Mineral Luster

Luster is how a mineral surface looks in reflected light - the quality and character of the light it sends back to the eye. It depends fundamentally on the balance between how much incident light is absorbed, how much is transmitted, and how much is reflected. Minerals dominated by ionic and covalent bonding are generally transparent, allowing most light to pass through; minerals with metallic bonding are opaque, reflecting or absorbing light at the surface. This difference in optical transparency is what produces the two broad luster categories: nonmetallic and metallic. ^[1]

Nonmetallic Luster

A mineral with nonmetallic luster may be brilliantly shiny, but it lacks the look of a metal because it is transparent - at least in thin pieces - and only a small fraction (as little as 5%) of the incident light is reflected off the surface. Most light enters the mineral and is refracted, absorbed, or transmitted. The quality and character of the nonmetallic luster depends on the mineral’s index of refraction and on surface texture. A higher index of refraction (slower light velocity) produces a more brilliant luster. ^[1]

The varieties of nonmetallic luster, in order of increasing brilliance, are: ^[1]

Vitreous luster - the luster of glass - is the most common nonmetallic luster because most silicate minerals have indices of refraction in the 1.5-2.0 range. Adamantine luster (diamond-like) is diagnostic of minerals with very high refractive indices, such as diamond itself, cassiterite, and sphalerite. Earthy and silky lusters arise from surface texture rather than intrinsic optical properties - earthy from fine grains that scatter light, silky from parallel fibres that reflect it in a directional sheen.

Metallic Luster

Minerals with metallic luster reflect light the way polished metals do and are typically opaque - even thin fragments do not transmit light. The proportion of reflected light determines the quality of the metallic luster: ^[1]

• Normal metallic (20-50% reflection): the familiar appearance of polished gold or steel. ^[1]
• Splendent (>50% reflection): an exceptionally strong, mirror-like metallic appearance. ^[1]
• Submetallic (<20% reflection): transitional toward adamantine nonmetallic luster. ^[1]

The specific wavelengths that are most strongly reflected determine the colour associated with the metallic luster - gold reflects red and yellow wavelengths most efficiently, giving its characteristic colour. ^[1]

Index of Refraction and Luster

The velocity of light in a mineral is expressed through its index of refraction (n) - the ratio of light velocity in a vacuum to light velocity in the mineral. The velocity of light in a vacuum is about 3 × 10¹⁷ nm/sec; in most minerals light travels at between 1.5 × 10¹⁷ and 2 × 10¹⁷ nm/sec, giving most minerals indices of refraction in the range 1.5 to 2. This range corresponds to the vitreous to adamantine bracket. ^[1]

References

1. Nesse, W. D. (2018). Introduction to Mineralogy, 3rd ed. Oxford University Press.

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