Lazulite

Lazulite (Mg,Fe2+)Al2(PO4)2(OH)2 – named in 1795 by Marten H. Klaproth from the term “Lazaward”, which means heaven in Arabic, in allusion to its colour. Lazulite is in a solid solution series with the mineral scorzalite. The lazulite-scorzalite series ranges from the magnesium rich lazulite to the iron rich scorzalite. The rarer scorzalite does not differ appreciably, except that it tends to be darker, less transparent and denser than lazulite.

Lazulite forms during high grade metamorphism of high silica quartz rich rocks, and in pegmatites. It occurs in association with quartz, andalusite, rutile, kyanite, corundum, muscovite, pyrophyllite and dumortierite, in metamorphic terrains, and with albite, quartz, muscovite, tourmaline and beryl in pegmatites.

Freßnitzgraben in Austria is the type locality of lazulite, with the noteworthy localities of Fischbach, in Styria. In Italy, lazulite comes from the Vizze pass, Bolzano Province; and from Monte Folgorito, Pietrasanta, Lucca Province. Some famous localities in the U.S. are Graves Mountain, Georgia and the Champion Mine in Mono County, California. The locality of Rapid Creek (and nearby Crosscut Creek), in the Yukon Territory of Canada is also well-known for its outstanding transparent dark blue lazulite crystals.

It may be confused with lazurite (a blue feldspathoid and a member of the sodalite group) or azurite (deep blue copper carbonate; alexstrekeisen.it).

 

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Vivianite group minerals

The vivianite group forms a group of monoclinic phosphate and arsenate minerals that have very similar structures. Named after one of their more common members, the group is generally very colorful. All members of this group can be very brilliantly colored. Vivianite is famous for its bluish-green color. Erythrite is famous for its red-purple color, and annabergite for its apple-green color.

A general formula of vivianite group minerals stands as:

A2+3(XO4)2 × 8H2O,

where X = P or As and A2+= Co, Fe, Mg, Mn, Ni or Zn.

vivianite
Schematic representation of the structure of vivianite viewed along the b axis. Black spheres: H2O; white spheres: O2- (after Mori and Ito, 1950)

There is known 11 minerals of vivianite group:

Annabergite

Ni3(AsO4)2 · 8H2O

Arupite

Ni3(PO4)2 · 8H2O

Babánekite

Cu3(AsO4)2 · 8H2O

Barićite

(Mg,Fe)3(PO4)2 · 8H2O

Erythrite

Co3(AsO4)2 · 8H2O

Hörnesite

Mg3(AsO4)2 · 8H2O

Köttigite

Zn3(AsO4)2 · 8H2O

Manganohörnesite

(Mn,Mg)3(AsO4)2 · 8H2O

Pakhomovskyite

Co3(PO4)2 · 8H2O

Parasymplesite

Fe2+3(AsO4)2 · 8H2O

Vivianite

Fe2+3(PO4)2 · 8H2O

Annabergite

Annabergite Ni3(AsO4)2 · 8H2O – named by Henry J. Brooke and William Hallowes Miller in 1852 after one of the co-type localities, Annaberg, Saxony, Germany. It has a wonderful, bright green color. This characteristic color is easily noticeable and was used to spot veins of nickel-bearing ore. Annabergite, or Nickel Bloom as it is called by miners, is often found as a green alteration coating on other nickel minerals.

 

Erythrite

Erythrite Co3(AsO4)2 · 8H2O, also called Cobalt Bloom, arsenate mineral in the vivianite group, hydrated cobalt arsenate. Erythrite, which is used as a guide to the presence of cobalt-nickel-silver ores because of its crimson or peach-red colour, occurs as radiating crystals, concretions, or earthy masses in the oxidized zone of cobalt and nickel deposits. It forms a complete solid-solution series with annabergite, in which nickel replaces cobalt in the erythrite structure. As the nickel content increases, the colour lightens to white, gray, or pale green.

Vivianite

Vivianite Fe2+3(PO4)2 · 8H2O – named by Abraham Gottlob Werner in 1817 after John Henry Vivian (August 9, 1785 – February 10, 1855), an English (Welsh-Cornish) politician, mine owner, and mineralogist living in Truro, Cornwall and discoverer of the mineral.

Usually found as deep blue to deep bluish green prismatic to flattened crystals, most crystals rather small to microscopic, larger ones are rather rare. When fresh the mineral may be colourless, or nearly so, and, once exposed, will oxidize with the Fe2+ converting to Fe3+ with a concurrent darkening to dark blue or blue-green.

Santabarbaraite – vivianite alteration product

Santabarbaraite is relatively new amorphous ferric iron hydroxy phosphate hydrate described from Valdarno, Tuscany, Italy. A simplified chemical formula for the type material can be given as Fe3+3(PO4)2(OH)3·5H2O.

The mineral is the result of in situ oxidation of vivianite, occurring as pseudomorphs after vivianite crystals. Santabarbaraite is brown to light-brown in hand specimen, but appears yellowish amber under the microscope and has a similar streak. It is translucent with a distinct vitreous to greasy luster. It is brittle with a distinct parting along the perfect cleavage of vivianite.

Analysis of santabarbaraite showed that all the Fe in santabarbaraite is trivalent, associated with the presence of both H2O and hydroxyl. This is consistent with an oxidation series from vivianite through metavivianite to santabarbaraite, involving progressive oxidation of Fe2+ accompanied by conversion of H2O ligands to OH ions. Such a process leads to a gradual collapse of the vivianite structure as hydrogen bonds are eliminated. Santabarbaraite is the end product of this process and can be thus considered the phosphorus analogue of ferrisymplesite Fe3+3(AsO4)2(OH)3 · 5H2O (Pratesi et al. 2003).

Originally described from Kerch peninsula, Ukrainekertschenite” – supposed hydrated phosphate of iron which is an alteration product of vivianite. Later studies have shown an intergrowth of vivianite or metavivianite with varying amounts of amorphous ferric iron phosphate (santabarbaraite) and bobierrite.

Melilite group minerals

Melilites – a group of tetragonal sorosilicates with a disilicate anion (Si2O7)6- or an Al or B-bearing derivative thereof and the general formula:

Ca2M(XSiO7)
M = Mg, Al, rarely Fe, B, Zn, Be, Si, etc.
X = Si, Al, rarely Be or B

where M denotes a small- to medium-sized divalent or trivalent cation (mostly Mg and Al, or rarely Fe, B, Zn, Be, Si, etc) and X is Si, Al or rarely Be or B. In general, Al or B replace one Si atom when M is a trivalent ion, but the charge can also be balanced by coupled substitution of Ca2+ with a monovalent ion, especially Na, and M3+ with M2+, such as in Alumoåkermanite, where Al3+ is still dominant on the M-site but the mineral is far from end-member composition (mindat.org).

Structure of melilite
A view down onto the sheets of the Ca2M(XSi2O7) melilite structure. The Ca atoms are yellow spheres, the M sites are pale orange tetrahedra, and the XSi sites are blue tetrahedra. Oxygen atoms (not shown) are on the corners of the tetrahedra; wikipedia.org

In petrology “melilite” usually refers to minerals in the åkermanitegehlenite series, by far the most abundant members of the group. Melilite with compositions dominated by the endmembers akermanite and gehlenite is widely distributed but uncommon. It occurs in metamorphic and igneous rocks and in meteorites.

Typical metamorphic occurrences are in high-temperature metamorphosed impure limestones. For instance, melilite occurs in some high-temperature skarns.

Melilite also occurs in unusual silica-undersaturated igneous rocks. Some of these rocks appear to have formed by reaction of magmas with limestone. Other igneous rocks containing melilite crystallize from magma derived from the Earth’s mantle and apparently uncontaminated by the Earth’s crust. The presence of melilite is an essential constituent in some rare igneous rocks, such as olivine melilitite – extremely rare igneous rocks contain as much as 70% melilite, together with minerals such as pyroxene and perovskite (wikipedia.org).

Åkermanite Ca2Mg(Si2O7) was first described from samples of slags from furnace iron production found at 3 furnace localities: Hofors, Löfsjöen and Mölnbo, Sweden (Vogt 1884).
Named by the Norwegian geologist, professor Johan Herman Lie Vogt (1858-1932) in honor of Anders Richard Åkerman (1837-1922), Swedish metallurgist. During his comprehensive study of the mineralogy of slag products, Vogt discovered a new Ca-Mg-silicate, and named the mineral Åkermanit. Åkerman had kindly given J.H.L.Vogt access to Stockholms Bergskolas large collection of slags. It was in samples from this collection Vogt discovered the mineral.

Gehlenite Ca2Al(AlSiO7) was named in 1815 by Johann Nepomuk von Fuchs in honor of Adolf Ferdinand Gehlen [5 September 1775 Bütow, Outer Pomerania, Prussia (Bytów, Poland) – 16 July 1815]. Gehlen was editor and publisher of publisher of Neues Allgemeines Journal der Chemie (1803–06), Journal für Chemie und Physik (1806-10) and the Repetitorium für die Pharmacie. He was initially a chemistry professor at the University of Halle (Martin-Luther-Universität Halle-Wittenberg) and later chemist at the Bavarian Academy of Sciences. He died early due to arsenic poisoning. Type locality for gehlenite are Monzoni Mts in Italy.

 

Alunite supergroup minerals (part I): Alunite group

ALUNITE SUPERGROUP – INTRODUCTION

At the present time, there is more than 50 representative minerals form the alunite supergroup, the general formula of which is given by

DG3(TX4)2X’6

where:

  • D: Th; Ce, La, Nd, Bi; Ca, Sr, Ba, Pb, Hg; Na, K, Rb, Ag, Tl, NH4, H3O; □;
  • G: Sn4+; Al , Fe3+, V3+, Cr3+, Ga; Cu2+, Zn2+, Mg;
  • T: S, Cr+6; P, As, Sb; Si;
  • X: O; (OH), F; (H2O).

The numerous possible combinations, depending on the elements present at D-, G– and T-sites, led to classify minerals in different groups. The classification proposed by Mills et al. (2009) and refined by Bayliss et al. (2010) consists in a subdivision of the alunite supergroup into various four groups: the alunite group, the plumbogummite group, the dusserite group and the beudantite group.

This division is based on the element that predominates in T-site. Thus, minerals in the alunite group are those for which TO4 is dominated by SO4. Cations in D-site are usually monovalent ions and compounds, with Al3+ as predominant element in G-site, are referred to alunite family, and those with Fe3+ to jarosite family. In the beudantite group, one SO42- is replaced by PO43- or AsO43-, in the plumbogummite group, TO4 is  predominated by PO4 and in the dusserite group by AsO4. For these groups, as PO43- or AsO43- increases at the expense of SO42-, charge is compensated by addition of a trivalent cation such as Ce3+, or by protonation of some of the hydroxyl groups in the structure (Maubec et al. 2012).

Alunite family structure exemplified by the atomic arrangement DG3(TO4)2(OH)6 in a projection along the c axis. Remarks: TO4 tetrahedra are grey, GO6 octahedra are dark grey, D atoms are black (after Sato et al. 2009).

Crystals in the group are generally small, imperfect, and rare. Habit is usually either tabular {0001} or pseudo-cubic (cube-like rhombohedron) to pseudo-cuboctahedral, or acute-rhombohedral. Acicular habits are extremely rare.

ALUNITE SUPERGROUP – Alunite Group

Alunites are a group of minerals, which form part of the alunite supergroup. In the general formula [DG3(TO4)2(OH,H2O)6] the D sites are occupied by monovalent cations such as K, Na, NH4, H3O+ and others, divalent cations such as Ca, Ba, Sr, Pb, trivalent cations for example Bi; and G is the trivalent cation either Al of Fe3+; and T is S6+. Alunites can be divided into alunites and jarosites simply depending on whether the concentration of Al is > Fe (alunites) or Fe > Al (jarosites). Of course, solid solution formation can exist across a wide range of concentrations and substitutions (Frost et al. 2006).

Common members of the alunite group are alunite KAl3(SO4)2(OH)6, natroalunite NaAl3(SO4)2(OH)6, ammonioalunite NH4Al3(SO4)2(OH)6, sclossmacherite (H3O+,Ca2+)Al3(SO4)2(OH)6 from alunite family and jarosite KFe3+3(SO4)2(OH)6, natrojarosite NaFe3(SO4)2(OH)6, plumbojarosite Pb0.5Fe3+3(SO4)2(OH)6 or argentojarosite  AgFe3+3(SO4)2(OH)6 from jarosite family.

Turquoise

Turquoise Cu(Al,Fe3+)6(PO4)4(OH)8·4H2O is a secondary mineral occurring in the potassic alteration zone of hydrothermal porphyry copper deposits. May be also formed by the action of meteoric waters, usually in arid regions, on aluminous igneous or sedimentary rocks (as vein filling in volcanic rocks and phosphatic sediments).

The turquoise group has the general formula:

A0-1M6(PO4)4-x(PO3OH)x(OH)8·4H2O, where x = 0 – 2,

and consists of six members:

  • planerite Al6(PO4)2(HPO4)2(OH)8·4H2O,
  • turquoise Cu(Al,Fe3+)6(PO4)4(OH)8·4H2O,
  • faustite (Zn,Cu)Al6(PO4)4(OH)8·4H2O,
  • aheylite (Fe2+,Zn)Al6(PO4)4(OH)8·4H2O,
  • chalcosiderite CuFe3+6(PO4)4(OH)8·4H2O
  • and an unnamed Fe2+-Fe3+ UM1981-32-PO:FeH analogue Fe2+Fe3+6(PO4)4-x[PO3(OH)]x(OH)8·4H2O (after Foord & Taggart, 1998).

The structure of turquoise group minerals contains distorted MO6 polyhedra (M = Zn, Cu), AlO6 octahedra and PO4 tetrahedra. The dominant structure element is an [Al2MAl2(O,OH,H2O)18] polyhedral cluster formed by a central MO6 polyhedron sharing four edges with two edge-sharing Al(1)O6-Al(2)O6 dimers. Further corner-linkage via the two non-equivalent PO4 tetrahedra forms a three-dimensional frame-work. Hydrogen bonds provide additional links between the polyhedral components (Koltish & Giester, 2000).

Screenshot-2018-1-19 12-koli 905 914 - MM64_905 pdf
View of the turquoise structure along the a-axis (the unit cell is outlined). PO4 tetrahedra are marked with crosses, AlO6 octahedra with parallel lines, and the distorted CuO6 polyhedron is shaded. The large grey sphere represents the partially occupied site (‘X’) at the position (½,0,½). The small grey spheres are H atoms (after Koltish & Giester, 2000)

Turquoise had enormous impact on human civilizations. Joseph E. Pogue (1887-1971) – american geologist, petroleum engineer, and economist, in his book “The Turquoise. A study of its history, mineralogy, geology, ethnology, archaeology, mythology, folkore, and technology”  wrote about turquoise: “Turquois is a mineral prized for its perfection of color; for, being opaque, it lacks the brilliant luster that forms the chief attraction of the transparent gems. When of finest quality it possesses a blue tone, soft and pleasing, like the color of clear sky; but often its value is lessened by a greenish cast, and in many stones the green predominates. The mineral is but slightly harder than glass, and may be worked with ease, even by primitive people possessing the crudest tools. (…) At the present day, among civilized nations, the turquois is outranked in value by the diamond, ruby, emerald, sapphire, and some other gems, although over some minds the wonderful blue of that precious stone wields a fascination shared by no other. With many semicivilized peoples, however, the turquois takes foremost rank, and its value depends not only upon its intrinsic worth but also upon the mystic properties and religious signification it is supposed to possess.”

Screenshot-2018-1-19 The turquoise a study of its history, mineralogy, geology, ethnology, archaeology, mythology, folkore,[...]
Front page of “The Turquoise. A study of its history, mineralogy, geology, ethnology, archaeology, mythology, folkore, and technology” by Joseph E. Pogue (1915)

 

Chrysoberyl

Chrysoberyl Al2BeO4 was discovered in 1789 and described and named by Abraham Gottlob Werner, in 1790. The name ‘chrysoberyl’ comes from the Greek and means ‘gold-colored beryl’. Despite the similarity of their names, chrysoberyl and beryl are two completely different gemstones. Together with alexandrite, chrysoberyl forms an independent gemstone category. Chrysoberyl comes in colors between lemon and greenish yellow, in honey colours, and shades from mint green to brownish green. The colour of yellow chrysoberyl is due to iron substitution. They are mostly found in the gemstone deposits of Brazil, Sri Lanka or East Africa.

In chrysoberyl a common repeated twinning (penetration trilling), especially in the alexandrite variety, causes a stellate appearance and the simulation of hexagonal symmetry. These twinned crystals have a hexagonal appearance, but are the result of a triplet of twins with each “twin” oriented at 120° to its neighbors and taking up 120° of the cyclic trilling.

chrysoberyl-crystals-goldschmidt
Examples of typical alexandrite twinning (penetration trilling), Izumrudnye Kopi area, Russia, acc. to “Atlas der Krystallformen” Goldschmidt 1918
If only two of the three possible twin orientations are present, a “V”-shaped twin results. Frequently an inclusion known as a stepped twin can be seen.

The best-known special effect of chrysoberyl is an eye, which is displayed when certain specimens of this gem are cut in a dome shape. Cat’s-eye chrysoberyl has a pupil-like band of light that sweeps across its dome. The “eye” is caused by fibrous inclusions that reflect the light in a sharply defined pattern. Chrysoberyl cat’s eyes are genuine rarities which are found only in a few deposits in the world, together with other varieties of chrysoberyl.

 

Alexandrite, the chromium-bearing variety, is extremely rare and highly prized as a gemstone because of its unique colouring. It is green by daylight and red under ordinary incandescent illumination; it is also strongly pleochroic.

Sulfosalts

The term “sulfosalt” (or “thiosalt”) was created by chemists during the XIXth century, by analogy to complex salts of oxygen, such as sulfate, phosphate, arsenate, antimonate, arsenite and antimonite. Oxysalts generally correspond to the combination of a simple cation with a complex anion (MeOm)n; this has been confirmed by crystal-structure studies and bond-valence calculations. In sulfosalts, S is considered to play the role of oxygen to similarly form complex anions. Sulfosalt minerals form a genetically well-defined group encountered in specific conditions of ore formation, usually referred to as hydrothermal processes (Moëlo et al. 2008).

Table presents the hierarchical structure of the system used by Moëlo et al. 2008. Whenever possible, the system is based on the level of structural relationships among mineral species. Thus, for a large number of species the system is essentially a modular classification.

Classification hierarchy of sulfosalts (Moëlo et al. 2008)
Classification hierarchy of sulfosalts (Moëlo et al. 2008)

More than 260 mineral species belong to the “sulfosalt group” (sulfosalts and other chalcogeno-salts). There are also about 200 incompletely defined minerals (so-called “UM” – unnamed minerals) in the literature related to this vast group (Smith & Nickel, 2007), mainly because the chemical composition alone was determined by EPMA, which is generally easier to obtain than crystallographic data (Moëlo et al. 2008).

The “sulfosalt group” is as heterogeneous from a crystal-chemical point of view as, e.g., the silicate group. Consequently, a rigorous classification and nomenclature of sulfosalts is much more complicated than that of more restricted mineral groups which have been reexamined in the past by specific committees of the IMA (amphiboles, micas, zeolites…). As the bulk of natural thioarsenites, thiostannates, etc. corresponds structurally to homeotypes of simple sulfides (e.g. arsenopyrite), the term “sulfosalt” is usually limited to the vast group of chalcogeno-salts containing trivalent As, Sb or Bi, as well as (exceptionally) Te4+. The S2− anion may be replaced by Se2− or Te2− (chalcogeno-salts). Thus, the general chemical formula can be given as:

(Me+, Me2+, etc.)x [(Bi, Sb, As)3+, Te4+]y × [(S, Se, Te)2−]z

At the present stage of research, some groups of these sulfosalts can already be neatly classified on a crystal-chemical basis, whereas others await further discoveries for achieving the same depth of classification. The latter are grouped on purely chemical principles (Moëlo et al. 2008).

Small dictionary:

The crystal structure of two compounds are isotypic if their atoms are distributed in a like manner and if they have the same symmetry. One of them can be generated from the other one if atoms of an element are substituted by atoms of another element without changing their position in the crystal structure. The absolute values for the lattice dimensions and the interatomic distancesmay differ, and small variations are permitted for the atomic coordinates. The angles between the crystallographic axes and the relative lattice dimensions (axes ratios) must be similar. Two isitypic structures exhibit a one-to-one relation for all atomic positions and have coincident geometric conditions (Müller 1991). Isotypic structure means, that mineral have analogous composition and closely similar crystal structure, but is not capable of intercrystallizing to form solid solutions. Examples are calcite and soda niter; galena and NaBr (mindat.org).

Two structures are homeotypic if they are similar, but fail to fulfil the aforementioned conditions for isotypism because of different symmetry, because corresponding atomic positions are occupied by several different kinds of atoms (substitution derivatives) or because the geometric conditions differ (different axes ratios, angles, or atomic coordinates) (Müller 1991).

A homology is a series of structures built on the same structural principle with certain module(s) expanding in various  dimension(s)  by  regular  increments.  This  could be  through  the  addition  of  a  layer  or  row  of  atoms  on  a given  module.  If  the  module  assembly  principle  is  constant and only the module size and volume evolve, then we can view the series as a set of isoreticular compounds (Kanatzidis 2004).

Merotypic structures (merotypes; meros = part) are composed of alternating layers (blocks), in the same way as do the homologous series. However, one set of these building layers (blocks) are common to all merotypes (i.e. they are isotypic, homeotypic or they are mutually related via homologous expansion/contraction) whereas layers (blocks) of the other set(s) differ for different mesotypes (Merlino 1997).

Plesiotypic structures (plesiotypes; plesios = near, close) form a group, that is built on the same overall principles. It means that (a) they contain fundamental structural elements (blocks, layers) of the same general type(s) and (b) mutual disposition/interconnection of these elements in all plesiotypes follows the same general rules. However, unlike the homologuous series, (1) the plesiotypic structures may contain additional structural elements that differ from one member of the family to another; (2) details of fundamental elements may may also differ between distinct members of one plesiotypic family; within this family, such elements may be interrelated by means of homologous or non-homologousexpansion, truncation, slip planes across them or in their interior, etc.; (3) details of the relationships sub (b) may differ as well (Merlino 1997).

References:
Moëlo Y., Makovicky E., Mozgova N.N., Jambor J.L., Cook N., Pring A., Paar W., Nickel E.H., Graeser S., Karup-Møller S., Balic-Žunic T., Mumme W.G., Vurro F., Topa D., Bindi L., Bente K. & Shimizu M. (2008): Sulfosalt systematics: a review. Report of the sulfosalt sub-committee of the IMA Commission on Ore Mineralogy. Eur. J. Mineral. 20, 7–46.