General text about the lack of minerals can be found here.
Silicon is a chemical element that occurs in our body only in trace amount. However, it has significance for proper bone metabolism, it affects the quality of a newly formed connective tissue, it slows the aging process (helps to prevent the oxidative stress) and protects the cardiovascular system. Silicon in the diet is present rather in plants; higher content of silicon is for example in apples, cereals, nuts, oranges, onions and carrots. Daily recommended intake of silicon in adult humans is estimated to be about 15 milligrams.
Silicon deficiency is visible in deterioration of the skin, hair and nails. The hair becomes brittle, it loses its shine and falls out, and the nails are brittle as well. The skin becomes thinner and the person suffers from slower wound healing. As silicon acts against the oxidative stress, its deficiency may accelerate the atherosclerosis of the arteries and increase the risk of diseases of the cardiovascular system. Bone metabolism is affected by disrupted mineralization and by osteoporosis. The bones are weaker and they break more easily. There have been also observed unspecific symptoms such as fatigue, loss of appetite and mood swings.
Lack of silicon can be solved by increase of the dietary intake or its administration in food supplements. However, it is not advisable to use too high doses as it is possible to overdose.