Microcline feldspar

Locale: La Mancuerna, Tatatila, Veracruz, Mexico

Approx size: 1–2"

Approx weight: 18–70g

Microcline is a type of feldspar that forms opaque blocky geometric crystals, the groupings often resembling cities and architectural forms. The mineral is near-white with parallel striations; this is called "perthitic" texture and is common in microcline, caused by the intergrowth of two varieties of feldspar. 

Feldspar likes to grow as twinned crystals, meaning that two or more microcline crystals have intergrown and share the same crystal lattice. I'm not hip enough on my crystallography to tell you what types of twinning we are looking at, but the directionality of the perthitic texture can help you trace individual crystals and find the seams where crystals merge.

Feldspar might seem dull since it is near-white and opaque, but under bright light the surface has a flashy sheen. The name 'feldspar' actually historically originates from people observing flashes off cleaved pieces of the mineral in their fields: 'feld-' is 'field' while 'spar' was a term for rocks that easily broke along flat planes (cleaved) and had a shiny luster. Parallel lines that appear to extend through a feldspar crystal represent cleavage planes.

These specimens from the Tatatila region of Mexico commonly include smoky quartz, which provides a dramatic contrast; small crystals can be found on several of these options. Rusty orange-brown coloration is iron oxide staining.

UV reactivity microcline feldspar can have slight red fluorescence which was present in many of these specimens; best viewed in dark conditions with a strong 365nm UV light.