The salt domes of Iran, particularly those of the islands of Hormuz, Qalat-e Payeen and Larak, present an exceptional mineralogical richness that has attracted the interest of geologists for several centuries. The Hormuz complex, composed of evaporitic and volcanic rocks, has been the subject of numerous studies since the 17^th century, but it is only recently that major discoveries have been made, particularly concerning zunyite. This rare mineral, described for the first time in the United States, has been discovered in these salt diapirs in new shapes, colors and sizes, with some of the largest specimens ever observed. Hormuz and Larak islands, as well as the Qalat-e Payeen dome, have yielded zunyite crystals with complex morphologies, including rare twins and pseudo-octahedra, illustrating the geological diversity and complex formation processes of these regions. These findings open new perspectives for the understanding of the mineralogy and dynamics of salt diapirs in this part of the world.

The Hormuz Salt Formation, named by Blanford in 1872, refers to a geological complex composed of salt-bearing rocks associated with sedimentary and igneous rocks. This formation is mainly exposed on Hormuz Island, located south of the Zagros Mountains in Iran. It represents a varied lithological sequence of evaporitic and volcanic rocks, influenced by multiple geological processes. These processes have contributed to the mineralogical diversity of the Hormuz complex, although investigations into the chronological and genetic relationships of the outcrops are made difficult by intense tectonic activities and diapiric movements. Indeed, the movements linked to the rise of the salt domes have profoundly modified the distribution of the rocks, making any precise geological interpretation of the site difficult. Today, the latter offers a unique landscape worthy of the greatest science fiction films with stunning colors. From the rainbow valley to the red beach, from white salt efflorescences to yellow to red ferruginous formations and gray-blue volcanic rocks, the color palette is incredible !

Red Beach of Hormuz Island, Iran © TPM TEAM - Travel Peacock Magazine

The first recorded geological observations in Iran date back to Tavernier's description in 1642 of the salt on Hormuz Island. In the 19^th and 20^th centuries, numerous geological studies were conducted, mainly in connection with oil exploration, which improved the understanding of the salt formations in the region. However, these studies focused more on the structural and tectonic aspects of the salt domes, leaving the mineralogy of the deposits relatively unexplored. More recently, work on salt diapirs has revealed an impressive variety of minerals, some of which were previously unknown in these contexts, such as zunyite.

Zunyite is a rare aluminum silicate Al₁₃Si₅O₂₀(OH,F)₁₈Cl that was first described by Hillebrand in 1885 at Zuni Mine in Colorado (USA). Its formation is generally associated with advanced alteration environments of igneous rocks, such as rhyolites, or in aluminiferous shales. In April 2017, a team of geologists led an excursion to Hormuz Island, followed by additional visits in 2018 and 2019, revealing specimens of zunyite particularly remarkable for their diversity of crystal forms, colors and sizes, including the largest crystals ever discovered for this mineral species.

Map of southern Iran showing the localities according to Hairapetian V., Pelckmans H. & Basirat H. (2020)

The island of Hormuz is located north of the Strait of Hormuz, between the Persian Gulf and the Gulf of Oman. It is dominated by a circular saline dome 7 kilometers in diameter, made up of colored evaporite rocks (white, green, red). The margins of the dome host clastic sedimentary rocks as well as volcanic rocks, such as rhyolites and tuffs, often very altered. It is in the northern part of the island that an exceptional mass of zunyite was discovered, made up of hundreds of well-formed, interconnected crystals, measuring 6 to 8 mm on each side. These crystals, gray-red in color due to the presence of hematite inclusions, represent a unique configuration with faces of positive and negative tetrahedra as well as cubes.

Right photo : Prismatic hematite from Hormuz Island, Iran

Larak Island, located north of the Strait of Hormuz, has a geology similar to that of Hormuz Island, with a salt dome, rhyolites and tuffs. Mineralogists have discovered zunyite crystals on this island, associated with hematites in crystals up to more than 7 cm and well-developed fluorites of 1.5 cm. Zunyite specimens found on Larak are distinguished by complex shapes and unusual secondary growth phenomena, creating fascinating and unique mineral structures, such as pseudo-octahedra formed by the equal development of tetrahedral shapes on a single crystal.

Three series of faces develop on zunyite crystals from Iran. The most common forms are tetrahedra, the latter being truncated by the faces of the cube and the faces of the negative tetrahedron.

The faces of the positive tetrahedron {111} are 4 in number: (111), (-11-1), (-1-11) and (1-1-1)

The faces of the negative tetrahedron {1-11} are also 4 in number: (1-11), (-111), (11-1) and (-1-1-1)

The faces of the cube {100} are 6 in number: (100), (001), (010), (00-1), (-100) and (0-10)

According to the development of each series of faces, we observe a large number of differently truncated tetrahedra, but also truncated cubes and even pseudo-octahedra.

Two twins are listed on these deposits, they are both very rare and represent only a tiny proportion of the crystals collected :

The Star of David : this is the most common twin present on these deposits with a record size of 2.5 cm. Note that it has not been described on the island of Hormuz. It is a twin by interpenetration with a rotation around the axis [111]. When it is complete it forms a 6-pointed star but this is only rarely the case, it often happens that one of the points is missing.

The cross twin : although it is described on the three deposits mentioned above, it is the rarest of the two twins. It is a twin by interpenetration with a rotation around the [100] axis. When it is complete it forms a perfect cube whose 6 faces form a cross.

 Be careful, many interpenetrating tetrahedra are not twins, although sold as such most of the time, they do not respect any rule of symmetry.

The acquisition of zunyite and Iranian minerals more broadly has become particularly complex in recent years, largely due to the tense geopolitical context surrounding Iran. Economic sanctions imposed by several Western countries, notably the United States and the European Union, have had a direct impact on trade, restricting imports and exports of goods, including minerals. These restrictions not only complicate international financial transactions, but also logistics, making it difficult to transport mineralogical specimens outside Iran’s borders.

In addition, diplomatic tensions and visa restrictions make travel to Iran more complicated for mineral collectors, researchers and traders. Those wishing to travel to Iran to explore or purchase specimens often face lengthy and uncertain administrative processes, or even travel bans. These obstacles have reduced the opportunities for mineralogy enthusiasts to access Iran’s rich mines and geological sites, including the famous salt diapirs of Hormuz and Larak.

The internal political context in Iran also plays a role, as local authorities impose strict regulations on the prospecting and export of certain minerals. Control over natural resources, often seen as a matter of national sovereignty, results in additional limitations on the commercialization of minerals. As a result, obtaining authentic Iranian minerals on the international market has become a daunting task, making these specimens even rarer and sought after by collectors worldwide.