The different types of plagioclase?

Question

The fractions of albite $\\ce{NaAlSi_3O_8}$ and anorthite $\\ce{CaAl_2Si_2O_8}$, which I call $\\ce{Ab}$ and $\\ce{An}$ in the following list, present in a plagioclase are often used to define it in the following way: - Anorthite: $\\ce{An}=90\\%-100\\%$, $\\ce{Ab}=0\\%-10\\%$; - Bytownite: $\\ce{An}=70\\%-90\\%$, $\\ce{Ab}=10\\%-30\\%$; - Labradorite: $\\ce{An}=50\\%-70\\%$, $\\ce{Ab}=30\\%-50\\%$; - Andesine: $\\ce{An}=30\\%-50\\%$, $\\ce{Ab}=50\\%-30\\%$; - Oligoclase: $\\ce{An}=10\\%-30\\%$, $\\ce{Ab}=70\\%-90\\%$; - Albite: $\\ce{An}=0\\%-10\\%$, $\\ce{Ab}=90\\%-100\\%$.What I am not sure of is whether these fractions are intended as applying to mass, molarity, volume.

Answer 1

Hi! What you are describing in your question is what is known as a "solid solution". Many minerals can substitute different ions into their crystalline structure. Feldspars are just one of these mineral species. When discussing the general composition of a mineral, we will typically use the % of the endmembers that are in the crystal chemistry that you are analyzing. An endmember is another term for a mineral or substance that is purely using one ion in a location in its structure. For example, albite contains 90-100% Na, while anorthite contains 90-100% Ca.

Now, most feldspars will contain some sort of mixture of Na and Ca. If you were to calculate the true chemical formula of a feldspar, it may look something like this:

(Na_0.3,Ca_0.7)Al_1.7Si_2.3O₈

Notice how the Na and Ca add up to 1 in that site in the crystal structure. The rest of the chemical formula needs to be balanced according to the substitution of the Ca and Na so the molecule is still neutrally charged.

Then, to answer the question: When describing a mineral in the format of Ab₃₀An₇₀ (such as the example above), You are describing the endmember molarity % that is in that site for the chemical formula. It essentially expresses the chemical formula in a shorter version. Someone reading this format only needs to know what the general mineral formula is that is being discussed.