Geology Q&A

What do you call a mineral that acquires a charge when it is compressed, twisted, or distorted?

Piezoelectric crystals can generate an electric charge with the application of pressure or vibrations. By putting piezoelectric material under mechanical stress, positive and negative charges in the material move to opposite sides, which causes an external electrical field. Conversely, they can change physical dimensions with the application of an electric field.

Quartz is the most popularly used substance of this group of minerals. Industrially, it is used for its piezoelectric properties in transducers, e.g., phonograph cartridges, microphones, and strain gauges, which produce an electrical output from a mechanical input, and in earphones and ultrasonic radiators, which produce a mechanical output from an electrical input. Other piezoelectric crystals, though less effective, include tourmaline, kyanite, labradorite, and turquoise.


What is causing the ongoing eruption of Mount St. Helens in Washington state?

A convergent plate boundary (subduction zone). Convergent plate boundaries come in several flavors, but they share one thing in common -- plate collisions!

In a contest between a dense oceanic plate and a less dense, buoyant continental plate, guess which one will sink? The dense, leading edge of the oceanic plate actually pulls the rest of the plate into the flowing asthenosphere, and a subduction zone is born! Where the two plates intersect, a deep trench forms.

Geologists aren't sure how deep the oceanic plate sinks before it completely melts, but we do know that it remains solid far beyond depths of 100 km beneath the Earth's surface.

When the subducting oceanic plate sinks deeper than 100 kilometers, huge temperature and pressure increases make the plate "sweat." Well, not exactly, but the uncomfortable conditions force minerals in the subducting plate to release trapped water and other gasses. The gaseous sweat works its way upward, causing a chain of chemical reactions that melt the mantle above the subducting plate.

This hot, freshly melted liquid rock (magma) makes its way toward the surface. Most of the molten rock cools and solidifies in huge sponge-like magma chambers far below the Earth's surface. Large intrusive rock bodies that form the backbones of great mountain ranges such as the Sierra Nevada form by this process.

Some molten rock may break through the Earth's surface, instantly releasing the huge pressure built up in the gas-rich magma chambers below. Gasses, lava and ash explode out from the breached surface. Over time, layer upon layer of erupting lava and ash build volcanic mountain ranges above the simmering cauldrons below.

An example of this kind of convergence produces the spectacular volcanic landscape of the Northwest. Off the coast of Oregon, Washington, Alaska and Canada a small plate, the Juan de Fuca, dives beneath North America. This type of convergent plate boundary, called a subduction zone, is known for producing historic earthquakes of great magnitudes (and the current eruption of Mount St. Helens).

~from U.S. Geological Survey.


What is "Venus hair" -- and does it come from outer space?

Venus hair is the name for needlelike crystals of reddish-brown or yellow rutile, forming tangled swarms of inclusions in quartz.

Rutile is a common accessory mineral, made up of titanium and oxygen. When found within other minerals, the mineral is said to be "rutilated", as in our rutilated quartz points. Rutilated quartz may also be called Venus-hair stone or sagenite. Metaphysically, it is said to intensify the power of the quartz crystal and provide insights into getting to the root of a problem.


What do we mean when we refer to a rock or mineral's habit? Is the mineral trying to give up smoking?

Habit (ha-BIT) : the characteristic crystal form or combination of forms of a mineral (Dictionary of Geological Terms). This is the shape or form in which a mineral naturally "grows," such as crystalline (quartz), rhombohedral (dolomite), cryptocrystalline (dioptase), acicular (goethite), concretionary (turquoise), tabular (staurolite), or platy (bismuth).

Many minerals may be found in more than one habit, depending on the conditions when they formed. For example, galena may be found in euhedral crystals or in a massive/granular form, and malachite is usually botryoidal (meaning grape-like), but may also be fibrous or stalactitic.


What terms are used by geologists to describe the appearance of a mineral, and what do they mean?

Luster (LUS-tur) : the appearance of a mineral in reflected light. A mineral is often labeled with more than one descriptive term, and lusters may be combined (e.g., adamantine-metallic, resinous-greasy) for minerals that don't fit clearly into one category.

metallic (having the brilliant appearance of metal) -- pyrite, hematite (polished)
vitreous (having the luster of glass) -- quartz
resinous (appearing like resin) -- smithsonite, amber
pearly (having the iridescent appearance of a pearl) -- lepidolite
greasy (appearing to be covered with a thin layer of oil) -- talc
silky (looking like silk) -- serpentine
adamantine (having a hard brilliant luster) -- diamond
waxy (having the appearance of wax) -- turquoise
earthy (looking like dirt or dried mud) -- hematite (rough)
fibrous (appearing to have fibers) -- malachite
dull (having a non-reflective surface) -- tektite, goethite


What do you call a mineral that steals another mineral's form, but replaces the previous rock's composition with its own chemical compound?

A pseudomorph is a mineral compound resulting from a substitution process in which the appearance and dimensions remain constant, but the mineral which makes up the chief component of the compound is replaced by another. The name literally means false form. (definition from Wikipedia)

There are three kinds of pseudomorphs:

  • A paramorph (also called allomorph) is a mineral changed on the molecular level only. It has the same chemical composition, but with a different structure. The mineral looks identical to the original unaltered form. This occurs in the aragonite to calcite change.

  • An infiltration pseudomorph is when one mineral or other material is replaced by another. The original shape of the mineral remains unchanged, but color, hardness, and other properties change to those of the replacing mineral. This occurs in petrified wood when silica (either quartz or opal) replaces the wood.

  • An incrustation pseudomorph results from a process by which a mineral is coated by another and the encased mineral dissolves. The encasing mineral remains intact, and retains the shape of the original mineral or material.


What is the common name for the cryptocrystalline quartz that fills fissures, seams and cavities in rocks, replaces wood fibers in petrified wood, is found at the center of geodes, and may be red, white, blue, grey, green, brown, purple, pink, black or clear?

Chalcedony (kal-SEHd-nee) : a translucent grey or milky-colored quartz having a crystalline structure and a waxy luster ( Of uncertain origin -- from the Greek khalkedon, a mystical stone, perhaps originating with Chalcedon, a port in Turkey where the first deposits were found.

While the dictionary definition makes this seem fairly simple, we actually use the term chalcedony for a huge range of minerals with the same chemical composition as quartz, but with more impurities. The impurities result in a wide range of colors -- iron oxides result in a clear red chalcedony is known as carnelian or sard; a green variety colored by nickel oxide is called chrysoprase; a bright to emerald-green chalcedony with drops of red jasper is known as bloodstone or heliotrope; when layered, we call it onyx. Chalcedony also may be used as a catch-all synonym for agate or jasper.


What's the proper name for the cat's eye effect found in tigereye, cat's eye chrysoberyl, fiber optic glass, and other rocks and minerals?

Chatoyant (shuh-TOY-ent) : having a changeable luster or color with an undulating narrow band of white light. From the French, chatoyer: to shine like a cat's eyes.1

Alternately, gemstones that show a star pattern of light (like star sapphire or star diopside, a.k.a. "Star of India") are called asteriated, from the Greek for "starry."

1From Merriam-Webster Dictionary online.

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