Margarite

Hand Specimen identification: Margarite is characterized by its micaceous nature, brittleness, and pearly luster. Association and brittle-ness usually distinguish it from other white micas.

Chemical Composition : CaAl2(Al2Si 2O10)(OH)2

Hardness : 31 2 to 5

Specific Gravity3.1

Cleavage/Fracture : perfect basal (001)/uneven

Luster/Transparency : vitreous/transparentto translucentvitreous/transparentto translucent

Color : gray-yellow, pinkish silver

Streak : white

Optical Properties : Margarite is colorless in thin section, resembling muscovite and other white micas. It is distinguished by a higher index of refraction, lower birefringence, and a 6° – 8° extinction angle. Biaxial 1- 2, a = 1.635, b = 1.645, g = 1.648, d = 0.013, 2V = 45°.

Crystallography : Monoclinic, a = 5.14, b = 9.00, c = 19.81, b = 100.8°, Z = 4; space group C2/c; point group 2/m.

Habit : Margarite is typically found in massive, micaceous books and aggregates; individual crystals are rare.

Structure and composition : Margarite has a structure transitional between mus-covite and chlorite. Because the interlayer site is occupied by Ca2 + rather than K+, bonds between lay-ers are stronger; thus, margarite is more brittle. Minor Na+ and K+ may replace Ca2+; charge balance is maintained by replacement of Al3+ by Si4 +. Be, Ba, Sr, K, Mn, Fe, Mg, and excess OH may also be present.

Occurrence and associations : Margarite, typically found with corundum and dias-pore, is an alteration product of corundum.

Varieties :

Related Minerals : Margarite is a member of the brittle mica group. Other members include clintonite and xanthophyl-lite,1both2 having compositions Ca1Mg,Al22 – 31Al2Si22O OH . Margarite also shares similarities with 10 2 1 2 1 2 stilpnomelane, K Fe,Al 10Si12O30 OH 12.

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