Profile of Mica

Mica is an inorganic material found in a wide variety of cosmetic and personal care products. Its use has been reported in more than 7,100 products,¹ making it one of the most important mineral ingredients used in cosmetics. Mica is associated primarily with the formulation of color cosmetics, as it is a critical component of hundreds of metal oxide-based inorganic pigments that provide a diverse palette of colors and optical effects.^{2, 3} Its utility as an optical modifier and tactile modifier have also led to its incorporation in a wide variety of skin and hair care products^.2

Chemistry and Manufacture

The term mica refers broadly to a group of closely related hydrous aluminum silicate materials of varying chemical composition; Table 1">Table 1 lists the chemical formulas of some of the most common types of mica.^{4, 5} Although mica is a naturally occurring mineral, it should be noted that synthetic micas have been produced and are utilized in cosmetics.⁶ According to INCI convention, the name Mica only applies to the naturally occurring mineral, and man-made micas are designated as synthetic minerals, e.g., Synthetic Fluorphlogopite. The present article will focus on naturally occurring micas that conform to the INCI definition for mica. The general chemical formula for most micas is W(X, Y)_2–3Z₄O₁₀(OH, F)₂. For the most common mica types, W is typically potassium (K⁺), sodium (Na⁺) or calcium (Ca⁺); the X, Y site is occupied by any two ions of aluminum (Al³⁺), magnesium (Mg²⁺), iron (Fe^{2+, 3+}), or lithium (Li⁺); Z is mainly silicon (Si⁴⁺) or Al³⁺; and the hydroxide (OH^-) ions may be partially or completely replaced by fluoride (F^-) ions. The most commonly occurring and commercially important types of mica are muscovite and phlogopite; mica-based cosmetic ingredients rely primarily on the muscovite form.

The general chemical formula for most micas is W(X, Y)2–3Z4O10(OH, F)2. For the most common mica types, W is typically potassium (K+), sodium (Na+) or calcium (Ca+); the X, Y site is occupied by any two ions of aluminum (Al3+), magnesium (Mg2+), iron (Fe2+, 3+), or lithium (Li+); Z is mainly silicon (Si4+) or Al3+; and the hydroxide (OH-) ions may be partially or completely replaced by fluoride (F-) ions. The most commonly occurring and commercially important types of mica are muscovite and phlogopite; mica-based cosmetic ingredients rely primarily on the muscovite form. Micas belong to a group of minerals known as phyllosilicates, or sheet silicates, which are characterized by well-defined parallel sheets of interconnected silica tetrahedral with a basic structural unit of Si2O5; however, in mica, one-fourth of the Si atoms are replaced by Al. In mica, the basic sheet structure comprises two tetrahedral (t) silica layers separated by an octahedral (o) silica layer, as shown in Figure 1">Figure 1.

The X and Y cations are located in the o layer, and the W cations occupy sites between pairs of the anionic t-o-t silicate sheets. These lamellar units extend indefinitely in the a and b dimensions and are stacked in the c dimension. The stacked lamella are only weakly attracted to each other on the molecular level by van der Waals interactions between adjacent t layers, allowing crystalline mica to be easily cleaved in the a-b plane to yield thin, perfectly flat sheets.

Mica is an abundant mineral found in a wide variety of geological formations and rock types including clay deposits, granite, pegmatite and schist.⁷ Mica is mined in two principle forms, sheet mica and flake or scrap mica, i.e., the scrap material from sheet mica production. Sheet mica is isolated from large pockets of mica crystals as “books” that can be cleaved into flat sheets having areas ranging from 4.5 cm2 to 645 cm2 and thicknesses ranging from 0.03 mm to 0.2 mm. It is produced in relatively smaller volumes and employed primarily in the fabrication of electronic devices and optical components. The higher volume flake mica is used to produce the ground mica that is utilized in a diverse range of products including joint compounds, plastic composites, drilling muds, paints and coatings, pigments and cosmetics. Among the leading producers of flake mica are China, Russia, Finland, the United States and South Korea.

Flake mica is typically obtained by open-pit mining methods, which yield crude rock ore. Mica is recovered from this crude ore along with other minerals such as kaolin, quartz and feldspar.5 The ore undergoes a number of processing steps collectively referred to as beneficiation to isolate and purify the mica fraction.⁸ The exact processing schemes employed vary depending on the source of the mica-bearing ore and the desired mica properties but generally involve the following steps.

Initially, ore is crushed to a fine powder to liberate the various mineral components of the ore, and then it is slurried to form an aqueous mineral dispersion. This crude dispersion is deslimed and separated according to particle sizes of the dispersed solids using a variety of mechanical classifiers. Desliming involves the addition of process chemicals such as sodium silicate to disperse slimes of hydrated clays, e.g., kaolin, which interfere with processing operations.

The separated fractions are then subjected to froth flotation to isolate the mica flakes from the kaolin, quartz and feldspar byproducts. Froth flotation entails diluting and agitating the mineral slurries in solutions of surfactants under acidic, pH = 2.5–4.0, or alkaline, pH = 7.5–9.0, conditions to entrain the desirable mica fractions in the resulting foam or froth. The mica-laden froth is then separated, concentrated and dried to recover the mica flakes, while the byproducts may undergo further treatment and isolation steps for use in other applications.

Flake mica may be converted to ground mica by dry or wet grinding.^{4, 5} Wet grinding is typically employed to obtain the higher quality ground mica used in cosmetics. The wet process yields exceedingly flat mica flakes with small particle sizes, high aspect ratios and smooth edges. In wet grinding operations, mica flake is ground in the presence of 20–35% water, dewatered, dried and then screened on sieves to segregate the various particle size fractions prior to bagging. Micronization techniques may be employed to produce even more finely ground mica particles. In this milling process, mica particles are propelled into each other at high speeds using jets of superheated steam or compressed air, causing a grinding action that effectively reduces particle size and thickness.

Properties

Cosmetic-grade mica^9–11 is typically supplied as dry, free-flowing, white to off-white powder. These micas exist as thin platelets with exceptionally smooth, flat surfaces. The platelets typically exhibit particle size distributions ranging from 2–50 μm in diameter, with peak average values of 15–25 μm and thicknesses of 100–300 nm. The density of muscovite mica is 2.8–2.9 g/cm3; however, the bulk densities of ground mica powders tend to be significantly lower after milling, ranging from 0.1–0.3 g/cm3. Mica particles are hydrophilic and easily dispersed in aqueous media. Mica is generally resistant to chemical degradation except in the presence of strong acids or at extremely high temperatures; for example, muscovite mica is reported to be stable at temperatures up to 500°C.⁴

Mica is regarded as nontoxic due to its chemically inert nature. The US Food and Drug Administration (FDA) lists mica as a Generally Recognized As Safe (GRAS) indirect food additive, such as a colorant or filler for polymers used in food-contact applications.¹² Mica is also exempt from batch certification by the FDA when used as a colorant for drugs and cosmetics, provided it meets the requirements listed in 21 CFR 73.1496.¹³

Technology and Applications

Perhaps the most important role of mica in cosmetics is its use as a substrate in metal oxide-on-mica pigments, which are used to provide brilliant color and visual appeal to products both in the package and upon application. The primary use of these pigments is in color cosmetic applications; however, mica-based pigments may be added to a variety of rinse-off and leave-on formulations to give the products a shimmering pearly appearance.

The utility of mica in manufacturing such pigments is quite evident based on the hundreds of mica-based colorants listed as trade name mixtures in the Personal Care Product Council’s On-Line INFOBASE.¹⁴ By varying the type, thickness and number of nm-scale titanium dioxide or iron oxide layers deposited on the surfaces of mica platelets, pigment manufacturers are able to achieve distinct colors and unique optical effects, such as pearlescence and color travel.^{3, 15} Ground mica is an ideal substrate due to its platelet morphology, lack of color and relatively low refractive index (RI)—for muscovite, RI = 1.5–1.6—as the optical performance of these so-called interference pigments is highly dependent on the difference in RI between the mica and the metal oxide.

In addition to its function as a colorant, mica provides a host of other benefits in color cosmetic products.^9–11 The platelet morphology of mica particles imparts a silky feel to leave-on products, from emulsions to powders, and it can act as a texture enhancer. Mica can be used as an optically neutral filler in loose and pressed powders, where it also functions to improve payoff, enhance skin adhesion and minimize caking. Due to its small particle size, mica is also reported to reduce the appearance of wrinkles by filling in fine lines on the skin.

It can be difficult to obtain uniform dispersions of mica in nonaqueous media, such as hydrocarbon or triglyceride oils, esters and silicone fluids, due to the hydrophilic nature of the mica surface. To overcome this problem, surface-treated micas have been developed to offer improved compatibility with these hydrophobic liquids.^{10–11, 16–17} Examples of agents used to treat mica include carnauba wax, lauroyl lysine, jojoba esters, magnesium myristate, methicone, perfluorohexyl ethylphosphates and combinations of isopropyl titanium triisostearate and triethoxyoctylsilane.

Surface treatments render the mica surface hydrophobic and enable dispersion in a wide range of nonpolar fluids. This variety of surface-modified micas provides formulators with a great deal of flexibility for utilizing mica and offers the ability to provide subtle changes in skin feel based on manipulation of the mica surface chemistry and its dispersion in the product matrix.

References

1. Compilation of Ingredients Used in Cosmetics in the United States, 1st edn, JE Bailey, ed, Personal Care Products Council, Washington, DC (2011)
2. Mica, Monograph ID 1634, in the International Cosmetic Ingredient Dictionary and Handbook, 14th edn, Personal Care Products Council, Washington DC (2012)
3. N Horiishi et al, Specialty pigments, ch 5 in Industrial Inorganic Pigments, 3rd ed, G Buxbaum and G Pfaff, eds, Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim, Germany (2005) pp 195–295
4. RJ Benbow, BHWS De Jong and JW Adams, Mica, in Ullmann’s Encyclopedia of Industrial Chemistry, vol 23, B Elvers, ed, published online: Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim, Germany (Jun 15, 2000) pp 131–143
5. JT Tanner, Mica, in Kirk-Othmer Encyclopedia of Chemical Technology, published online: Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim, Germany (Dec 4, 2000) pp 1–20
6. US 5094852, Synthetic mica powder, manufacturing method thereof and cosmetics having the synthetic mica blended therein, K Ohno et al, assigned to Toby Kogyo KK and Shiseido Co Ltd (Mar 10, 1992)
7. JC Willet, Mica (natural), Mineral commodity summaries, US Geological Survey, published online at https://www.usgs.gov/centers/national-minerals-information-center/mineral-commodity-summaries(Jan 2012)
8. JS Browning, Mica beneficiation, Bulletin 662, US Bureau of Mines, Washington, DC (1973)
9. RonaFlair Functional Fillers, Rona Product Brochure US W503171, EMD Chemicals Inc, Gibbstown, NJ (Jun 2009)
10. KoboMica S-25, Technical Bulletin KMS25-001, Kobo Products Inc, South Plainfield, NJ (Jan 29, 2010)
11. Mearlmica Mica-Based Performance Minerals, in BASF BeautyCare Ingredients, Product Portfolio 2009, BASF Corp, Florham Park, NJ (2009)
12. Indirect food additives, Code of Federal Regulations, 21 CFR 176.170, 177.2600 and 178.3297, (Apr 1, 2011)
13. Listing of color additives exempt from certification: Mica, Code of Federal Regulations, 21 CFR 73.1496 and 73.2496 (Apr 1, 2010)
14. Mica, trade name mixtures, Personal Care Products Council On-Line INFOBASE, https://online.personalcarecouncil.org/jsp/IngredientDetail.jsp?monoid=1634 (Accessed Jul 2, 2012)
15. P Linz and Q Peng, Color-travel cosmetic pigments: Interference to the max, Cosm Toil 118(12) 63–66, 68, 70 (2003)
16. US 20100136065A1, Natural ester, wax or oil treated pigment, process for production thereof, and cosmetic made therewith, D Schlossman and Y Shao, assigned to Kobo Products Inc (Jun 3, 2010)
17. US 20100003290A1, Oil and water repellent cosmetic powder and methods of making and using the same, D Schlossman and Y Shao, assigned to Kobo Products Inc (Jan 7, 2010)