Sphalerite is needed for Indium. Currently, besides coins (and mining? I'm not sure), it can only be obtained from the T5 Microminer, at a rate of 64 sphalerite ore per T5 microminer. The T5 microminer is very expensive for something as important/widely used (as I understand it to be) as sphalerite.

Exa recently considered replacing something more useless from an earlier miner with sphalerite ore, so that there is a viable option other than coins. A conversation on Discord with some context is here.

In dev, it appears as if the T3 mission has one empty slot for another ore. It may be a good idea to add Nether Sphalerite Ore to this microminer. (Copper from the T2 miner could also be replaced, as Exa considered in the conversation cited above).

I'd honestly be more inclined to fill the "empty" spot in the T3 mission output with another stack of sapphire ore. Unlike Almandine (the other ore we added to missions in 1.2.2) which pulverizes into 12 dusts, sapphire only doubles so you get far less return per mission.

Copper ore is pretty worthless by the time you get to the T2, and the mission throws three stacks of it at you. I really think it makes the most sense to replace 1-2 stacks of the copper with Sphalerite.

That sounds fine to me as well! Would it be worth changing the title? / Would a pull request be helpful?

No worries about the issue title, I got it. Feel free to PR if you like, I'm currently testing other things at the moment. It should be fairly trivial to implement this.