Introduction
Crystals are a regular and repeating arrangement of atoms. Sometimes this arrangement will contain defects. One important type of defect is called twinning, and in the simplest sense may be thought of as a mirroring of the atomic arrangement. It may be caused by a change in environmental factors during growth, such as temperature or pressure, or mechanical stresses. Another theory is that the twinning is present at the initial nucleation of the crystal.
Energy conditions must be satisfied at the twin boundary, and atomic spacing must be satisfied, just as they must when atoms pack in an untwinned crystal. As such, only some minerals twin. Of those that do, they can do so only at certain angles with respect to the crystallographic axes of the mineral. Because they can only occur at specific angles, the different twin angles are referred to as twin laws. Many common twin laws have names such as Brazil-law or Japan-law twinned quartz, and Carlsbad-law twinned orthoclase. Just as some minerals do not twin, others twin very easily, and twinning may be more common than not. Some minerals are virtually always twinned, including quartz, but it may not be visually apparent. Some minerals are polysynthetically twinned, or repeatedly twinned through the whole crystal. That is not really of interest for this page.
Many mineral collectors like twins because they offer something a bit out of the ordinary. In fact, twins of some minerals may be quite rare. Other collectors may enjoy twins to study the crystallography. In some cases, the twin causes the various faces of a crystal to grow at different speeds resulting in unusual shapes that would otherwise not be found. Whatever the case, twins are always enjoyable. Below is a list of minerals from Nova Scotia that have been found as twinned crystals, where the twin is externally visible.
Twinned Examples from Nova Scotia
Minerals that have been found in euhedral, free-growing twinned crystals include: arsenopyrite, bertrandite, calcite, cassiterite, cerussite, chabazite, chalcocite, copper, gmelinite, gypsum, langite, laumontite, marcasite, microcline, muscovite, posnjakite, quartz, rutile, and staurolite.
Arsenopyrite - FeAsS
This is a common mineral in the slates and shales along the Eastern Shore. Dumps from old gold mines have produced large quantities and at times it was isolated for use in the insecticide industry. Most of the crystals are penetration twinned, from micro crystals to larger miniature sized specimens.
Bertrandite - Be4Si2O7(OH)2
This micromineral forms beautiful twins that look like crystalline butterflies. At East Kemptville, these twinned bertrandites were common in one small breccia zone. In fact the twinned crystals were at least as common as untwinned ones.
Calcite - CaCO3
Calcite has four known twin laws. These are very nicely explained in an article by Richards. Using the notation of the structural unit cell, the four twin laws are {00.1}, {01.8}, {10.4}, and {01.2}. The angle between the c-axes for each of these, rounded down to the nearest degree is 180, 127, 90 and 53 respectively. The first two twin laws are common worldwide, while the last two are rare. I have observed three of the twin laws in local material from around the Minas Basin.
The picture below shows a twin on {00.1}, with equally sized twin components. Sometimes these crystals twin twice down the length of the crystal. The twinned section in the middle of the crystal is typically thin, and often wedge shaped. Sometimes it does not even appear to cover the whole cross sectional area of the crystal.
Twins of scalenohedral crystals on {01.8} are often referred to as fish-tail twins. The photo below may appear somewhat different than the drawing in Figure 3b. The photo is taken such that you would be looking down at the drawing from above, making it difficult to distinguish from Figure 3a. Also, the actual crystal shows notches along the twin plane due to additional faces of the scalenohedral form.
The photos below show two examples of calcite twinned on {01.2}. This twin law is in general rare, but about half a dozen examples were found near these. The pocket containing the specimen on the right had about four twins itself. Unfortunately, a couple had already cleaved off when found and had started to heal.
Cassiterite - SnO2
The tin ore at the East Kemptville mine was not common as good crystals on the dumps, though even in the few examples I have seen, twinning is present on most of them.
Cerussite - PbCO3
This secondary mineral is found as micro crystals at the Dunbrack Mine in Halifax County. Cerussite from here displays simple elbow shaped twins. In rare cases the twins are cyclic producing a crystal with six 'arms', like the one shown below.
Chabazite - (Ca,Na2)(Al2Si4O12)·6H2O
This zeolite is another mineral that is very easily twins. The most conspicuous are the penetrations twins, however, even a simple looking rhomb may be twinned. This is observed by looking for striations on the surface of a crystal. If untwinned the striations on a given face will all be in the same direction. More often than not though, they will go in two directions, indicating twinning. Contact twins are also known on {10·1}, however these are far less common than the first two types.
Chalcocite - Cu2S
This copper sulfide has been found in attractive striated, twinned crystals at Swan Creek. Chalcocite is known to twin in three ways with twin planes {110} (pseudohexagonal twins), {032} and {112}. The angle between twin components for {110} twinning is 60.4 degrees. Because it is so close to 60 degrees, cyclic twinning according to that law results in pseudohexagonal crystals.
Copper - Cu
Copper may be frequently twinned, but because of the complex morphology of the arborescent growths, the twinning and be very difficult to observe. I was very happy to find the crystal below, which shows repeated twinning, observed by the changing direction of the striations.
Ferberite - FeWO4
Contact twins of ferberite with the compositional plane {023} are common worldwide (Handbook of Mineralogy). Below is a small bladed example from the Keddy Pegmatite. For the {023}-law, the angle between c_primary(001) and c_twin(011) is 60.0°. A rough measurement using the striations in the photo gives 59.1°, which is a good match.
Gmelinite - Na2CaAl2Si4O12·6(H2O)
Pirsson (1891) describes two type of twinning in gmelinite from the Nova Scotia. His figure is shown below.
Gypsum - CaSO4·2H2O
Nova Scotia has enormous quantities of gypsum, but good crystals are difficult to find. That said, I have not spent much time collecting in gypsum quarries, so there may be more than I am aware of. Twins of gypsum crystals are very common worldwide and can follow one of several twin laws. Henry How reports on some from the Clifton Quarry in Windsor. Discussing masses of mirabilite, he writes,
"Many masses were penetrated by perfect crystals of selenite, of various sizes, simple and in macles."
where 'macles' is the French term for twin. The use of the word today, in English, is generally restricted to twinned diamonds.
The specimen below is a large cleavage that was reportedly collected at Milford. It is a contact twin with a clear, vertical twin plane.
An excellent review of the twin laws of gypsum is given by Rubbo et al. (2011). They summarize five distinct twin laws. Two of them are shown in the drawing above. These two both happen to have an angle between the twin members of 105 degrees, but the orientation of the primary and twin is different in the two cases. Rubbo et al. write that the only way to conclusively distinguish the two is to look at the optical extinction angle, but by assuming some common forms, we can make a guess. I also wonder if measuring interficial angles of the modifying forms could provide the additional distinguishing information.
The Milford specimen appears to be the left hand twin, though an extra termination form changes the shapes of the component crystals. Using the photograph, we can crudely measure the angle of the notch on the computer, which gives 107 degrees. That is close to the expected value of 105 degrees, given the uncertainty of the method. The other three twin laws, not shown, have angles of 126.34, 132.18, and 117.68 degrees. Further proof can be gleaned by noting that in the drawing there are vertical edges, and that several vertical edges can also be seen in the photo (often leading to a bevelling face).
In contrast, the image below shows a prismatic twinned crystal. This is a visual dead-ringer for a swallow tail twin (on the right in the drawing), but because the crystal is very small, it is hard to do good measurements.
Langite - Cu4(SO4)(OH)6·2H2O
Langite is another micromineral found at the Dunbrack Mine that forms cyclic twins. In this case they form tabular star shaped aggregates.
Laumontite - CaAl2Si4O12·4(H2O)
Even with the tremendous amount of laumontite in the North Mountain, no twinned laumontite crystals are reported in the historical literature. In 2021, Terry Collett found a pocket with many "swallow tail" twins, which are twinned on {100} and produce notches at the end of the crystals. They were transparent when collected. Tschernich (1992) writes that such twins are common, but based on photos submitted to Mindat.org, they do not seem to be, with the exception of the Bishop Mining District in California which is a classic laumontite locality with huge crystals that are nearly always twinned. Perhaps that contributed to their large size.
Marcasite - FeS2
The photo below shows a tabular marcasite, also from the East Kemptville mine. It is micro in size and is growing on drusy quartz. Marcasite twins on the {101} plane. Using the cell parameters for the species (a=4.436, 5.414, c=3.381) we can calculate the angle of the twin components to be 74.6 degrees. Marcasite is well-know to form cyclic twins or fivelings. The fifth component of the cyclic twin end up being incomplete because 5*74.6 = 373 degrees, which is 13 degrees more than a complete revolution.
Looking at the photo above, we can make some observations about the twinning. The horizontal component extends right across the crystal. Knowing the angle between twin components is 74.6 degrees, we can see that the crystal starts forming a cyclic twin from each end. On the left side there is a twin component above and below the horizontal component. That is shown below with red arrows. The same is true on the right, shown with green arrows. These angle between the twin components on the left and right do not align crystallographically, so those components simply grow to abut the adjacent component, and we see a notch at the top between the two twin sections. I've measured the angle between the twin sections and the horizontal component, by simply drawing lines over the striations. Using this crude method, the angles are 74.6, 74.1, 73.9, and 73.2, for an average of 74.0 degrees. That is reasonably close to the true 74.6 degrees, considering the error of this approach and the fact the photo is not likely to be perfectly perpendicular to the plane of the twin.
K-Feldpsar (Microcline and Orthoclase) - KAlSi3O8
Feldspars (the K-Feldspars microcline and orthoclase, along with plagioclase) twin in multiple ways, usually at a microscopic level. Macroscopic twins are the only types discussed here. Macroscopic twin laws include Carlsbad, Baveno, and Manebach twins. I'm not sure what conditions are needed for their development, but considering how common feldspar is in our granites, I would not be surprised if examples of all could eventually be found. That said, I have only observed the Baveno twin shown below.
At Bayer's Lake some crystals of feldspar were Baveno-twinned. They tended to be heavily etched but made interesting specimens. The specimen below is a prism with a square outline. The twinning is clear when looking at the termination of the crystal, with growth/etching lines at or near 90 degrees. The twinning probably accelerated growth - this crystal is about 9 cm long.
Muscovite - KAl2(AlSi3O10)(OH)2
Books of muscovite with a pseudohexagonal outline and often showing the star-shaped twinning, were fairly common at Bayer's Lake. The twin axis is [310]. Diagrams showing how the crystals are twinned are shown below.
Posnjakite - Cu4(SO4)(OH)6 · H2O
This mineral is common at the Coxheath mine in Cape Breton County but discrete crystals are rare. As is typical for this mineral, the good crystals show twinning in the form of branches at 60 degrees to the main crystal. An example is shown below.
Quartz - SiO2
Quartz is known to twin according to 15 different laws. The Brazil and Dauphiné laws (or combinations of the two) are common but generally difficult to spot because a single crystal is twinned within itself. The others, including the Japan and Reichenstein-Grieserntal twin laws are more conspicuous, but are rare to very rare.
A lucky find at Boylston, Guysborough Co., produced a small find of crystals with modifying s faces {11·1}. If the s form was ideally developed, it would produce a small diamond shaped face on every second top corner of the quartz prism. As luck would have it, one of the crystals with s faces happened to have three s faces but not on every second corner. Instead, two reasonably large s faces were directly opposite each other, and a much smaller thin s face was next to on of the larger ones. Two possible explanations exist. The first is that the s form {11·1} and its complimentary form {-1-1·1} are both present. The other is that the crystal is Dauphiné twinned. Considering that Dauphiné twins are common (even if rarely recognized) it is likely that the crystal is twinned.
Below is another manifestation of a Dauphiné or Brazil twin. In this case, the faces show lustrous and dull patches due either to etching or a thin coated (to be determined). To make the following description simpler, let us assume that the dull face that makes up the majority of the large lefthand face, is the r form. All of the r faces are dull and in the twin, the r faces are also dull. On the other hand, the z and z faces are lustrous.
Rutile - TiO2
Some minerals twin very easily and are thus commonly or even usually found as twins. Rutile is one such mineral. The most common are contact twins on {011}. The result is a branching or an elbow shaped crystal. Twinned rutiles have been found at both the East Kemptville tin mine and at Moose Point near Guysborough. Sometimes twinning of the rutile results in a mesh or network as shown in the photo below. In this case the habit is referred to as reticulated.
Staurolite - Fe2Al9Si4O22(OH)2
Two types of twinning are generally observed in staurolite. The most common is the {231} twin in which the crystals cross at an angle of about 60 degrees (Handbook of Mineralogy). The other is the {031} twin, producing an almost 90 degree angle between the two crystal components. Crystals of staurolite twinned by this second law are often informally referred to as fairy-crosses. Twinned staurolite can be found on the South Shore. Sometimes they are found weathered from the softer rock matrix, leaving floaters, as below.
Conclusions
Collecting twinned crystals is a specialty that many collectors enjoy, and there are many examples from Nova Scotia. Other minerals generally known to form twins such as aragonite, fluorite, stibnite, sphalerite and galena are also found in Nova Scotia. However, to my knowledge no twins of these species have yet been identified in this area.
References
Anthony, J., et. al., Handbook of Mineralogy, Volume II - Silicates, Mineralogical Society of America.
Gault, H.R. (1949) The Frequency of Twin Types in Quartz Crystals. American Mineralogist, vol. 34, pp. 142-162. http://www.minsocam.org/msa/collectors_corner/arc/qtztwin.htm [2004].
Goldschmidt, V. (1913) Atlas der Krystallformen.
Goldschmidt, V. (1923) Atlas der Krystallformen.
Henderson, W., Jr. (1975) The Bertrandites of Connecticut. The Mineralogical Record, pg. 114-123, May-June.
How, H. (1857) On the Occurrence of Natro-Boro-Calcite with Glauber Salt in the Gypsum of Nova Scotia. The Edinburgh New Philosophical Journal, Vol 6, pg. 54-60.
Hurlbut, C.S., Jr. (1956) Muscovite from Methuen Township, Ontario. vol 41, no 12, pg 892.
Lewis, W.J. (1899) A Treatise of Crystallography. Cambridge University Press, 615 pp. [Online 2019]
Nelson, S. (2003) Twinning, Polymorphism, Polytypism, Pseudomorphism, Tulane University.
Pirsson, L.V. (1891) Gmelinite from Nova Scotia. American Journal of Science, Series III, vol. 42, art. 8, pp. 57-63. [Online 2015].
Richards, P. (1999) The Four Twin Laws of Calcite and How to Recognize Them. Rocks and Minerals.
Rubbo, M., et al. (2011) The Five Twin Laws of Gypsum (CaSO4*2H2O): A Theoretical Comparison of the Interfaces of the Contact Twins. Crystal Growth and Design, American Chemical Society, pg 264-270. [Online 2012]
Tschernich, R.W. (1992) Zeolites of the World. Geoscience Press, Phoenix. [Online 2015]
Disclaimer: This page is intended for information purposes only. The localities described are not necessarily open to collecting and are not necessarily safe.