Spodumene and petalite are lithium-bearing feldspars, and lithium is the flux that lowers thermal expansion. That property is why they appear in bodies meant to survive thermal shock and in glazes that need to fit a body they otherwise would not.
What lithium does
Lithium is a powerful flux, so a little lowers a glaze's melting point noticeably. More usefully it lowers thermal expansion, which means a lithium-bearing glaze contracts less as it cools. Where a glaze is crazing on a particular body, substituting some of the flux with a lithium source is one of the standard corrections.
Petalite and flameware
Petalite carries lithium with more silica and less alumina than spodumene, and bodies containing a high proportion of it can survive direct flame and rapid temperature change far better than ordinary stoneware. That is the basis of flameware, and it is the one part of studio ceramics where a body is engineered for thermal shock rather than for working properties.
Choosing between them
They are not interchangeable by weight, because their alumina and silica contents differ. Recipes name one or the other, and swapping requires recalculating rather than substituting. Where a supplier is out of stock of one, the published analyses are what you compare, exactly as with frits.
Cost and handling
Lithium materials cost more than common feldspars, so they appear in recipes where their specific property is needed rather than as a general flux. They are fine powders and carry the same weighing precautions as everything else on this shelf.
Questions people ask about spodumene
What is spodumene used for?
As a lithium-bearing flux that lowers a glaze's melting point and, more importantly, its thermal expansion, which helps a crazing glaze fit.
What is petalite for?
The same lithium contribution with a different silica and alumina balance, and it is the key ingredient in flameware bodies that survive direct heat.
Can I swap one for the other?
Not by weight. Their analyses differ enough that the recipe needs recalculating.