KMg3

KMg3(AlSi3 Oio) (0H)2 —contain sheets in which every fourth silicon atom is replaced by an aluminum atom. Because of the planar arrangement of its aluminosilicate network, mica is easily broken into flakes. Figure 9-19 includes a photograph of mica. 

Examples are the mica group (e.g., KMg3(OF[)2Si3A10io) of which biotite, (K(Mg,Fe)3(OH)2Sl3A10io) (Figure 1.55) and talc (Mg3(OFl)2Si40io) are members, and contains a sandwich of two layers with octahedrally coordinated cations between the layers and clay minerals such as kaolin, Al4(OH)8Sl4C3io. 

The potassium ions are located between the flat aluminosilicate sheets (Fig. 7.4). Crystals of micas cleave easily parallel to the sheets, and the thin transparent flakes can be used for electrical insulation (e.g., in capacitors) or as furnace windows. Phlogopite, KMg3(OH)2[Si3A10io], has a similar structure but with Mg2+ in octahedral environments instead of Al3+. 

More complex (and more common) structures result when some of the Si4+ ions in silicates are replaced by Al3+ ions to form the aluminosilicates. The missing positive charge is made up by extra cations. These cations account for the difference in properties between the silicate talc and the aluminosilicate mica (Fig. 14.44). One form of mica is KMg3(Si3AlO10)(OH)2. In this mineral, the sheets of tetrahedra are held together by extra K4 ions. Although it cleaves neatly into transparent layers when the sheets are tom apart, mica is not slippery like talc. Sheets of mica are used for windows in furnaces. 

Type II (TOT) silicates include pyrophyllite, AlSi2Os(OH) (Section 10.3.25) and talc, Mg3Si40io(OH)2 (Section 10.3.24). Muscovite, KAl2(AlSi3)Oio(OH)2 (Section 10.3.27), and phlogopite, KMg3(Si3Al)Oio(F, OH)2 (Section 10.3.28) are Type II minerals with K+ ions in interlayer sites. 

The Crystal Structure of Phlogopite [KMg3(Si3Al)Oi0F2]... 

KMg3(OH)2Si3A10io phlogopite KAl2(OH)2Si3A10io muscovite... 

Aluminosilicates form when a Si ion is replaced by Af plus additional cations for charge balance. One example is mica, KMg3(Si3A10 o)2(OH)2. The extra cations hold the sheets of tetrahedra together, accounting for the hardness of these materials. Feldspar is a silicate material in which more than half the silicon is replaced by aluminum. 

For smaller angles a synthetic fluorphlogopite, KMg3[Si3A10io/F2], is available as SRM 675 with i/ooi = 9-98104(7) A at 25 °C (Table 4.9). This mica material should possibly be oriented parallel to the sample surface (strong... 

Muscovite KAl2(AlSi30,o)(OH)2 Phlogopite KMg3(AlSi30,o)(OH)2 Biotite K(Mg,Fe)3(AlSi30,o)(OH)2... 

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