Difference Between Quartz and Feldspar

The key difference between quartz and feldspar is that the major chemical element present in quartz is silicon whereas, in feldspar, it is aluminum.

Quartz and Feldspar are minerals that we can find prominently in the earth’s crust. More than 60% of the earth’s crust consists of Feldspars. Feldspar forms when magma solidifies into igneous rocks. On the other hand, quartz is an oxide of silicon that is abundant on the earth’s crust. Some of the varieties of quartz are “semiprecious gemstones”. Because of their structural similarities, many people remain confused between these two mineral forms. Therefore, this article attempts to highlight the difference between quartz and feldspar, which are two most important rock-forming minerals.

CONTENTS

1. Overview and Key Difference
2. What is Quartz
3. What is Feldspar
4. Side by Side Comparison – Quartz vs Feldspar in Tabular Form
5. Summary

What is Quartz?

Quartz is a mineral compound containing silicon and oxygen atoms. It contains silicon dioxide (SiO2) molecules. Moreover, it is the most abundant mineral on Earth’s crust. Though it contains SiO2, the repeating unit of this mineral is SiO4. It is because, in the chemical structure of quartz, it contains one silicon atom bonded to four oxygen atoms surrounding it. Hence, the geometry around on silicon atom is tetrahedral. However, between two tetrahedral structures, one oxygen atom is shared. Therefore, the crystal system of the mineral is hexagonal.

Furthermore, quartz crystals are chiral. That means; the quartz exists in two forms as the normal α-quartz and the high-temperature β-quartz. The alpha form can transform into the beta form at around 573 °C. Looking at their appearances, some quartz types are colourless and transparent while other forms are colourful and translucent. The most common colours of this mineral are white, grey, purple and yellow.

What is Feldspar?

Feldspar is a mineral compound containing mainly aluminum, silicon and oxygen atoms. It has alumina and silica units in combination with each other. We can categorize it as a silicate mineral because, as in quartz, it has SiO2 units. Feldspars form when magma solidifies into igneous rocks. However, it also occurs in many metamorphic and sedimentary rocks. Further, we can give the chemical formula of this mineral as KAlSi₃O₈ – NaAlSi₃O₈ – CaAl₂Si₂O₈. It is the repeating unit of this mineral.

The mineral streak of feldspar is white, and the common crystal systems are triclinic and monoclinic structures. It is available in different colours such as pink, white, grey and brown. Moreover, the lustre of this mineral is vitreous. According to the composition, there are three types of minerals as potassium feldspar, albite or sodium feldspar and anorthite or calcium feldspar. Furthermore, the mineral weathering of this material forms clay minerals such as kaolinite.

What is the Difference Between Quartz and Feldspar?

Quartz is a mineral compound containing silicon and oxygen atoms, and feldspar is a mineral compound containing mainly aluminum, silicon, and oxygen atoms. Therefore, the key difference between quartz and feldspar is that the major chemical element present in quartz is silicon whereas in feldspar it is aluminum. As another important difference between quartz and feldspar we can say that the repeating unit of quartz is SiO₄ while the repeating unit of feldspar is KAlSi₃O₈ – NaAlSi₃O₈ – CaAl₂Si₂O₈.

Furthermore, quartz is harder than feldspar. Also, we can identify a difference between quartz and feldspar from the colors. That is; quartz mostly appears in light colors whereas feldspar can be found in a variety of colors, even dark-colored like brown and purple because of the presence of impurities. More differences are shown in the below infographic on difference between quartz and feldspar.

Summary – Quartz vs Feldspar

Quartz and feldspar are rock-forming minerals. Both these contains silicon and oxygen atoms. However, their compositions are different from each other. Therefore, the key difference between quartz and feldspar is that the major chemical element present in quartz is silicon whereas in feldspar it is aluminum.

Reference:

1. “Feldspar.” Minerals Education Coalition. Available here 
2. “Feldspar.” Wikipedia, Wikimedia Foundation, 8 Nov. 2018. Available here  

Image Courtesy:

1.”Quartz Brésil”By Didier Descouens – Own work, (CC BY-SA 4.0) via Commons Wikimedia  
2.”Feldspar-Group-291254″By Rob Lavinsky, iRocks.com (CC BY-SA 3.0) via Commons Wikimedia