Star muscovite

From Minas Gerais

Although muscovite mica, KAl₂(AlSi₃O₁₀)(OH)₂, crystallizes in the monoclinic system, its molecular arrangement leads to frequent crystal intergrowths (twins) that mimic hexagonal and orthorhombic forms as well as the five-point stars you see in this specimen from the famous Jenipapo pegmatite field at Itinga, Minas Gerais, Brazil.

The figures that follow are by Harry Groom, from Hurlbut (1956, Muscovite from Methuen Township, Ontario: Am. Mineral. 41, 892-898), citing Peacock and Ferguson (1943, The morphology of muscovite in relation to the crystal lattice: Univ. of Toronto, Geol. Ser., No. 48, 65-82). Hurlbut concluded that twins like this are actually penetration twins, rather than contact twins as Peacock and Ferguson thought.  

In any case the result is a remarkable star shape that really does not reflect the underlying monoclinic symmetry of muscovite.

The pegmatites around Itinga, including Jenipapo and nearby Taquaral, are attributed to collisions that assembled the cratons of Brazil, called the Araçuaí Orogeny (Pedrosa-Soares and others, 2009, Field Trip Guide, Eastern Brazilian Pegmatite Province: 4^th International Symposium on Granitic Pegmatites). This was about 600 to 500 million years ago (Neoproterozoic to Cambrian) and I’d call it an aspect of the Pan-African Orogeny that brought many small blocks together to form the supercontinent Gondwana.

The name muscovite is from Muscovy Glass, transparent sheet mica derived from Muscovy Province, Russia, where Moscow is located. The first use of the word as a mineral name was probably in 1794, although historical names we associate with muscovite, including glimmer and isinglass, were applied to other materials as well, including for isinglass, the pearly, flaky bladders of sturgeons that served as a flexible, semi-transparent window covering.  “Mica” is probably from smicka, ultimately from Latin micare, to flash or glisten.