You can't simply sub somethiong like spodumene for lithium carbonate.
The issues here are understanding that the raw mateial is not what is present on the pot when you open the kiln, the concept of the relative molecular weights of compounds, and the fact that the chemical compounds in different raw materials can provide different compounds in the final melt.
Lithium carbonate has the chemical formula LiCO3. In the kiln it converts to Li2O ...... which is what is the flux on silica.... (the carbonate is NOT a flux....it supplies the flux ... important distinction). When you weigh out X grams of lithium carbonate you are weighing out a certain amount of lithim atoms.....but ALSO one carbon and three oxygens per lithium atom. The carbon and some oxygen are given off in the kiln as CO2 gas.
If you subsitute spodumene directly for the weight of lithium carbonate in the recipe, the amount of Li atoms in that same X grams in the recipe is going to be WAY different that what you had in the original. And in fact you are also going to ALSO be adding other ceramic active components like alumina and silica. The approximate formula for spodumene is 1 Li2O - 1 Al2O3 - 4.1 SiO2.
One molecule of lithium carb gives you one half of a molecule of lithium oxide in the final melt. One molecule of spodumene gives you one molecule of lithium oxide in the melt..... but also one molecule of alumina and 4 molecules of silica.
The molecular weight of spodumene is 380.4. The molecular weight of lithium carbonate is 29.8. Relative weights are 380.4/29.8 . Spodumene is almost 13 times heavier than lithium carbonate. If you weigh out the same amount of spodumene as was called for as lithium carbonate..... you are going to get about 1/13th the atoms of lithium as the original recipe,.... plus some alumina and silica that were not there in the first place.
This is why one learns glaze calculation.....or at least how to use glaze calc software.
Professor of Ceramics; New Hampshire Insitute of Art
Guest Professor, Wuxi Institute of Arts and Science, Yixing, China
Former President and Past President; Potters Council