The Basics About Quartz Twinning

Quartz is one of the most common minerals on earth and also, one of the minerals that most of the general populace can recognize in various shapes and colors.  The word quartz comes from the word twarc, possibly originating from the polish twardy, which means “hard”.  Quartz is Moh’s hardness of 7, which makes it perfect for almost all jewelry uses.  Beads and carvings of quartz have existed for centuries.  The fact that you can find minorly weathered quartz downstream of a deposit miles and miles away, shows how tough it is.  Other stones turn to dust well before quartz!  Quartz is composed of Silicon and Oxygen, Si02, which is why it is found in such abundance around the world.

One of the more fascinating things about quartz is the crystal’s habits and structures.

CHLORITE in QUARTZ Crystal from Pakistan

Twinned quartz crystals have always been held in high regard as collectable mineral specimens for their rarity and unique aesthetics. Mineral collectors always go crazy over new finds of twinned quartzes. What they may not realize is the exact mechanism behind twinning in quartz and the true diversity of twinning types in this mineral. Most varieties of quartz twins are actually a bit difficult to spot and are also extremely rare. Although we call them twins, the answer is based more upon physics than genetics!

Before we understand the ways quartz crystals can twin, we have to understand the properties and a little of the crystallography of quartz itself. Quartz is an extremely unique mineral. Chemically it is made from silicon dioxide arranged in a framework of SiO4 tetrahedra connected at the corners.

the quartz unit cell is composed of a central tetrahedron of SiO4 surrounded by four other tetrahedra with the oxygens of the central SiO4 bridging between silicons. The diamond outline divides the outermost SiO4 tetrahedra in two because half of their oxygens serve as a bridge between the silicon in different unit cells. If you look closely at the unit cell of quartz, specifically at how the Si-O bonds are oriented towards and away from you, you will notice that the unit cell is asymmetrical. This property gives quartz its unique shape and electromechanical properties. If you look at a quartz crystal from the termination down, it may look hexagonal but is actually trigonal. It is very difficult to be able to determine this just by looking at the shape of the crystal- the structure of the unit cell and the fact that it itself is not hexagonal shaped is the basic reason why.

When you look at a cluster of ordinary quartz crystals, notice that the borders between the crystals are only physical contacts. There is no molecular connection between the individual crystals. Twinning on the other hand represents two or more crystals with a molecular connection between them. The mechanism behind it is actually quite simple and occurs across a boundary between unit cells called a twinning plane.

As a quartz crystal grows, SiO4 tetrahedra link up and the volume increases from one initial unit cell to many. Typically, unit cells of only one enantiomer will form a crystal. This doesn’t mean that cells of another enantiomer can’t be present- It is rare, but very possible for unit cells of two quartz enantiomers to bond to each other. When they do, twinning occurs. In the image above, we notice an array of blue diamonds next to each other with a small layer of red at the top. This array represents a very basic structure of a simple quartz twin. Let’s imagine a quartz crystal forms from a fluid and initially consists of the blue enantiomer. Later in the growth of the crystal, the red enantiomer starts crystallizing out of solution but instead of forming their own crystals, they bond to SiO4 tetrahedra present in the initial crystal formed from the blue enantiomer. This boundary between the red and blue unit cells is the twinning plane. The formation of the twinning plane usually forms very early in crystallization, but it can form later. This depends on many factors surrounding the thermodynamics of the crystal solution as well as properties of the crystal surfaces. As crystal growth progresses many different kinds of twinning can result from additional twinning planes forming. Many of the rarer kinds such as Dauphiné and Brazil twins have multiple twinning planes. Two or more types of twinning can even occur in the same crystal although this is extremely rare.

Japan law Twinning-

Japan law twins are the simplest and most recognizable twins. A Japan law twin is formed when a two crystals of opposite chiralities form contact twins with the twinning plane being derived from the very rare zeta form of the quartz crystal. They are the most recognizable form of twin and are known for their “mitten-like” shape composed of two quartz crystals twinned at approximately right angles. Some Japan twins can have very stout crystals and appear heart or nearly square shaped because of this. Originally, the Japan twin was known as the La Gardette twin for its discovery at the La Gardette Mine- a famous rock crystal and gold mine in the French Alps near the border with Italy. However, due to the abundance of these twins in Japan, they were renamed. Whether it was an intentional renaming, I am unsure but it is certainly more appropriate considering the plentiful nature of this kind of twin at some Japanese localities.

Japan law twinned quartzes are rare in general but have been recovered from many localities worldwide. Strangely enough, they are very uncommon from Australia. I have never seen a single example of Japan law twinned quartz from this continent. The following is a brief description of Japan law twin occurrences from worldwide localities continent by continent, but first with mention of their occurrence in their namesake locality- Japan.

Japan law twinned quartzes are regionally quite abundant in Japan in comparison to other international locations. Perhaps the most famous locality for these twins is “Suishodake” on Naru Island in Kyushu, Japan. Japan law twins have been found as especially well formed and clear examples to 3cm in large quantities. They occur in simple low temperature quartz veins in a tertiary sandstone with sparse pyrite. Quartz crystals from here are rarely untwinned- this is highly unusual, as many localities known for Japan twins produce them as only a rarity. The reason for the high proportion of these twins at Naru island is unknown. Sometimes, they even have additional Dauphiné or Brazil twinning too as indicated by surface etching. They are of diverse morphology but are typically tabular and occur in interlocking groups of up to six crystals. Unfortunately, collectors at this locality have caused so much damage to the surrounding landscape that the site has been closed and no more examples will be recovered.

The Otome Mine in Kofu City, Yamanashi Prefecture, Honshu, Japan also has produced fine Japan twins quite similar to those on Naru Island. This locality is also famous for unusual ferberite pseudomorphs after scheelite similar to specimens found in Trumbull, Connecticut and Rwanda.

Japan-Law twins are typically found most abundantly in hydrothermal vein deposits and several locations in both North and South America have produced fine crystals. In the US, Denny Mountain in Washington and Mina Tiro Estrella in New Mexico have both produced some large and striking examples- perhaps the finest in the US. At Denny Mountain, they typically form as twins of large flattened crystals with a twinning face that is typically small compared to the c-axis of the crystal; they are most often L-shaped but more tabular examples similar to the specimens from Naru Island have been found too. Although Denny Mountain is most famous for its amethysts, to my knowledge no Japan twinned amethysts have been found at this location. Several other locations in Washington have produced nice examples as well.

Although the US does not have an abundance of locations producing Japan law twins in great volume, the various hard to access localities for smoky quartzes in Lincoln County, New Mexico have produced some of the finest examples in the world. In addition to being very nicely formed, they are also smoky and sometimes intensely so. Mina Tiro Estrella in the El Capitan Mountains has produced the best specimens in this area. They are typically almost square, deep smoky tabular twins approaching a tremendous size of about 10cm! The quartz here occurs in a very simple crumbly vein of quartz and feldspar hosted in alaskite. In the 1980s, many crystals with an unusually deep color were marketed by Dick Jones. The color was so intense compared to previous examples that some people have questioned it is natural.

Fine Japan law twins in the US have also been found near Hot Springs, Arkansas where they are transparent and quite rare but sometimes sizeable, and at Washington Camp, Arizona where they can reach around 15cm in width.

Not many fine Japan law twinned quartzes have been found in Canada, but some nice examples have been recovered from Harrison Lake, British Columbia. These twins are smaller and somewhat resemble those from Japan but are less abundant.

In South America, the finest Japan twin quartzes come from a plethora of locales in Peru and Bolivia. In Bolivia, the Siglo XX mine has produced some very nice well formed Japan law quartz crystals. This famous mine is more popular for its tin production, fine cassiterite, rare phosphates, and historical massacre of workers via General René Barrientos, but has produced a fair amount of Japan law twinned quartzes. They end to be very clear and are more spindly than others, which makes sense considering the needle-like shape of many of the quartzes from this mine. They occur in hydrothermal veins associated with cassiterite, sulfides, sulfosalts and phosphates. They may not be the most aesthetic Japan law twins in my opinion, but they are very important for Bolivia. Specimens are often associated with yellow wavellite.

Peru has many localities for Japan law twinned crystals, most of which are mines that produce metallic sulfides. They typically are similar to the Bolivian crystals in form and occurrence. Good specimens from the Mundo Nuevo mine are well known and are typically thin and elongated in form although more tabular crystals are found too. The Pasto Bueno district more famous for the world’s best hubnerite crystals has produced some fine, colorless Japan law twins as well.
The finest are from the Rosario Mabel claim in the Huancavelica Department. They occur with epidote, andradite, and other classic skarn minerals. They typically reach about 4cm in size, occur in clusters, and are clear and have a flattened, tabular morphology.

There are also a multitude of other less well known localities that have produced a multitude of Japan law twins in Peru. I really like pieces from Pentadora, Castrovirreyna Province.

Africa does not have many localities for Japan law twinned quartzes, but the few it has are extremely good. Madagascar in particular produces very fine amethyst scepters at Andilamena, Toamasina Provence where the first generation of quartz is often Japan law twinned. The second generation of quartz in these specimens is typically amethystine, very gemmy, and often included with red goethite spheroids and needles. Notice the amethyst at this locality does not form Japan law twins; I am not sure it does at any locality.

In Zambia, extremely large and fine citrine Japan law twins were recovered from the Mansa District, Luapula District. They are huge- often over 10cm in width and are a light to dark caramel color. They range in form to L shaped to tabular but are more often the former.This is the only locality that produced citrine Japan law twins in abundance.

Asia aside from Japan is very diverse in terms of the variety of mineral localities there, however it is not well known for Japan law twinned quartzes. The Fengjiashan mine in Hubei Province, China is probably the most well known locality for Japan law twinned quartzes. Usually they are quite large and typically L shaped. They are associated with calcite, apophyllite, inesite, hubeite, and many of the other minerals found at this unique mine. Some exceptional specimens of amethyst sceptered Japan law twins have even been recovered.

In Russia’s far eastern region, very sporadic Japan law twinned quartz has been found at Dal’Negorsk. The mines in this region have produced an astounding variety of colors and habits of quartz so it is not surprising that some Japan-law twinned crystals have been found. They are quite rare though and not terribly spectacular compared to other worldwide locations.
Europe is the original discovery site of Japan law twinned quartzes at the La Gardette mine in France as previously mentioned, and although Japan law twinned crystals are found sporadically throughout the continent, they are not abundant. Perhaps the locality best known to produce Japan law quartz crystals in Europe aside from the La Gardette mine is the molymetallic deposits within the Madan ore field, Bulgaria. The crystals from these localities are very similar to those from Peruvian localities such as the Pasto Bueno district- they are typically L shaped and occur on beds of thin, almost needle like quartz crystals. Still, Madan is not well known for them in comparison to other minerals such as galena and sphalerite which are found there.

Recently, a find of fine Japan law twins was made at an outcrop of Wollastonite in Thrace, Xanthi, Greece. These crystals aren’t numerous or well known and are quite small, but form clear, aesthetic hear-shaped twins. They are some of the finer Japan law twins from Europe.
Brazil and Dauphiné Twinning-

Both of these modes of twinning are extremely rare and are formed from cyclic twinning of quartz. They are both very similar in terms of how the twinning occurs however Dauphiné twins are formed from only one enantiomer of quartz and Brazil twins are formed from both.

In Dauphiné twinning, only one enantiomer- only right or left handed quartz can be present in the twin, which is composed of subunit crystals each separately rotated around the c axis at 60° intervals relative to each other forming a kind of cyclic twin. The individual constituents are often intergrown significantly additionally making it difficult to distinguish if a quartz crystal in question is a Dauphiné twin or just a macromosaic. Due to the presence of only one enantiomeric form of quartz in Dauphiné twins, the subunits cannot be distinguished via a polarizer. The subunits of a Dauphiné twin are also electrical twins and produce an electric potential between them when pressure is applied.

The schematic diagram above represents the inside of a very general Dauphiné twin when examined down the c-axis. Each color is an individual sub-crystal which is cyclically twinned. Also notice how each subunit is intergrown into the next. Often in Dauphiné twins, this intergrowth is apparent only if there is etching present on the crystal. The etching will increase relief and reveal the margins between each twinned crystal which is often otherwise indistinguishable.

This twinning style and the chirality of the twin can also be distinguished from accessory faces which cause a chiral “beveling” of sorts between the prismatic faces on the side of the crystal and the termination. These faces are typically designated as the x-face. If they are on the right side of the prismatic face, the twin is composed of the right handed quartz enantiomer and vice-versa. Notice these faces in the figure below. They are not usually as prominent on most crystals and are typically very tiny and triangular in form if even externally visible.
Dauphiné twins are hard to spot and so are therefore quartz crystals exhibiting this twinning mode are quite rarely found although they may be more common than mineralogists realize. They are found at many localities worldwide though but I have noticed they are especially abundant in granitic environments such as pegmatites and occasionally in alpine style clefts. They have been reported as fine smoky macromosaic crystals from miarolytic sections of the Conway granite in North Conway and Albany in Carroll County, New Hampshire, from the many mines around Hot Springs, Arkansas as large, clear twins with accessory faces, from miarolyic granites in the Erongo Mountains of Namibia, in the many quartz crystal localities of Bahia and Minas Gerais, Brazil, as well as many other pegmatitic, miarolytic granitic, and low temperature hydrothermal quartz veins worldwide.

Brazil twinned quartzes are perhaps the rarest form of twinned quartz. Often, these crystals are hard to spot because their twinning features are not visible from faces present on the outside of the crystal. They can be spotted though by viewing the crystal down the c-axis with a polarizing filter and illuminating it from the other end. Crystallographically speaking, the Brazil law twin is formed from cyclic, usually intergrown, and sometimes polysynthetically twinned domains of quartz with opposite chiralities forming a larger crystal. The degree of intergrowth in these twins is usually quite great and the chirality always reverses with the presence of a twinning plane making Brazil law twins slightly akin to heterocyclic Japan law twins. The individual domains are typically arranged at 60° angles, which corresponds to the geometry of the quartz crystallization. The alternating chirality of quartz twins can result in unusual accessory faces being formed between the termination and prismatic faces of the quartz, however, often the presence of Brazil twinning in quartz is usually only revealed through viewing through a polarizer or via chirally selective in situ etching of the quartz after its formation. Sometimes, right and left handed x faces indicate the presence of Brazil twinning too. In the figure below, we see a schematic of two Brazil law twins sliced perpendicular to the C axis. Notice how the twinning can either be present discretely or as intergrown units of the same enantiomer. Red and blue coloration correspond to the intergrowth of the quartz.

Brazil law twins are the rarest form of twin in quartz but are present in highly variable quantities from many quartz crystal localities worldwide. They are named for their occurrence at the rock crystal mines in Bahia and Minas Gerais, Brazil where they are found in hydrothermal veins. Despite this rarity, many amethysts are polysynthetically Brazil law twinned in a scheme corresponding to the left schematic above. This twinning is only apparent if etching increases relief of the twinning or a polarizer is used to spot iy.

Gwindels are often confused for twinned quartz and deserve special mention due to their collectibility. They are found in two forms- open and closed. The open form appears as if it is composed of many distinct terminated crystals and the closed form looks like a large, tabular, twisted crystal. Twists can be either right or left handed due to the chirality of quartz. The reasons for formation of each type is not well understood, but closed gwindels are far less common than open gwindels.

Gwindel quartz is very rare. Even at locations known for gwindel quartzes, they are extremely uncommon. There has only been one exception- the Scheuchzerhorn alpine-type smoky quartz locality in the Grimsel area of Switzerland. In 1997, a large cleft was opened where over 50% of the quartzes were gwindels. This is for an unknown reason and is a true geologic anomaly. Gwindels are most common worldwide in the European alps, with the best known examples coming from France, Switzerland, and Austria. Most of these crystals are slightly smoky and associated with other alpine-type minerals like adularia, chlorite, rutile, and titanite. Gwindels are also found in the Ural Mountains of Russia and the Himalayas.

Hopefully now you understand the different modes of twinning in quartzes and how to spot them. Twinned quartzes are likely more common that most collectors realize but distinguishing them from regular quartz crystals is very difficult. This article serves as a tool kit to make the process of spotting twins easier as well as being a primer on a very interesting and important part of crystallography. In synthetic crystals, twinning plays a significant role in growth of materials for the electronic industry. Twin planes in synthetic crystals such as quartz and silicon renders them useless by interrupting the homogeneity of electromagnetic energy flowing through them. It is important that constant temperature and pressure processes for crystal growth are ensured to avoid the formation of twinning planes in these crystals. For us mineral collectors, twinning is desirable though! That is, unless you plan on using quartz crystals in your next electronics project!

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