I want to create a convincing world where gunpowder is a rare and expensive commodity, allowing swords and plate armour to still be used effectively despite the world having current level of scientific knowledge is similar to that of earth in the early 1800s.
I reckon sulfur is the best candidate in terms of being a rare ingredient of those that are used to make up gunpowder, so I figure the most reasonable way to do this is by altering the sulfur cycle from that of earth (https://commons.wikimedia.org/wiki/File:SulfurCycle_copy.jpg#/media/File:SulfurCycle_copy.jpg)
My biology knowledge is poor, but if I understand this correctly, elemental sulfur isn't used for much in nature, so it's not unreasonable to say there are little to no sulfur mines. The formation of mineral sulfur comes from sulfate - and reduced sulfur, which comes from sulfate. Thus, the explanation I currently have in mind is that a new 'competitor' is present in the environments where sulfate would typically be deposited in minerals, in the form of some bacteria/algae/plant life that has a rapid sulfate uptake.
My questions are:
Is this feasible? If so, which out of bacteria/algae/plant/other would be a good way to fill this role, and in the case of plants/other, what form might the plant life or other alternative take.
How might the world look different from ours as a result of this. In particular, are there sulfur compounds that had many useful applications in the 1800s
Are there any alternative ways sulfur might have been produced in large quantities, for example from biological sources, using chemistry known in the 1800s, that might have developed in the absence of sulfate minerals
Obviously little of this will be explicitly stated in the book, but I would still like my solution to be scientifically plausible, and am very interested how a world altered in such a way might look different to our own in ways besides the desired scarcity of gunpowder.
If anyone has any other suggestions about how a gunpowder might be rare in a world whose scientific knowledge is comparable to ours in the 1800s, I'm all ears
Edit: Tried and failed to fix the image link