don't you have a colleague you could say you bought it for? maybe if you have a garden/plant you could say it's some sort of fertilizer. only way I can think of proving that it's not cocaine is by offering it to her in a way that she can have it tested on a lab or something, and honestly unless you are a drug dealer it unlikely you'll have a 500g or 1kg bag of cocaine laying around. also the crystals are quite different, I don't think you can mistake them for one another.
another option to prove it would be by showing her the receipt. if I'm not wrong, you also bought it from Mercado Livre, your purchase account history could easily accessed and it's possible that she will believe it if you show her.
according to wikipedia SN is used on the textile industry, photography and to prevent metal corrosion, you could explore one of these, depending on your hobbies and interests to make an excuse.