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Geology is an intriguing and complex field of study, and rocks and minerals are just a fraction of it. When rockhounding, you may run into the occasional sample that may be difficult to identify, especially with specimens as similar to those in the feldspar category and quartz. But how do you tell the difference?
The most significant difference between feldspar and quartz is that feldspar is a group of minerals, and quartz is an individual mineral. When comparing specific specimens in the feldspar group to quartz, the differences found have to do with:
- Gemstone Classification
Quartz is the most common mineral on earth, and the minerals in the feldspar group make up almost 60% of the earth’s crust. Both quartz and feldspar are classified as silicates since their chemical composition includes silicon. In addition, both mineral types are often found in the same rocks, but despite many similarities, there are simple ways to tell the difference between them.
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Classifying Feldspar & Quartz
Before reviewing the differences between feldspar and quartz, it’s essential to understand the similarities that often have rockhounds confusing the two with one another: that being that they are both classified as minerals.
For a material to be classified as a mineral, it must meet five requirements:
- Naturally Occurring
- Definite Chemical Composition
- Ordered Internal Structure
Both quartz and feldspar are classified as minerals due to their relevant characteristics:
- They are naturally occurring molecules.
- Neither quartz nor feldspar contains both carbon and hydrogen.
- They are neither a gas nor a liquid.
- They have a specific chemical composition.
- The atomic structure of quartz is SiO4; there is one silicon atom and four oxygen atoms. When those molecules bond, they create an ordered internal structure.
However, even though feldspar and quartz are both minerals, they do have their own distinct features.
Feldspar is a group of minerals with extremely similar structures, chemical composition, and physical properties. Since they are so similar, they are collectively known as “feldspar.”
Combined, all the feldspar minerals make up almost 60% of the earth’s crust; it is most commonly found in igneous and metamorphic rocks, and occasionally in sedimentary rocks.
When looking at rocks, being able to identify that there is feldspar in it is more important than identifying the precise feldspar mineral.
To do that, it’s worth knowing that feldspar mineral is made up of aluminum silicate with some combination of the following:
Minerals Classified as Feldspar
In total, there are around 26 minerals that are considered feldspar. Within the broader feldspar classification, there are also two main subgroups:
- Plagioclase Feldspar
Contains calcium and sodium. All the minerals in this group are a combination of albite and anorthite:
- Albite (90-100% Ab and 0-10% An)
- Andesine (50-70% Ab and 30-50% An)
- Anorthite (0-10% Ab and 90-100% An)
- Bytownite (10-130% Ab and 70-90% An)
- Labradorite (30-50% Ab and 50-70 % An)
- Oligoclase (70-90% Ab and 10-30% An)
- Potassium Feldspar (alkali feldspar, k-spar, potash feldspar, potspar)
Contains potassium. These minerals are formed from the same compounds but have different crystal structures. The minerals in this group are:
The difference between plagioclase feldspar and potassium feldspar is that plagioclase feldspar contains no potassium, and when broken, striations can be seen on one cleavage (more on this later). Although there are differences between plagioclase and potassium feldspar, they are similar enough that they are often grouped together.
The color of feldspar depends on the exact chemical compounds, making it up. Feldspars most often are white but can also be:
- Red Black
Each specific feldspar mineral can vary in color, so there is no single color that feldspar always is.
Most feldspar minerals are not appreciated for their appearance. On occasion, under the right circumstances, the mineral form is gorgeous, and its appearance and rarity make it a gemstone. Feldspar mineral gemstones include:
Uses of Feldspar
Due to its abundance, feldspar is a highly used mineral. It is typically crushed into a powder and used in factories to create:
- Ceramics and Pottery
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- The Crystal Bible
- Smithsonian Handbooks: Rocks & Minerals
- Gemstone & Crystal Properties (Quick Study Home)
- Ultimate Explorer Field Guide: Rocks and Minerals (National Geographic Kids)
Quartz is the most abundant individual mineral on earth; it can be found in igneous, metamorphic, and sedimentary rocks.
Varieties of Quartz
There are many variations of quartz, but all variations have the same chemical formula. What makes them unique has to do with the conditions in which they form.
For example, some quartz crystals will have other compounds trapped inside; the trapped compounds are called impurities.
The impurities, along with formation conditions, will change the color and opaqueness of the quartz.
Well known varieties of quartz include:
- Milky Quartz
- Rose Quartz
- Smokey Quartz
Many gemstones are varieties of quartz. Certain quartz-based gems are rare because the impurities or conditions which make it are less likely to occur. Nowadays, many gems can be created in a lab.
Uses of Quartz
Like feldspar, due to its abundance, quartz is commonly used for manufacturing. Its strength and heat-resistant properties make it useful in:
- Hydraulic Fracturing
- Time Keeping
TIP: Are you interested in how the quartz is formed? This is a very interesting process that you can read more about in the article below:
What’s the Difference Between Feldspar & Quartz?
As you can see, feldspar and quartz have a few similarities, including:
- Being Silicates
- Often Found in the Same Rocks
- Overlapping in Color
But, the similarities are only limited to these three characteristics.
The most significant difference between feldspar and quartz is that a feldspar is a group of minerals, and quartz is one specific mineral. However, they also differ in the following ways:
- Gemstone Classification
Labradorite, a plagioclase feldspar, has the chemical formula ((Ca, Na)(Al, Si)4O8) while orthoclase, a potassium feldspar, has the chemical formula KAlSi3O8.
The minerals in the feldspar group are individual minerals that just happen to all be incredibly similar—so similar, that people lump them together.
The varieties of quartz have the same structure, but what causes variations has to do with the formation process, including the temperature, and other chemical compounds present. Compounds such as iron get trapped inside the crystal structure as it forms.
It’s as though the crystal structure is cement. If you stick pieces of glass into the cement, the cement doesn’t change; it becomes cement with glass (also called a mosaic).
One of the fundamental ways to identify a mineral is by how hard it is. Since two minerals can look almost identical, tricks like testing their hardness can help you figure out what they are.
The Mohs Hardness scale is based on ten unique minerals, each with different hardness. If you rub one mineral on another and it leaves a scratch, you know the mineral doing the scratching is harder than the mineral receiving the scratching. The minerals in order of hardness on the Mohs Hardness Scale are:
Talc is the softest mineral, so it can be scratched by everything higher on the list. Diamond is the hardest; no other mineral can scratch a diamond.
On the list are quartz and orthoclase. (Orthoclase is one of the most abundant types of feldspar.) By looking at the scale, it is clear that quartz with a hardness of 7 is harder than the orthoclase feldspar with a hardness of 6.
Since there are multiple types of feldspar, the hardness does vary, but the range is small, between 6 and 6.5.
If you had a sample of feldspar and a sample of quartz that looked similar, you could use the scratch test to determine which one was which: quartz will be the one that would scratch feldspar.
Minerals are crystalline; the atoms in crystalline structure bond in a specific pattern that is repeated no matter how many atoms there are. A quartz crystal that is large will have the same arrangement of atoms as a small quartz crystal; there are simply more atoms in the large crystal.
The crystal structure of an atom describes the geometry and atomic arrangement. The crystal system is just geometry. There are seven different crystal systems:
Each system differs in:
- The number of axes it has
- The length of the axes
- The angle at which the axes meet
Since feldspar and quartz have different atomic makeups, their crystal systems and structures are different. Minerals in the feldspar group have a crystal structure that is triclinic or monoclinic, and quartz has a hexagonal crystal structure.
When minerals form, they always take on the same shape, their crystal structure. If a mineral is smashed, it will break apart.
Some minerals will shatter into pieces of different shapes and sizes, while other minerals break in a specific way. How this takes place is referred to as a mineral’s “cleavage.”
Feldspar and quartz have different cleavages. When you hit a quartz sample with a geologist’s hammer, it shatters into pieces that all look different.
Quartz has a cleavage that is indiscernible. There is no way to know what the broken pieces will look like. In contrast, when you hit feldspar, it will always break the same way: feldspar has two perfect cleavages that meet at a 90-degree angle.
In other words, even if you start with a large specimen, and continue breaking it into smaller and smaller chunks, the shape of the pieces will always look similar.
When looking at minerals, never rely on looks alone. A mineral can have many variations depending on the conditions in which it forms.
The stereotypical quartz is colorless, but not all quartz is colorless, and many other minerals are colorless. If you just look at the appearance, you can easily mistake one mineral for another.
With feldspar and quartz, there is plenty of overlap between colors. Since they are both highly abundant crystals, there are plenty of variations. In many cases, though not always, feldspar is cloudy in appearance, while quartz is clear.
If you held up a piece of quartz to a light source, the light would shine through. If you do the same with feldspar, much less light would be seen.
A mineral can qualify as a gemstone if it has a color or pattern that people find attractive. Some minerals are considered semi-precious gems; this is because they are more common.
Both feldspar and quartz can form gemstones, but of 200+ recognized gemstones, at least twelve are varieties of quartz while only four are feldspar.
Part of what makes quartz varieties more likely to be considered gemstones than feldspar has to do with its hardness. While soft minerals can be beautiful, if they scratch or break easily, they become less valuable.
TIP: Soft minerals are usually less valuable. But even soft rocks and minerals can be really valuable. These 6 signs help you to recognize valuable rocks and minerals, read about them in the article below:
What Rocks Contain Quartz and Feldspar?
Our earth has what is called the rock cycle, and there are three types of rock:
- Igneous Rocks
Inside the earth is magma, basically liquid rock. When that magma breaks through the earth’s crust, it is called lava. When magma or lava cools, it hardens into rock. There are two types of igneous rocks:
- Intrusive – This type of igneous rock forms when magma cools inside the earth’s crust. Cooling happens slowly and allows large crystals to form.
- Extrusive – This igneous rock forms from lava outside the earth’s crust that cools. Cooling happens quickly, so crystals have less time to form and are small.
Igneous rocks can be transformed into metamorphic rock or erode and form sedimentary rock.
- Metamorphic Rocks
When an igneous or sedimentary rock is subjected to heat and pressure, the structure changes. Metamorphic rocks form like metamorphoses: just as a caterpillar goes through a metamorphosis to turn into a butterfly, rock A goes through a transformation into rock B. Metamorphic rocks can melt and cool to form an igneous rock or erode and form a sedimentary rock.
- Sedimentary Rocks
Wind, rain, and ice cause mountains and rock outcrops to erode. The small particles get washed down rivers. As the water slows, the particles settle to the bottom.
When sediments such as rock pieces or minerals from the shells of oceanic creatures settle down to the ground, over time, they get compacted.
With enough pressure and time, these sediments create a new type of rock. Sedimentary rocks can erode and form other sedimentary rocks, melt and cool as igneous rock, or be transformed into a metamorphic rock.
Quartz and feldspar minerals can be found in all three types of rock, although they are most abundant in igneous rocks since this is where the minerals first form. If an igneous rock containing quartz and/or feldspar is changed into a metamorphic or sedimentary rock, then the quartz and feldspar will be found in the new rock.
Igneous Rocks with Quartz and Feldspar
Many igneous rocks contain both quartz and feldspar, but not all of them. Here are a few common rock specimens and which of the two minerals they contain:
- Diorite – both feldspar and quartz
- Gabbro – feldspar
- Granite – both feldspar and quartz
- Peridotite – both feldspar and quartz
- Rhyolite – both feldspar and quartz
Note: The igneous rocks listed above also contain minerals besides feldspar and quartz.
Metamorphic Rocks with Quartz and Feldspar
Any igneous rock that is formed with quartz and feldspar can be heated and pressurized to create a new rock. Metamorphic rocks with quartz and feldspar are:
- Amphibolite – both feldspar and quartz
- Gneiss – both feldspar and quartz
- Quartzite – quartz
- Schist – both feldspar and quartz
- Amphibole – both feldspar and quartz
Sedimentary Rocks with Quartz and Feldspar
Many sedimentary rocks are formed from the shells of animals in the ocean. These types of sedimentary rock would contain little or no quartz or feldspar. Since quartz is the most abundant mineral on earth, it is found in more sedimentary rocks than feldspar is.
Sedimentary rocks with quartz and feldspar are:
- Quartz Sandstone – quartz
- Sandstone – can contain both feldspar and quartz
- Chert – quartz
- Conglomerate – can contain both feldspar and quartz
Rocks such as conglomerate and sandstone, are made up of pieces of other rocks. A sample of conglomerate could contain feldspar, quartz, both, or neither. It is more likely that a sedimentary rock will form with quartz and feldspar if it is in an area where those minerals are part of the bedrock.
Feldspar and quartz have some similar properties, but overall are very different. For example, feldspar is not an individual mineral, but a group of minerals, whereas quartz is one mineral.
If you were to hit a sample of reach mineral with a hammer, you would find that they have different cleavages: the quartz would shatter into random pieces, but the feldspar will break into smaller pieces with the same shape.
Feldspar and quartz are the most abundant minerals on earth. No matter where you go, you can find a rock with one or the other, or maybe even both.
However, as long as you remember the differences between the two, you can easily discern one from the other on your next rockhounding trip.
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