Sodalite and lapis lazuli

Sodalite can be confused with the rarer and more expensive lapis lazuli (shortened or casual version of this word is lapis), which is also blue. This post deals with how to tell them apart.

Sodalite is a mineral. It is named for its sodium content, consists of the elements sodium, aluminum, silicon, oxygen, and chlorine. It belongs to a group of minerals called the feldspathoids, which resemble feldspars but have a different crystalline structure, a much lower silica content (i.e., feldspathoids are never found in rocks congaing primary quartz), and contain sulfur or chlorine. Sodalite is an ornamental gemstone and is commonly used in jewelry or in making bookends, etc. It is best known for its blue color, but it can also be gray, yellow, green, and commonly mottled in color. It commonly has white veining. It rarely has inclusions of pyrite, and it is not opaque (thus light can transmit through its edges).

Bookends made of sodalite. They are 13 cm hight.

Other side of the bookends shown above.

A small piece of sodalite (5 cm maximum dimension) with a polished surface.

Sodalite has poor cleavage, therefore, it is useful for making carvings of animals. This mineral is commonly found as vein fillings in plutonic igneous rocks (such as nepheline syenites). Associated minerals are microcline, albite, calcite, fluorite, and baryte (barite). It is found in Canada (Ontario, Quebec, and British Columbia), as well as in Maine and Arkansas. 

Sodalite is a "poor man's lapis lazuli."

Lapis lazuli is a metamorphic rock. The most obvious and important  component of this rock is the mineral lazurite, a feldspathoid silicate mineral consisting of sodium, calcium, aluminum, silicon, oxygen, chlorine, and sulfur. It is the presence of sulfur that gives lazurite its intense deep blue color. Most lazurite also contains the minerals calcite (white), sodalite (blue) and sparkling pyrite, as well as small amounts of mica, hornblende, etc.  

The gem form of Lapis lazuli has been prized since antiquity for its deep-blue color. This rock has been mined for thousands of years in Afghanistan and Pakistan (note: "lapis" is an Arabic word). It is opaque, thus light does not transmit through its edges. Pyrite is commonly present, but in minor amounts.

A small piece of polished gem-quality lapis lazuli (3 cm maximum
 dimension). Notice the flecks of pyrite.

Flip side of the lapis lazuli shown above. Notice the vein of calcite
with some pyrite veinlets.

Lapis lazuli takes an excellent polish and can be made into jewelry, carvings, mosaics, ornaments, small statues, and vases. 

Crystals That Show Twinning

In my March 31, 2017 post concerning a distinctive granite, I showed an orthoclase crystal with twinning, which occurs when two separate crystals of the same substance share some of the same crystal lattice. Instead of a normal single crystal, the crystalline structure appears doubled. 

In this new post, I show some other common examples from my personal collection of twinned crystals. They are aragonite, pyrite, gypsum, quartz, and staurolite. 

aragonite (40 mm length)

gypsum (37 mm length)

pyrite cubes (47 mm length)

quartz (47 mm length)

staurolite (18 mm length)

pseudomorph of staurolite (40 mm)

pseudomorph of staurolite (40 mm length)

       
















pseudomorphs are formed
when a mineral is replaced
by a foreign substance

Pyrite cubes

Pyrite is a mineral that most people have either heard about or seen. It superficially resembles gold, yet the chemical and physical properties of pyrite make it easy to distinguish it from gold. The main differences are listed below:

Pyrite is an iron sulfide, with the chemical formula FeS2. Gold's chemical formula is simply Au.

Pyrite crystal system is isometric (cubic), and crystals formed under perfect conditions will be cubes (as shown below). Gold is rarely found as crystals; rather, it occurs in nuggets, irregular blobs, or small flakes. It cannot occur in cubes.

Single cube of pyrite, width 2.9 cm (from Spain).

Cluster of intergrown pyrite cubes, total width 5 cm long (from Spain).

Pyrite is harder with a value of 6.5 on the Moh's Hardness Scale [i.e., a scale with talc and graphite the softest minerals (value of 1), and with diamond the hardest mineral (value of 10). Gold has a value of 2.5. Gold is very soft; so much so that other elements (e.g., copper, nickel, or platinum) have to be added to it (in the form of an alloy) in order to make jewelry.

Note: An ordinary steel knife (hardness value of 4.5) cannot scratch pyrite but can easily scar gold. The superior hardness and brittleness of pyrite also cause it to smash into bits if struck with the tip of a high-quality knife or shatter into small pieces, if hit with a hammer.

Pyrite's streak (its powdered from when scratched across an unglazed porcelain plate, called a streak plate), is black. Gold's streak is brassy yellow.

Small irregular piece of pyrite with its characteristic black streak on a "streak plate.

Pyrite is less dense, and small flakes normally wash away when placed under running water. Gold flakes are very dense and will sink. This is why "gold panning" works so well for finding gold.