Biotite cannot scratch glass on its own.
In this article, we’ll examine both materials in detail to get a better sense as to why biotite cannot scratch glass.
Can Biotite Scratch Glass? (EXPLAINED)
To understand why one material can scratch another, we generally start with the hardness and chemical makeup of a material.
Amateur geologists and rockhounds are familiar with the Moh’s scale of hardness, which assigns a value between one and ten to various materials.
The higher the number (closer to 10), the harder the substance is, and the less likely it is to be scratched by other materials.
In comparing biotite and glass, you have to compare the Mohs hardness of each substance.
Glass varies, but typically is found to have a hardness of a 5.5-7 (towards hardest to scratch), and biotite a 2.5-3 (towards easiest to scratch).
Since glass is generally harder than biotite, biotite does not scratch glass.
Let’s Learn More About Glass
Let’s start with glass and its history.
In addition to stone tools, our early ancestors also used obsidian, a naturally occurring glass formed by volcanic activity.
Fast forward to the dawn of civilization, humans began manufacturing glass, but in small amounts.
The ancient-Roman historian, Pliny the Elder, described the making of glass on the Phoenician coast (now modern Lebanon), and noted that the discovery of glass making came from Phoenician merchants.
However, further studies suggest that the actual origins of this material were in the Palestinian area, with the inventor’s name lost to history.
In addition to the Phoenicians, ancient Egyptians also took part in glass manufacturing.
Glass back then wasn’t as we know it today, as glass blowing hadn’t been invented yet, it could only be used for whetted beads, special bottles, or valuable treasures that only the very rich could afford.
Later on, the technique of glass-blowing came into the world via the Syrians around the 1st century BC, which was later followed up by the Romans when they spread the practice throughout their entire empire.
Could an early, simpler form of glass be scratched by biotite?
Considering how most early glass was made of silica sand, or silicone dioxide (SiO2), which has a Mohs hardness of 7, then the answer is no.
Then what about the earliest form of glass, obsidian?
When comparing the hardness of the two materials, obsidian is the harder substance, with a 5-6, and would not be scratched by the 2.5-3 that biotite has.
When we talk about modern glass, you could probably point out an example of where you’re sitting right now.
The screen on your phone, a nearby window, or even the light bulbs that illuminate your room, glass is just about everywhere in our daily lives.
However, some forms of modern glass are more common than others.
The type that has the largest amount produced in the world today is soda-lime glass, approximately 90% of all glass made.
Like the ancient jewels and treasures of our ancient past, silica sand makes up the majority of soda-lime, up to 71-75%.
Other materials are added to make it more malleable and transparent, such as the 12-16% sodium bicarbonate (Na2O) and 10-15% lime (CaO) content, with the addition of small amounts of dye to give the end result some color.
From food jars, sheets for glass tables, beer bottles, soda-lime is just about everywhere.
Other kinds of modern glass include crystal and borosilicate.
Like soda-lime, crystal glass has a high silicone dioxide content, around 54-65%, with the additions of 13-15% alkali oxide and other oxides as well.
This kind of material serves a more decorative purpose, taking forms such as fancy dishware, wine glasses, or even ashtrays.
Borosilicate also has a large amount of silica sand, about 70-80%, as well as 7-13% boron trioxide, 4-8% sodium and potassium oxide, and 7% aluminum oxide.
This species of glass is used in a lot of “heatproof” applications and anything requiring high resistance to chemicals such as pharmaceuticals.
Then could it be the case that certain modern types of glass can be scratched by biotite?
Again, the answer is no, as biotite’s 2.5-3 hardness could not damage the average 6-7 hardness carried by most glass used today.
Now it’s time to discuss our other material, biotite.
It is part of the phyllosilicate minerals, specifically the mica group, with the chemical composition (K(Mg,Fe)3AlSi3O10(F,OH)2).
Sometimes called “black mica,” biotite usually has a rich iron content, giving it dark colors such as greenish-brown, yellow, or even blackish-brown.
Most biotite is part of a group known as “sheet silicates,” a class of minerals that make up around 90% of the Earth’s crust.
Like most mica, biotite has a “highly perfect basal cleavage,” making it very easy to break apart into flexible sheets or lamellae.
Biotite can range from transparent to opaque, has a grey-white streak when dragged across an un-weathered surface, and a vitreous to pearly luster.
It typically forms within metamorphic and igneous rocks.
During the early days of research on the optical properties of mica, the name “biotite” was given in 1847 by J.F.L. Hausmann, a German mineralogist, after Jean-Baptiste Biot, a French Physicist.
To find this particular mineral, you could try your luck by exploring various pegmatite veins in North Carolina, Bancroft and Sudbury in Canada’s Ontario, or Virginia’s New England. Various places in Italy are known to have it as well, such as in dry lava flows of Mount Vesuvius or even the western Dolomite mountains.
In terms of modern-day usage, biotite is very useful in determining the ages of rocks, specifically through argon-argon or potassium-argon dating methods.
However, seeing that argon can escape very easily from biotite crystal structures under extreme temperatures, such methods might not be as effective in determining rock ages as other dating techniques.
Biotite is also used to assess the historical temperatures of metamorphic minerals, as the partitioning of magnesium and iron between garnet and biotite is quite sensitive to temperature.
Like several other micas of today, biotite is just too soft to scratch glass, ancient or modern.