Falcondoite, a member of the

sepiolite Sepiolite, also known in English by the German name meerschaum ( , ; ; meaning " sea foam"), is a soft white clay mineral, often used to make tobacco pipes (known as meerschaum pipes). A complex magnesium silicate, a typical chemical formula ...

group, was first discovered in the

Dominican Republic The Dominican Republic is a country located on the island of Hispaniola in the Greater Antilles of the Caribbean Sea in the Atlantic Ocean, North Atlantic Ocean. It shares a Maritime boundary, maritime border with Puerto Rico to the east and ...

, near the town of

Bonao Bonao is a city in the Cibao region of Dominican Republic. It is the capital of the Monseñor Nouel province. The city is located in the center of the country, to the northwest of the national capital Santo Domingo. The city is known as "Villa ...

. The mineral was found in a deposit mined by Falconbridge Dominica, and so was named "falcondoite" after the company. Falcondoite is frequently associated with sepiolite,

garnierite Garnierite is a general name for a green nickel ore which is found in pockets and veins within weathered and serpentinized ultramafic rocks. It forms by lateritic weathering of ultramafic rocks and occurs in many nickel laterite deposits in the ...

talc Talc, or talcum, is a clay mineral composed of hydrated magnesium silicate, with the chemical formula . Talc in powdered form, often combined with corn starch, is used as baby powder. This mineral is used as a thickening agent and lubricant ...

, and serpentine, and is commonly

nickel Nickel is a chemical element; it has symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive, but large pieces are slo ...

-bearing. While the chemical formula for falcondoite can vary, the mineral must contain more nickel than

magnesium Magnesium is a chemical element; it has Symbol (chemistry), symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 ...

to be considered its own species. The ideal chemical formula for falcondoite is (Ni,Mg)₄Si₆O₁₅(OH)₂·6H₂O.

Occurrence

Falcondoite occurs in an extensive deposit of

laterite Laterite is a soil type rich in iron and aluminium and is commonly considered to have formed in hot and wet tropical areas. Nearly all laterites are of rusty-red coloration, because of high iron oxide content. They develop by intensive and prolo ...

near the town of Bonao in the Dominican Republic. This deposit is part of a serpentinized

harzburgite Harzburgite, an ultramafic, igneous rock, is a variety of peridotite consisting mostly of the two minerals olivine and low-calcium (Ca) pyroxene ( enstatite); it is named for occurrences in the Harz Mountains of Germany. It commonly contains a ...

massif, which is composed of limonitic and serpentinitic ore. It's found in whitish-green veins of garnierite that is rich in sepiolite material. These

veins Veins () are blood vessels in the circulatory system of humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are those of the pulmonary and fetal c ...

are often only millimeters to centimeters thick, and can be formed by the infill of tension fractures. Some commonly associated minerals include sepiolite, garnierite, talc, and serpentine. While falcondoite is most closely associated to sepiolite, it's characterized by containing more nickel.

Physical properties

Falcondoite is a whitish-green to green colored translucent mineral with a resinous luster and a white streak. It exhibits a hardness of 2–3 on the

Mohs hardness scale The Mohs scale ( ) of mineral hardness is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of minerals through the ability of harder material to scratch softer material. The scale was introduced in 1812 by the Germ ...

. Falcondoite is a

brittle A material is brittle if, when subjected to stress, it fractures with little elastic deformation and without significant plastic deformation. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. ...

and

friable In materials science, friability ( ), the condition of being friable, describes the tendency of a solid substance to break into smaller pieces under stress or contact, especially by rubbing. The opposite of friable is indurate. Substances tha ...

mineral with distinct

cleavage Cleavage may refer to: Science * Cleavage (crystal), the way in which a crystal or mineral tends to split * Cleavage (embryo), the division of cells in an early embryo * Cleavage (geology), foliation of rock perpendicular to stress, a result of ...

. It occurs as aggregates of microscopic, fibrous crystals that are typically smaller than 10 μm in size, which have a refractive index of less than 1.55. Under

cross-polarized light Polarized light microscopy can mean any of a number of optical microscopy techniques involving polarized light. Simple techniques include illumination of the sample with polarized light. Directly transmitted light can, optionally, be blocked wi ...

, these crystals experience extinction parallel or nearly parallel with positive elongation along the c-axis. The mineral has a measured

density Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...

of 1.9 g/cm³ and a

specific gravity Relative density, also called specific gravity, is a dimensionless quantity defined as the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for solids and liquids is nea ...

of 1.9.

Chemical properties

Falcondoite is considered to be a variety of

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, ). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impuriti ...

-like, nickel-bearing hydrous

silicate A silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is also used ...

, which is a common component of garnierite mineralization. It is categorized as a

phyllosilicate Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust. In mineralogy, the crystalline forms of silica (silicon dio ...

whose structure consists of single tetrahedral sheets of six-membered rings that are connected by octahedral sheets or bands. Falcondoite and sepiolite form a solid solution series of garnierite, with sepiolite being characterized as containing more magnesium, while falcondoite generally contains more nickel. Falcondoite's chemical formula can range from i₈Si₁₂O₃₀(OH)₄(OH₂)₄·8(H₂O)to Ni₄Mg₄)Si₁₂O₃₀(OH)₄(OH₂)₄·8(H₂O)with the weight percent of

NiO Nio or NIO may refer to: * NI Opera, Opera company * Nio (Buddhism), guardians of the Buddha * Nio Inc., a Chinese electric automobile manufacturer * Nicaraguan córdoba, currency by ISO 4217 currency code * National Institute of Oceanography (d ...

being up to 38.03%. Some common impurities of falcondoite include

aluminum Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...

chromium Chromium is a chemical element; it has Symbol (chemistry), symbol Cr and atomic number 24. It is the first element in Group 6 element, group 6. It is a steely-grey, Luster (mineralogy), lustrous, hard, and brittle transition metal. Chromium ...

, and

iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

Chemical composition

Crystallographic properties

Falcondoite is in the orthorhombic

crystal system In crystallography, a crystal system is a set of point groups (a group of geometric symmetries with at least one fixed point). A lattice system is a set of Bravais lattices (an infinite array of discrete points). Space groups (symmetry groups ...

, with

space group In mathematics, physics and chemistry, a space group is the symmetry group of a repeating pattern in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of the pattern that ...

Pncn. The unit cell dimensions are a = 13.5 Å, b = 26.9 Å, c = 5.24 Å, and α = 90°, β = 90°, γ = 90°. The standard unit cell volume is equal to 1903 Å³. Falcondoite is biaxial positive with a refractive index value of 1.55, and a

birefringence Birefringence, also called double refraction, is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are described as birefrin ...

of δ=0.01 - 0.02.

X-ray diffraction X-ray diffraction is a generic term for phenomena associated with changes in the direction of X-ray beams due to interactions with the electrons around atoms. It occurs due to elastic scattering, when there is no change in the energy of the waves. ...

analysis of falcondoite shows a pattern similar to that of sepiolite. Some of the measured peaks end up broadened possibly due to poor crystallinity, so accurate d-values were difficult to obtain. Powder X-Ray Diffraction data:

References