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Mica: mirrors, reflections and light

I have a few crystals in my collection that contain mica in various forms, and so I thought it might be useful to spend some time looking at the different types of minerals that belong to the Mica group to gain an understanding of quite a complex stone group. The make up of a crystal gives us a good insight in to how a crystal can assist in therapeutic healing. I know many therapists will choose crystals by intuition, myself included, but it is also useful to have an understanding of why a crystal will help with a certain situation. I hope the study will be useful for other like minded souls.

Mica is a common group of hydrous potassium, aluminium, silicate minerals and is found in igneous, sedimentary and metamorphic rock. It is a phyllosilicate, (phyllo is Greek for leaf) or sheet silicate, forming in thin parallel sheets of silica, enriched with potassium and aluminum. Mica belongs to the monoclinic system, but has a tendency towards pseudohexagonal structure, and has a perfect cleavage and a vitreous or pearly lustre. It has a Mohs hardness of 2.5-3, and its colour can range from clear, grey, white, green, lilac, brown, and black. The colour of mica is determined by the amount of aluminium, iron and other chemical elements it contains. These chemical elements also determine the particular identity of the mica.

There are 28 known micas but as that would fill a book, I am going to write about the most common and visually identifiable ones. The two sub groups are categorised as dark and light.

Biotite - k(Mg,Fe)3(AlSi3)10(F,OH)2

There are various micas that come under the biotite umbrella, but they are hard to distinguish without being tested. Biotite, also called black mica, is abundant in igneous granite and pegmatite rocks, and metamorphic gneiss and schist rocks. It is rare in sediments as it alters easily during chemical weathering. It is named after the French chemist and physicist Jean Baptisite Biot (1774-1862), who drew attention to the optical differences in micas. Biotite contains iron and magnesium, which gives it its black, or dark to red brown colouring. it often grows amongst other minerals, such as quartz, tourmaline, beryls and feldspar. Biotite is dichroic, meaning it displays various colours and shades, depending on light absorbency. It is translucent to opaque.

Phlogopite - KMg3Si3AlO10(F,OH)2

Phlogopite is a type of biotite. Rich in magnesium, the name phlogopite is from the Greek, phlogopos, meaning ‘fire like’, and is such named because of its fiery golden red colour. It can also be yellow or green. Like biotite it is dichroic, displaying different colours at different angles due to the varying light absorbency. It will form in igneous and metamorphic rocks, and is found in carbonates and marbles. Phlogpite is heat resistant and is a poor conductor of heat. Tiny flakes of phlogopite have been mistaken for gold by newbies to gold panning due to the bright flash of bronze light when the sun hits the flake.

Muscovite - KAl2(Si3Al)O10(OH,F)2

Muscovite is common in silica rich igneous rocks of granite and pegmatite, sedimentary sandstones and shales, and metamorphic schists, shales and clay. The most common member of the mica group, and also known as white mica, muscovite is named after the former Russian province of Muscovy, where glass panes were once made out of large transparent sheets of muscovite mica. When held up to the light muscovite is transparent and sometimes colourless, but will often display a tint of yellow, brown, rose or green colouring. The colouring is due to small traces of other elements seeping in, such as magnesium, iron, lithium, chromium, or vanadium. When the trace element is enough to record, it is defined under another name, such as lepidolite or fuchsite. (see below).

Lepidolite - K(Li,Al3)(AlSi3)O10(OH,F)2

Lepidolite is rich in lithium and is either pink, red or purple in colour. However, lithium is not the cause of Lepidolite’s colour. The colouring is believed to be caused by small traces of manganese seeping in and substituting aluminum. Manganese is not listed in the chemical formula, most probably due to the minute amount present. Lepidolite is considered to be quite rare as it will only form in lithium rich environments, and during the final stages of crystallisation, due to the lithium ion being very small. Lepidolite forms mostly in pegmatites and hydrothermal quartz veins. It will often form in association with elbaite tourmaline, quartz, topaz, beryl and a variety of feldspars.