Mica

The word Mica is thought to derive from the Latin word “micare”, meaning to shine, in reference to its glittering effect when exposed to light. The history of mica dates back to pre-historic times, as mica was known to the ancient Egyptian, Greek and Roman civilisations, and even in the Aztec civilization of the New World. Mica is a phyllosilicate mineral that exhibits an almost perfect basal cleavage. Mica is a group comprising almost 30 members that differentiate from each other, primarily, by atom substitutions or vacancies in the crystal lattice. From an industrial standpoint, very few are mined: Muscovite, white mica, is by far the most common, Phlogopite – a dark brown mica – comes in second. To a lesser extent, lithium mica such as Lepidolite find some interesting applications in our industries. Amongst platy minerals, micas are unique due to the broad range of particle sizes naturally available from microns up to several centimetres. Mica elementary crystals are threelayer platy minerals: they consist of two tetrahedral sheets and one octahedral sheet, so called Te-Oc-Te to describe the fact that 2 layers consisting of 3/4 [SiO4]- and 1/4 [AlO4]- tetrahedrons enclose the [M(O4,(OH)2)]-octahedron layer (M = Al for muscovite and Mg for phlogopite with the exception of a few substitutions, primarily Fe). This elementary sheet structure is approximately 10 angströms thick. The Te-Oc-Te structure has a slight negative charge that is compensated by inter-layered cations (in natural form, predominantly K+ ions) located in the intersheet region. The bonding strength between inter-layered cations and tetrahedral sheets, together with the steric effect, make the extraction of those cations almost impossible under standards conditions and gives all micas very high chemical and weathering stability. Under selected process routes, thorough delamination is achievable to the benefit of final applications where high levels of film reinforcement and/or barrier properties are expected. Depending on the nature of their origin, micas contain a variety of accessory minerals in addition to muscovite/ phlogopite. These minerals may include quartz, feldspar, kaolin and pyroxene. The presence of these minerals in conjunction with the mica contained in the ore will impact upon both the industrial value of the deposit and the process complexity, reducing or increasing its value depending on the application. Mica deposits are either mined for mica only, when matrix yield is high enough, or in association with other minerals such as kaolin/feldspar. When Mica is present in soft rock, it is beneficiated through wet processing, typically blunging, gravity separation and flotation. Mica is subsequently dried to attain a moisture content below 1%. In hard rocks, the preferred process is crushing, directly after drying, followed by sieving and magnetic separation. According to the final application, mica needs to be milled to the required particle size distribution and simultaneously delaminated to enhance its aspect ratio (particle diameter / thickness). This is obtained either through merely sieving (flakes form), or dry milling (into powder and micronised powder form) or wet milling for the most engineered grades with high smooth sheet surface and particle edges. Processor knowledge is critical in the balance of size reduction / delamination ratio. For special applications, mica can be further calcined or surface treated (grafted with organic functions or coated with iron oxide) in order to enhance specific attributes such as compatibility with polar polymer matrix.