Granodiorite is an extremely common rock type, but many people have never even heard of it. Chances are good that you have seen and touched granodiorite many times in your life, whether or not you were aware of it at the time. Because of its innate strength, beauty, and variety, it is also one of the most popular rocks for amateur collectors and for decorative use in construction.
The biggest reason granodiorite is not as well-known as it should be is that it is often confused with granite. This is perfectly understandable, because granodiorite and granite are very similar rocks that share many of the same physical properties. It can be difficult to know exactly what granodiorite looks like and how to identify it, but it is relatively simple if you know what to look for.
Granodiorite is an intrusive igneous rock composed primarily of feldspar, quartz, and smaller amounts of mafic minerals. It has a phaneritic texture, meaning its interlocking crystals can be seen with the naked eye. It usually displays mottled colors of off-white, gray, and black with no layering or banding.
While granodiorite is a clearly defined rock type, it can be hard to distinguish from pure granite or pure diorite – especially if you don’t have access to sophisticated testing methods. I’ll walk you through how to identify granodiorite, what it looks like, and where it can be found.
What Does Granodiorite Look Like?
Granodiorite is one of the most common igneous rocks in the world. Most people have seen it many times in their life – probably without realizing it. It will come as no surprise that granodiorite gets its name because it is intermediate in composition between granite and diorite, which often makes it difficult to identify as a unique rock type.
In everyday life, it is relatively unimportant to be able to distinguish between granodiorite and granite, so people often just label everything that remotely meets the description as ‘granite’ and call it a day. But if you’re actually interested in geology and knowing what kind of rocks you have you probably want to be more specific than that. So, what does real granodiorite actually look like?
Granodiorite has a coarse-grained texture with large, interlocking crystals of translucent to off-white quartz and much more whiteish plagioclase feldspar than pink alkali feldspar. Darker, mafic minerals are present in small quantities. Its crystals are roughly equal-sized and it is relatively light in color.
While all granodiorite meets this general description, there is a pretty wide spectrum of granodiorite that can look significantly different from one another. The difference in their appearance is driven by the mineralogy of each type of granodiorite and, to a lesser extent, the subtle differences in their textures (crystal sizes).
Color is Driven by Mineralogy
The three major minerals present in all granodiorites are quartz, plagioclase feldspar and alkali feldspar. Each of these minerals can present itself in different ways. Feldspars, in particular, can look quite different from rock to rock depending on the specific type of feldspar that is present.
Quartz usually appears in granodiorite as translucent, light gray, or off-white. In polished granodiorite (as you have likely seen in ‘granite’ countertops) it sometimes feels like you can see below the surface of the rock. This is because the quartz crystals are transparent or translucent, allowing you to see through them. Some varieties may have more impurities which make the quartz crystals an opaque gray or white color. In any case, quartz is a significant contributor to granodiorite’s overall light color.
Feldspar has much more variety than quartz. It is actually a group of minerals that can be divided into two main types: plagioclase feldspar and alkali feldspar. These two types of feldspar can look quite different from one another, and can even vary significantly in appearance themselves. Both are present in granodiorite.
Plagioclase feldspar is somewhat more abundant in granodiorite than is alkali feldspar, but both are common. Being able to distinguish between the two is perhaps the most important (and most difficult) aspect of identifying a rock as granodiorite as opposed to granite. While this can sometimes be tricky, there are some easy-to-spot differences that can often help you see the difference and further understand your rock.
Both alkali feldspar and plagioclase feldspar can be white, but alkali feldspar is also very commonly salmon-pink or even darker pink, bordering on red. Plagioclase feldspar usually has visible striations in its crystals that may be visible if the individual crystals are large enough.
Some of the most common accessory minerals (minerals found in relatively low amounts) in granodiorite are mica and hornblende. The mica (usually biotite, a dark variety of mica), forms very flaky ‘books’ as crystals in the rock. Hornblende and pyroxene will almost always look like dark, blocky crystals interspersed throughout the granite.
The accessory minerals in granodiorite give it a ‘salt and pepper’ appearance, especially when there is very little pink alkali feldspar present.
Texture of Granodiorite
One of the defining features of granodiorite’s appearance is its texture. All granodiorite is coarse-grained, meaning that you can see the individual crystals in the rock. This texture is known as ‘phaneritic’, and it forms when magma cools slowly and allows the crystals enough time to grow before becoming completely solidified.
While all granodiorites are phaneritic, it is also possible (and even common) to describe their texture in other ways. Rocks can be described as having more than one texture as long as the texture types are not mutually exclusive.
Most granodiorite is ‘equigranular‘, which means that all of the crystals are approximately the same size. So, a granitic rock may be described as ‘phaneritic and equigranular’ if you can see similarly-sized crystals with the naked eye.
Some granodiorites can have a ‘porphyritic‘ texture. This indicates that there are larger crystals called ‘phenocrysts’ in a groundmass of smaller crystals. So if a granodiorite has large feldspar crystals surrounded by smaller (but still visible) quartz and amphibole crystals you could describe it as ‘porphyritic and phaneritic’.
Granodiorite may grade into its fine-grained equivalent, dacite. The two rocks have the same basic mineralogy but vary in their crystal sizes.
How to Identify Granodiorite
Granodiorite has a fairly distinctive look, but it is often confused for closely-related rock types. If you aren’t overly familiar with how it’s defined and what it looks like it can be easy to misidentify it as something else entirely. As with any rock, it is important to take a systematic approach when identifying granodiorite.
To identify granodiorite, first look to see if you can make out individual crystals with the naked eye. Try to identify large amounts of whitish quartz and plagioclase, with lesser amounts of pinkish alkali feldspar. Dark, mafic minerals make up less than 15% of the rock, and no layers or porosity should be present.
A rock must meet all of these requirements to be considered a granodiorite:
- Igneous – Formed from cooling magma, with interlocking crystal grains.
- Coarse-grained – Phaneritic texture with crystals visible to the naked eye
- Felsic Mineralogy – High quartz and feldspar content, with more plagioclase than alkali feldspar
- Massive – No internal structures or layering
If your rock meets all of those criteria then it is very likely a granodiorite, or at least something very closely related. Rocks with mineralogies very similar to granite are considered ‘granitoids’, and many people simply group them all together when a specific identification isn’t overly important.
Tip: This article is part of my igneous rock identification series. To read more about how to identify all igneous rocks, check out my article here.
When observing and identifying a rock like granodiorite it can often be useful to use a hand lens like this one from Amazon. This allows you to see the individual crystal grains more clearly and often helps you identify the specific species of minerals present in a rock.
What Is Granodiorite Made Of?
As I mentioned above, granodiorite is largely defined by a specific mineralogy. All rocks are made from one or more minerals, and in order to fully understand a rock like granodiorite you have to know what those minerals are.
Granodiorite is primarily made from large amounts of quartz and plagioclase feldspar, with smaller amounts of alkali feldspar and mafic minerals like amphibole and hornblende. The mineral composition of granodiorite consists of 20% to 60% quartz, and 35% to 90% of the total feldspar must be plagioclase.
This definition is very specific, but without sophisticated methods for measuring mineral types and percentages it is impractical (or even impossible) for us to determine if a granite-like rock falls into those specific percentage ranges.
The best you can do is to first estimate the percentage of quartz in the rock. If it is between 20 and 60% quartz then you know it is probably either granite or granodiorite. Then, estimate the percentage of the feldspar that is plagioclase vs alkali feldspar. If there is noticeably more plagioclase than alkali feldspar then you can confidently call your rock ‘granodiorite’.
It is useful to know what some closely-related rock types (granitoids) are called and what their mineralogy looks like.
Granitoids are granite-like rocks with slightly varying mineralogies:
Where Is Granodiorite Found?
Granodiorite is a fairly widespread igneous rock, but you might not realize just how often you see it because it is so easy to confuse with granite. But as common as granodiorite is, you can’t find it just anywhere.
Granodiorite is most commonly found in mountainous regions where basement rock has been thrust up to the surface, and in areas where erosion has stripped away shallower rock to reveal the granodiorite beneath. It is also common to find granodiorite in riverbeds downstream from large granitic exposures.
Because granodiorite forms beneath the surface of the earth, the vast majority of it isn’t visible or accessible to us. But, through the tremendous mountain-building power of plate tectonics, large mountain ranges of granitic rock are fairly common throughout the world. The large igneous intrusions are thrust upwards to the surface to form impressive mountains which endure for millions of years.
Granodiorite is composed of relatively hard minerals (quartz and feldspar) which make it incredibly durable. Once it is exposed to the surface it takes a long time for it to break down. Pieces of granodiorite can survive in a river for great distances, which is why it’s so common to find pieces downriver from its source.
You can look for granodiorite formations near you using this excellent interactive map from the USGS. I have a video about how to use this tool in my Practical Rock Identification System, plus even more information on how to identify diorite and other rocks.
In the United States, granodiorite is most often found in mountainous regions like the Rockies and Appalachians, plus the ancient batholiths in the Midwest.
How Does Granodiorite Form?
We’ve learned all about what granodiorite looks like, what it is composed of and generally where it’s found, but I have only briefly touched on how it’s actually formed. The creation of granodiorite is a fascinating process that always follows a few simple rules but can vary significantly in the details, which is the reason granodiorite can look so different from one sample to another.
Granodiorite forms from the slow cooling of felsic, silica-rich magma beneath the Earth’s surface. The felsic magma intrudes into shallower rock where it gradually cools, allowing the formation of relatively large crystals of quartz, feldspar, and accessory minerals like amphibole and hornblende.
Because the composition of the source magma is never exactly the same from one granodiorite to another and the surrounding environment is always different, there can be quite a lot of variation in how the magma cools and the crystals form.
If the surrounding rock is relatively cool then the crystals formed on the edges of the igneous intrusion are likely to be smaller. Similarly, if the intrusion is very large then the crystals in the center of the magma body are likely to become very large because they have such a long time to cool down and crystalize. Those exceptionally large crystals give some types of granodiorite a pegmatitic texture.
As the individual crystals of granodiorites’s various minerals cool down, they eventually run into each other and they run out of room. Crystallization will continue until all of the magma is solidified, leaving behind interlocking crystals with virtually no porosity between them.
Because granodiorite forms as one solid mass, it contains no internal structures like bedding or banding. It looks the same from all directions and you can’t tell which direction was ‘up’ at the time of crystallization. This is called a ‘massive’ rock.
What Is Granodiorite Used For?
Chances are good that you have seen plenty of granodiorite in your day to day life, but it can be easy to miss if you’re not paying attention. Because granodiorite is so similar to granite it should come as no surprise that they are used in many of the same applications. Granodiorite has been in use for thousands of years because of its durability, strength, prevalence, and striking appearance.
Granodiorite is most commonly used for interior and exterior building applications. Highly polished granodiorite is an extremely popular material for countertops, while rough-cut granodiorite is one of the most prevalent components of buildings, bridges, and other structures.
Like most rocks, granodiorite is very strong in compression. This means it can withstand a great deal of ‘squeezing’ without breaking, making it ideal for use in construction projects with large overburden stresses.
What really sets granodiorite (and granite) apart is its ability to accept a polish and its resistance to weathering. The highly polished ‘granite’ countertops you’re used to seeing in modern buildings are often actually made of granodiorite. They are made possible by the fact that quartz and feldspar are easily brought to a high shine.
One of the biggest reasons granodiorite and similar rocks are so popular as a building material is because it doesn’t break down in the rain, ice, and wind as easily as other rocks.
This article is part of my rock identification series. To learn more about identifying rocks, check out my full in-depth guide here.