Pulled a feldspar megacryst from a granite roadcut near Bishop this afternoon and the penetration twin is classic Carlsbad — the twin axis lines up with [001], and the {001}/{010} cleavages meet at about 90 deg under the pocket goniometer. Before I commit to a thin section, what’s your go-to for calling orthoclase vs microcline from macroscopic twinning and habit alone?
If you can pop a fresh {001} chip, wet it, and hit it with strong raking light under a 10x, look for the ultra-fine ‘tartan cross-hatch’ on the cleavage — if you see that, I’d call microcline; if it stays smooth with only the Carlsbad, lean orthoclase. Caveat: subsolidus inversion can muddy the waters, so it’s a tartan-vs-pinstripe call until the thin section. Do you see any cross-hatch on that Bishop piece?
Leaning orthoclase if it’s just a clean Carlsbad with no patchy perthite under 10×; microcline in those Bishop granites is usually a bit turbid with patchy perthite. Quick check: set a fresh {001} chip on a bright phone screen and view through polarized sunglasses while rotating — microcline gives a fine flicker/banding, orthoclase stays fairly even. Seeing any perthite blebs or cloudiness on that ‘twin axis [001]’ crystal?