The different types of plagioclase 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.
According to the pages Plagioclase composition (from the textbook: Introduction to Mineralogy Nesse, 2000), the percentage definitions are due to the mole percent of $\ce{Ca^{2+}}$ (from anorthite) and the presumed subsequent $\ce{Na+}$ (from albite) ions present in the sample.  

This is done under the assumption that the anorthite% + albite% = 100% in a plagioclase sample.

The mole percent are usually represented by percentage proportions of the plagioclase end member anorthite, often in the form of Anx, where 'x' is the mole percent of $\ce{Ca^{2+}}$ present in the plagioclase sample.

Consider the following diagram below - the relevent part for your question is across the base of the triangle:



Image source.

For example, oligoclase is An10%-30% means that it would possess a $\ce{Ca^{2+}}$ mole percent of between 10% to 30%, and a corresponding $\ce{Na+}$ mole percent of between 70% to 90%.