Minerals with silky luster are those with a shine that resembles satin or silk. Common examples include chrysotile, shattuckite, satin spar, adamantine spar, cyanotrichite, and mongolite.
Luster is nothing other than how minerals, rocks, or crystals reflect light quantitively or qualitatively. It describes how they will appear under reflected light. We use various terms to describe the appearance, including metallic, submetallic, greasy, resinous, adamantine, silky, pearly, vitreous, and dull or earthy.
Discover more about silky minerals, including what causes the shine. We will also give you some of the five common examples.
What is a silky luster?
A silky luster describes minerals, rocks, or crystals that reflect light like satin or silk, i.e., they have a shine that resembles that of silk or satin cloth or skein.
Most of the silky minerals will have minute fibrous structures, i.e., they tend to have tiny fibers arranged in a parallel manner, and if large, we describe such minerals as fibrous. Some may show this shine on certain cleavages or in fibrous crystal aggregate forms.
Also, minerals that display chatoyancy or cat’s eye effect may have a concentrated, narrow, wavy, silky band of reflected light or sheen that seems to move on the curved surface when cut correctly in cabochons. This optical phenomenon is due to minute parallel fibers, inclusions, or cavities.
Examples of minerals with a silky luster
Common examples of these minerals include:
1. Satin spar
Satin spar often refers to the translucent, whitish, or grayish-green fibrous variety of gypsum with a silky luster. When appropriately cut, en cabochon, satin spar shows chatoyancy or the cat’s eye effect. However, it can refer to fibrous, silky forms of calcite and aragonite.
2. Shattuckite
Shattuckite (Cu5(SiO3)4(OH)2), a copper silicate hydroxide, is a bluish or turquoise mineral with a silky luster but may be dull at times. It has a Mohs scale hardness of 3.5, a specific gravity of 4.1, and a refractive index of 1.752 to 1.815.
This rare silicate copper often forms a pseudomorph after malachite and sometimes serves as a gemstone.
3. Adamantine spar
Adamantine spar is a brownish corundum (a rock-forming crystalline aluminum oxide that often has trace elements of vanadium, iron, chromium, and titanium) with a silky luster and a Mohs scale hardness 9. The other primary corundum minerals are the adamantine to vitreous sapphire and ruby.
4. Chrysotile
Chrysotile (Mg3(Si2O5)(OH)4) or white asbestos is a translucent type of asbestos belonging to the serpentine’s subgroup phyllosilicates and not amphibole like others. This fibrous asbestos type has a silky luster, Mohs hardness of 2.5-3, specific gravity of 2.53, and a refractive index of 1.569-1.570.
Chrysotile comes in two crystal classes. The most common one is clinochrysotile with a monoclinic crystal class, while the other two, parachrysotile and orthochrysotile, are orthorhombic.
Like other asbestos, chrysotile is carcinogenic (cancer-causing) in the US and Europe, with Canada and Quebec governments ending their mining support in 2012.
5. Cyanotrichite
Cyanotrichite or lettsomite (Cu4Al2(SO4)(OH)12·2H2O) is a sky or azure blue mineral with a silky luster that forms a crystal aggregate of extremely fine fibers. Its Mohs scale hardness is 1-3, specific gravity 2.76, and a refractive index of 1.588 – 1.655. Some of the uses of cyanotrichite include micromounting and minor copper ore.
Besides the above, less common minerals with a silky luster include elyite, mongolite, foshagite, fraipontite, gonnardite, karlite, blatonite, kornelite, and laurelite. Others are nitrocalcite, wupatkiite, amianthus, microsommite, parachrysotile, rhodesite, boussingaultite, carpholite, and riversideite.
Besides the above, some minerals such as calcite, malachite, scolecite, ulexite, mesolite, and collinsite will show silky luster, but only if they are in fibrous form.
Many silky minerals often show other lusters
Luster is, without a doubt, an important optical property. However, it doesn’t have well-defined boundaries. Also, you will find many minerals whose specimens show different kinds of luster.
For instance, there are very many vitreous to silky minerals. Examples include prehnite, cummingtonite, scolecite, ulexite, crocidolite, pectolite, biehlite, tremolite, crocidolite, and erionite. Others are mordenite, riebeckite, inesite, tobermorite, artinite, prosperite, dawsonite, and langite. The list goes on.
Also, some minerals like uranopilite, shattuckite, cuprosklodowskite, odinite, and rutherfordine have a silky to dull or earthy shine, while sericite, aurichalcite, isokite are silky to pearly. Ludwigite is silky to submetallic, cesanite silky to greasy, and hewettite adamantine to silky. Still, you will find many minerals with several of the luster kinds.
Thus, luster isn’t a good diagnostic property of minerals. Nonetheless, it is important, especially when used with other properties.
References
- Anthony, J. W., Bideaux, R, A, Bladh, K. W., & Nichols, N. C., (Eds.) (2022). Handbook of mineralogy, Mineralogical Society of America, Chantilly, VA 20151-1110, USA. http://www.handbookofmineralogy.org/
- Manutchehr-Danai, M. (2009). Dictionary of gems and gemology (3rd ed.). Springer.
- Rutley, F. (1988). Rutley’s elements of mineralogy (26th ed.). Unwin Hyman. https://doi.org/10.1007/978-94-011-6832-8
- Okrusch, M., & Frimmel, H. (2020). Mineralogy: An introduction to minerals, rocks, and mineral deposits. Springer. https://doi.org/10.1007/978-3-662-57316-7
- Cairncross, B., & Mccarthy, T. S. (2015). Understanding minerals & crystals (1st ed.). Struik Nature (An Imprint of Penguin Random House South Africa (Pty), Ltd.
- Perkins, D. (2014). Mineralogy (3rd ed.). Pearson.
- Mccarthy, T. (2015). Understanding Minerals & Crystals. Struik Publishers (pty) Ltd.