Are you experiencing itching while using a "crystal" deodorant? There are a few different causes to underarm itching when using a "crystal" salt deodorant.
Itching with no redness
If you're experiencing itching with the crystal deodorant, but no visible irritation or redness, it could be the crystal deodorant basically "tanning" your hide.
Crystal deodorants are made of either potassium alum or ammonium alum. Alum has been used for hundreds of years in the leather industry to tan leather hides. When alum comes in contact with skin, the water molecule within the collagen is displaced with an aluminum ion. In theory, this would shrink the collagen cell, dry the skin out, and have a tightening effect. If your skin is itching while using a crystal deodorant, it may be that the outermost layer of your dermis is becoming "tanned" with the aluminum salt. This tightening might be creating itching with no visible redness.
Itching and redness in the crease of skin
If you're experiencing itching with redness in the crease of skin, you might have intertrigo. Basically, when you have moist skin rubbing against itself, the skin becomes broken, irritated, and red. Sometimes it can progress to infection and ooze or flake off. More on intertrigo here. You might need a deodorant with powders to help alleviate the friction.
Itching and pimple-like bumps
If you're experiencing itching and pimple-like bumps with a crystal deodorant, you might be getting heat rash. Heat rash shows up as little blisters or pimples that itch or burn like crazy. They're caused by perspiration getting trapped in your pores.
Itching with irregular patchy rash
An itching, irregular, patchy rash (sometimes with a ring around the edge) could mean you have a fungal infection on your skin. Usually to clear this up, you'd need an antifungal cream or powder. Fungal/yeast rashes can be quite stubborn sometimes, and usually need medical intervention. It may be that your skin was compromised by friction or other irritation from the crystal deodorant, and that's how infection crept in.