Howlite

Howlite, a calcium borosilicate hydroxide (Ca2B5SiO9(OH)5), is a borate mineral found in evaporite deposits.

Howlite

General

Category Inoborates

Formula Ca2B5SiO9(OH)5

IMA symbol How

Strunz classification 6.CB.20

Dana classification 25.3.5.1

Crystal system Monoclinic

Crystal class Prismatic (2/m)

(same H-M symbol)

Space group P21/c

Unit cell a = 12.82 Å, b = 9.351(1) Å

c = 8.608(2) Å; β = 104.84(2)°; Z = 4

Identification

Color White, colorless

Crystal habit Massive to nodular, occurs as tabular prisms flattened parallel to 

Cleavage None

Fracture Conchoidal, uneven

Mohs scale hardness 3.5

Luster Subvitreous, glimmering

Streak white

Diaphaneity Translucent

Specific gravity 2.53 – 2.59

Optical properties Biaxial (−), colorless (transmitted light)

Refractive index nα = 1.583 – 1.586 nβ = 1.596 – 1.598 nγ = 1.600

Birefringence δ = 0.017

2V angle 73°

History

Howlite was discovered near Windsor, Nova Scotia, in 1868 by Henry How (1828–1879), a Canadian chemist, geologist, and mineralogist. How was alerted to the unknown mineral by miners in a gypsum quarry, who found it to be a nuisance. He called the new mineral silico-boro-calcite; it was given the name howlite by the American geologist James Dwight Dana shortly thereafter.

Classification

In the outdated 8th edition of the mineral classification according to Strunz, howlite belonged to the mineral class of “silicates” and there to the department “neso-subsilicates”, where it is together with dumortierite, garrelsite, grandidierite, harkerite, kornerupine, melanocerite, painite and serendibite in the “dumortierite-grandierite group” with the system number VIII/A'.13.

In the Lapis classification system by Stefan Weiß, last revised in 2018 and formally based on the 8th edition of Karl Hugo Strunz 's old system, the mineral was given the system and mineral number VIII/F.28-010. This corresponds to the class of "silicates" and the division "chain and band silicates," where howlite is the only mineral forming an unnamed group with the system number VIII/F.28. 

The 9th edition of Strunz's mineral classification, last updated in 2009 by the International Mineralogical Association (IMA), classifies howlite in the class of "borates" and within it in the division "triborates." Here, the mineral is found in the subdivision "chain and ribbon triborates (inotriborates)," where it is the sole member of an unnamed group with the system number 6.CB.20.

In the Dana classification of minerals, which is primarily used in English-speaking countries, howlite has the system and mineral number 25.03.05.01. This corresponds to the class "Carbonates, Nitrates, and Borates" and the division "Anhydrous Borates with Hydroxyl or Halogen." Here, it is found within the subdivision "Anhydrous Borates with Hydroxyl or Halogen" as the sole member of an unnamed group with the system number 25.03.05.

Crystal structure

Howlite crystallizes monoclinic in the space group P 2 1 / c (space group no. 14) with the lattice parameters a = 12.82 Å; b = 9.35 Å; c = 8.61 Å and β = 104.8° as well as 4 formula units per unit cell. 

Characteristics

Howlite is often interwoven with brown-black veins, whose marbled pattern resembles the coveted matrix of turquoise. The crystal faces exhibit a vitreous luster, while the fracture surfaces are matte. Howlite fractures conch-like, similar to glass.

Howlite is very similar in appearance to magnesite (a magnesium carbonate) and can often only be distinguished by chemical analysis. Howlite melts into a gel when exposed to hydrochloric acid, whereas magnesite reacts by releasing gas. A density determination is often insufficient to distinguish between the two, as howlite, with a density of 2.5 to 2.6 g/cm³, is very similar to magnesite (2.9 to 3.1 g/cm³). Furthermore, magnesite is often even lighter due to air inclusions. 

Chemical characteristics

It was formerly classified as a calciumsilicatecontainingboronanions, but the study of its structure indicates that it is acalciumborate that also containssilicon. In addition to the elements in its formula, it usually contains impurities such assodiumandpotassium.

Geology

The most common form of howlite is irregular nodules, sometimes resembling cauliflower. Crystals of howlite are rare, having been found in only a couple localities worldwide. Crystals were first reported from Tick Canyon in the Sierra Pelona Mountains of California, and later at Iona, Nova Scotia. Crystals reach a maximum size of about one centimeter. The nodules are white with fine grey or black veins in an erratic, often web-like pattern, opaque with a sub-vitreous luster. The crystals at Iona are colorless, white or brown and are often translucent or transparent.

Its structure is monoclinic with a Mohs hardness of 3.5 and lacks regular cleavage. Crystals are prismatic and flattened on {100}. The crystals from Tick Canyon are elongated along the 010 axis, while those from Iona are elongated along the 001 axis.

Jewelry

Howlite is commonly used to make decorative objects such as small carvings or jewelry components. Because of its porous texture, howlite can be easily dyed to imitate other minerals, especially turquoise because of the superficial similarity of the veining patterns. Due to its low Mohs hardness of 3 to 3.5, it is very soft and must therefore be protected from damage by stabilization (coating with resin or plastic) to protect it from daily use. Howlite is also sold in its natural state, sometimes under the trade names of "white turquoise" or "white buffalo turquoise," or the derived name "white buffalo stone" and is used to produce jewelry similar to how turquoise is used. 

Manipulations and imitations

Because magnesite looks so similar to howlite, but is more common and therefore cheaper, it is often mistakenly sold as howlite. Both, however, are dyed blue and serve as imitations of the rare and valuable matrix turquoise.

Esotericism

In esotericism, howlite is said to have a special psychological and physical effect, for which, however, there is no scientific evidence so far.

Among other things, it is said to strengthen judgment and memory, reduce stress, feelings of pain, and anger, neutralize negative energies, and release blockages. Howlite water is said to have a generally positive effect on problems with bones, teeth, joints, and nails.

Sourced from Wikipedia