Purple soil - how would it be possible?

Question

For one of my projects, I'm developing an inhabited Earth-like world, with the key visual difference being that much of the land is purple, instead of the brownish colour it is on Earth. My question is: how would this be possible?

Constraints:

• The soil must be fertile, and capable of supporting both animal and plant life. I don't mind if it's toxic to humans, as long as it's feasible that life could evolve to be immune to it.
• It has to actually be purple. It can't just look purple (i.e. be covered in something that's purple, or speckled with purple rocks).
• It has to be purple on a permanent basis.
• It has to be purple to a depth of at least several metres.
• The soil needs to plausibly cover a significant proportion of the planet's surface (i.e. the bits not covered by deserts, ice caps, and oceans). So if there's a compound that would result in purple soil, but it would be extremely unlikely to find massive quantities of it on a planet, then that's a no.
• My preference is for a deep, Cadbury's-chocolate-wrapper purple, but if that's not doable, then any shade will do as long as it's visibly purple. Pink does not count.

I am aware of this similar question, and borrowed a few of my constraints from it, but I don't believe it's a duplicate because none of the answers mention purple soil, and blue is not the same as purple.

Answer 1

Amethyst is quartz turned violet with radiation.

Sand in most places is just broken down quartz. If your quartz is purple to start with then so is your sand. I figured there must be someplace where the sand was made of amethyst. Pfeiffer beach is such a place. https://californiathroughmylens.com/2011/08/pfeiffer-state-beach-in-big-sur/

https://i.pinimg.com/originals/4f/fb/55/4ffb55b6ee9e813464a26821ee85d3f2.jpg

Nothing to invent here: amethyst is not crazy rare, nor is sand.

Answer 2

According to the wiki:

Purple is a color intermediate between blue and red. It is similar to violet, but unlike violet, which is a spectral color with its own wavelength on the visible spectrum of light, purple is a composite color made by combining red and blue.

So if you just have many small, blue particles and many small, red particles in the soil, that should do the trick. Someone analyzing it up close may be able to see the differently colored particles. Some small patches of land may be either blue or red, or other shades of their mix (such as pink), depending on the concentration of the particles. But the more homogeneous the mix is, the closer to purple the ground would look. Even for someone standing right on top of it.

So how do you get red and blue particles?

Start with the answer for the question you link about how to have blue soil. Edit: Will has called to attention in the comments that Vanadium may be a more favorable choice than copper. You may have to consider the toxicity of Vanadium vs that of Copper.

Then add loads of iron to the soil, to make it red as well. Iron tends to redden things.

Answer 3

Can I interest you in some algae?

There are compounds that already produce shades of purple in algae- anthrocyanins and retinal.

Anthrocyanins are found in many plants and algae, either as an antenna pigment or as an antioxidant. They're mostly studied in higher plants like Arabidopsis but there's no reason that soil algae wouldn't produce them as well. If you'd like a little more justification, try having a "hotter" or bluer sun- that'll encourage the formation of purple antenna pigments in cyanobacteria and other algae that live in the soil.

On the other hand, we have retinal. This compound actually provides the basis for the Purple Earth hypothesis, as it's light-sensitive and can be used to convert light energy into chemical energy. If you take this route, the hotter sun will help justify it as well, and the algae in the soil will proliferate wildly.

Sometimes it's hard to get algae to grow densely across an entire planet. In that case, try pairing your favorite cyanobacteria with a fungus to produce a lichen, which is much more stable and can be found in many more places such as deserts. The coloration still comes through quite nicely.

Answer 4

I think you should solve this problem from the opposite angle.

What makes Earth's soil seem brown?

Not all dirt, even on earth, is brown. As mentioned in some other answers iron can add a reddish tint and other minerals can cause colorations running the entire spectrum, but the vast majority is brown-ish, or has a brown overtone.

This link gives a simple explaination: https://www.livescience.com/4045-ground-brown.htm

Basically brown is caused by decaying plant life releasing carbon into the soils. So, what I would do is ask: what do the plants and animals of my world release when they decay? How would that decaying matter impact soil color? If lots of red and blue colored minerals are part of that decay cycle, then the soils could (quite plausibly) be mostly purplish, no matter where you are.

Answer 5

Ever visit Death Valley? In particular the Artists Pallet?

The purple here is due to the manganese inside what's known as purple shale.

Excellent outcrops of deep purple, wine red, red, blue, turquoise, and green argillites of the Belt Supergroup can be seen in Glacier National Park in northwestern Montana and in Wolf Creek Canyon along US Interstate 15 in west-central Montana. – Argillite

There are actually quite a few purple shales as quite a few compositions will create a wide variety of purples from light ones to relatively dark purples.

The presence of hematite and limonite (hydrated ferric oxide) gives rise to reddish and purple colouring – britannica.com

No point reinventing the wheel, purples exist quite naturally.

Answer 6

Purple is not really a color in the way we think of it, it is an artifact of how our eyes are put together, primates are probably the only organisms that see purple. As Renan pointed out it is created by activating red cones and blue cones without activating green cones, which actually requires two separate wavelengths at the same time, thus the best way to produce it consistently is have two strong color sources.

Red Rocks and Blue Plants

An early oxygen atmosphere (that is to say an atmosphere that has just recently risen in oxygen content) like the permian and triassic makes red rock and soils extremely common, see red beds. So make your plants and soil bacteria blue instead of green and you will end up with a lot pf purples on land but really blue seas.

red rocks and blue rocks

Alternatively in the same type of atmosphere as above soils that a very rich in both iron and copper would give a variety of purple colors. The hard part is having enough copper, iron is fairly common but copper is not, especially on a galactic sense.

Answer 7

For purple soil you'll need a lot of pyrolusite. It doesn't seem toxic, but I'm not sure how well plants will grow in it since it is a crystal. Maybe if it's well mixed it would still be purple yet fertile.

Answer 8

Purple is the combination of blue and red, right? So, one way to get blue and red into the soil would be to have large amounts of something red (such as iron oxide - rust) as well as large amounts of something blue - cobalt or something of that nature. (Cobalt is pretty toxic to people, so that could be problematic, but that's neither here nor there).

Additionally, you could look at this link on colors and how they're derived chemically and find something that fits your needs.

Answer 9

There are purple rocks on Mars, as seen by Curiosity in this article. Geological processes can break rocks down into soil. As far as fertility goes, you could just have different microbes on your world, ones that thrive in this soil.

Answer 10

Lepidolite can definitely be purple, but as mentioned previously - dirt is primarily the decomposition of plants and animals.
In order to meet your requirement of:

The soil must be fertile, and capable of supporting both animal and plant life

You would need to make the local lifeforms Lepidolite based (instead of carbon based).

Answer 11

In some beaches of Crete in Greece the sand is pink to purple.

'Pink sand is a result of tiny red organisms that grow on the dead coral reefs and pieces of shells which fall to the ocean floor and is washed onto shore. It is also made of natural formulation of calcium carbonate from very small marine invertebrates that mixed to corals and shells and other marine things which has Foraminifera, microscopic amoeba that has a red or bright pink body shell. All of these are what causes the sand to have a pink hue.' source: http://www.chaniapost.eu/2015/06/24/the-pink-sand-beaches-of-elafonissi-and-balos-a-brief-explanation/

Answer 12

How about this: The local equivalent of chlorophyll is purple, or a first stage breakdown product is purple. Further, unlike chlorophyll, it is a very stable molecule and takes a long time to break down. This compound then would accumulate in the soil. If it's soluble it would leach into the water, dyeing lakes and oceans purple too.

Answer 13

The simplest answer: pretend that you've created a purple soil which contains all the qualities you want it to have. Bam, you're done! And it fits quite nicely with the scenario you've created.

Or, if you want to be 'real world' about it, give your world a different history and elemental make-up so the main components of the rocks and soils have red and blue in them, which would mean a world with different chemistry and hence a different origin and geologic history, maybe resulting in a different atmosphere and geologic make-up, and different plant and animal life! So, you get Prince's Purple World, but maybe we humans are not in it.