Orthoclase vs Plagioclase
Orthoclase and Plagioclase are commonly confused, and 4 measured properties separate them — cleavage most clearly.
Orthoclase vs Plagioclase
Orthoclase
KAlSi3O8
The potassium feldspar of granite — the most abundant mineral group in the Earth's crust, and the raw material of porcelain.
Plagioclase
(Na,Ca)Al(Si,Al)Si2O8
A continuous solid solution from sodium to calcium feldspar — the commonest mineral series in the crust, and a thermometer for the magma that grew it.
| Property | Orthoclase | Plagioclase |
|---|---|---|
| Cleavage Categorical values can be listed side by side but not ordered or subtracted. | perfect in one direction, good in a second at right angles | perfect in one direction, good in a second at about 86 degrees |
| Crystal system Categorical values can be listed side by side but not ordered or subtracted. | monoclinic | triclinic |
| Density | 2.55–2.63 g/cm³ | 2.62–2.76 g/cm³ higher |
| Lustre Only one of the two has a figure we can stand behind, so there is nothing to compare. | vitreous | Not known |
| Mohs hardness Mohs hardness is an ordinal scale: values can be ranked, but differences and ratios between them are not meaningful. | 6 | 6–6.5 higher |
| Streak Only one of the two has a figure we can stand behind, so there is nothing to compare. | white | Not known |
Why these get confused
the commonest feldspar identification problem; fine parallel striations on a cleavage face indicate plagioclase
The confusion is a real and documented one, not a resemblance we have invented.