. Regardless of its placement, it is important for petrologists to carefully consider the mineralogy of their samples and the implications of their interpretations. The ongoing debate highlights the complexity and nuance of petrology and the importance of continued research and discussion in the field.

FAQs

1. What is the QAPF diagram?

The QAPF (Quartz, Alkali feldspar, Plagioclase, and Feldspathoid) diagram is a classification system used by petrologists to identify and classify igneous rocks based on their mineral content.

2. What is albite?

Albite is a plagioclase feldspar mineral with the chemical formula NaAlSi₃O₈. It is a common mineral in igneous and metamorphic rocks and is often found in granites, syenites, and diorites.

3. Why is there a debate about the placement of albite on the QAPF diagram?

There is a debate about the placement of albite on the QAPF diagram because it shares characteristics with both the alkali feldspar and plagioclase groups of minerals, making it difficult to determine which side of the diagram it belongs on.

4. What is the traditional placement of albite on the QAPF diagram?

The traditional placement of albite on the QAPF diagram is on the plagioclase side, along with the other plagioclase minerals. This is because albite is a member of the plagioclase series and sharesmany of the same characteristics as the other plagioclase minerals.

5. What is the argument for placing albite on the alkali feldspar side of the QAPF diagram?

Some petrologists argue that albite should be placed on the alkali feldspar side of the QAPF diagram because it is a sodium-rich feldspar, like the alkali feldspar minerals, and can form solid solutions with them.

6. Is there an in-between option for placing albite on the QAPF diagram?

Yes, some petrologists suggest an in-between option for placing albite on the QAPF diagram to reflect its similarities to both the alkali feldspar and plagioclase groups of minerals. However, this option has not been widely adopted.

7. Why is the placement of albite on the QAPF diagram important?

The placement of albite on the QAPF diagram is important because it can affect the interpretation of the rock type and its characteristics. Depending on its placement, the rock may be classified differently and have different implications for its formation and history.