Hexagonal crystal system
HEXAGONAL CRYSTAL SYSTEM, including the trigonal subsystem, is a coordinate system with as many as four axes, three of which are equal and horizontally positioned crystallographic axes a1, a2 and a3, each rotated from the others by an angle of 120°.
The fourth coordinate axis, or crystallographic axis c, is different from them, either longer or shorter, and positioned vertically.
a1=a2=a3≠c; α1=α2=α3=120°, β=90°
In the hexagonal system there are as many as 12 crystal classes, so it is understandable that the crystal forms are numerous. Here we will present some representatives of different groups of forms.
A form is a set of symmetrically identical faces, i.e. faces that are connected to each other by symmetry elements, a centre of symmetry, axes of symmetry and/or planes of symmetry.
BERYL, Be3Al2Si6O18 – a relatively rare silicate mineral found in igneous and metamorphic rocks.
It crystallises mainly in forms of longer or shorter six-sided prisms bounded by the basal pinacoid. Forms of hexagonal and dihexagonal dipyramids are also often present, blunting the ends of the prisms.
Strong atomic bonds cause great hardness, which on the Mohs scale is 7 ½ to 8, ideal for cutting and use in jewellery.
Chemically pure beryls are colourless, goshenite, but they may contain certain trace elements that cause different colours, so there are very beautiful, colourful varieties popular in the world of gemstones.
The best known are aquamarine – clear, delicately light blue or greenish-blue, whose colour is caused by Fe2+, and emerald, with a deep green colour, which contains Cr3+ or V3+.
Morganite, in shades of delicate pink caused by Mn2+, is also highly valued, as are heliodor and golden-yellow beryl in shades of yellow and greenish-yellow caused by Fe3+ ions.
Especially valuable and rare is red beryl, with a deep pinkish-red colour caused by the presence of Mn3+ ions.
Net – beryl aquamarine
Net – beryl morganite
Net – heliodor
APATITE, Ca5(PO4)3(Cl/F/OH) – includes a group of phosphate minerals that represent a significant component of phosphorites and the main source of phosphorus needed for the development of living organisms. They are used to produce chemicals, phosphoric acid and fertilisers.
Apatite has a consistent hardness of 5 and serves as an index mineral on the Mohs hardness scale.
In nature it occurs in various shades of purple, pink, yellow, blue, brown and green, and it may also be colourless.
Larger apatite crystals can be found in cavities of pegmatites and hydrothermal veins in well-developed forms of hexagonal prisms and hexagonal dipyramids, and such crystals are valued among collectors.
Net
TRIGONAL SUBSYSTEM
CALCITE, CaCO3 – a very widespread and common mineral in the Earth’s crust, which is the main component of limestones, travertines, cave sediments, marble and carbonatites.
In rock cavities it occurs in beautifully developed crystals on which several hundred crystal forms have so far been identified, making it the most morphologically diverse of all mineral species.
Most often these are forms of various ditrigonal scalenohedra and rhombohedra, as well as hexagonal prisms and the basal pinacoid. Forms and combinations of forms belonging to the group of rhombohedra and scalenohedra are especially diverse.
The mineralogical collections of the Croatian Natural History Museum preserve numerous specimens of this morphologically interesting mineral, and with a little effort you can find them yourself already on the slopes of Medvednica, especially in the limestone rocks of the Bizek, Podsusedsko dolje, Gornje Vrapče and Donje Orešje quarries.
Net
Net 2
QUARTZ, SiO2, is one of the main rock-forming minerals, widely distributed and at first glance a simple mineral.
In nature it occurs in a large number of different varieties and growth forms. There are microcrystalline varieties, fibrous and granular, chalcedonies, agate, onyx, jasper, chert, and macrocrystalline varieties of quartz that occur as developed crystals visible to the naked eye.
These crystals are often found in different colours, for example purple amethyst, yellow citrine, brown smoky quartz, black morion, white milky quartz, and they may also be colourless rock crystal.
On quartz crystals we will most often find forms of hexagonal prisms, rhombohedra, trigonal trapezohedra and trigonal dipyramids, as well as other forms of its crystal class, developed and combined to different degrees.
If we also add different forms of crystal growth and intergrowth, quartz will surprise us with its complicated morphology.
Net – quartz amethyst
Net – quartz citrine
CORUNDUM, Al2O3 – a mineral widely known for its great hardness, as high as 9 on the Mohs scale, while position number 10 is held by the hardest mineral in nature – diamond.
Its exceptional hardness makes corundum an excellent abrasive, and when this hardness is found in colourful crystals, it also provides perfect material for cutting and polishing gemstones.
The best-known varieties of corundum are the red variety ruby, whose red colour is caused by Cr3+ locally replacing Al3+ ions in the structure, and the blue variety sapphire, whose blue colour is caused by Ti4+ and Fe2+.
Sapphire can, however, also occur in various shades of other colours, such as yellow, orange, greenish, pink, purple or brown.
Rubies and sapphires are often cut as rounded cabochons in order to highlight the phenomenon of asterism – the so-called six-rayed star effect created by the reflection of light on tiny needles of the mineral rutile, TiO2, oriented within the structure of this trigonal mineral.
Natural corundum crystals are often barrel-shaped, with rough and cloudy faces of hexagonal dipyramids, prisms, rhombohedra and the basal pinacoid.
Net – CORUNDUM, ruby
Net – CORUNDUM, sapphire
TOURMALINE – a group of minerals of borosilicate composition with a complex chemical formula.
Minerals of the tourmaline group have the same crystal structure and similar physical properties, but they differ greatly in chemical composition.
The wide range of chemical composition and colour zoning within individual crystals is the reason why tourmalines occur in more colours and colour combinations than any other mineral.
They crystallise in forms of trigonal and ditrigonal prisms and pyramids. Cross-sections of such elongated crystals are triangular or hexagonal in shape, with rounded edges.
They often very beautifully show multicoloured zoning from the centre toward the edges of the crystal, and colour transitions can also be observed along the length of the crystal, indicating a change in the concentration or composition of trace elements within an individual crystal during crystallisation.
Net – tourmaline
Beryl, corundum, tourmaline and quartz are wonderful examples of allochromatic minerals, that is, minerals whose colour is caused by different “impurities”, trace elements that enter the structure during crystallisation.
These are not the main chemical components in the mineral, but they play a key role in the colouration of the mineral. They absorb light differently and in this way cause different colours, i.e. they act as chromophores.
When we look at crystals of any mineral, we see that they are quite extraordinary creations. Most minerals present in the Earth’s crust do not show an external crystal shape, but occur as irregular grains in rocks.
Beautifully developed, larger crystals are actually very rare phenomena, and they probably survived for millions of years before someone discovered them.
For this reason, crystals should be treated with respect and great care; one careless moment is enough for them to be permanently damaged or even destroyed.
Crystals are the flowers of the mineral world!