I think the space argon would blow away in the solar wind.
I think the obsidian core would not affect how the moon looks, because it is on the inside. Maybe if there were a crater deep enough to unearth some of the obsidian it would be darker than adjacent moon, because obsidian is dark.
The surface of Luna is made of plagioclase feldspar.
Lunar rocks are in large part made of the same common rock forming
minerals as found on Earth, such as olivine, pyroxene, and plagioclase
feldspar (anorthosite). Plagioclase feldspar is mostly found in the
lunar crust, whereas pyroxene and olivine are typically seen in the
Feldspar is an aluminum containing mineral. Petalite is very similar and differs in that it contains lithium instead of potassium; I found petalite described as "lithium feldspar". The moon's surface is broken up into dust and tiny bits from impacts. When broken into tiny bits I do not think one can distinguish potassium feldspar from lithium feldspar by sight. Take a look.
As regards albedo and color I think the proposed petalite moon would look like Luna does now.
With one exception: this moon has 2 suns and so it would be lit from 2 angles, and so there would be 2 different crescents as the moon went through its phases: one crescent brightly lit by both stars, one dimmer crescent lit by just one star and the remainder dark and unlit. If anyone feels motivated to Photoshop 2 phases of the moon on top of each other, please feel free to add your image in an edit. MS Paint is not up to that job, it turns out.