Quartz; The minerals of Silica Group

Quartz is the collective name used for the minerals of Silica group. The Chemical composition of all minerals of quartz is Silicon and Oxygen. The chemical formula of these minerals is SiO2. Quartz is commonly found in vugs or pockets in pegmatite dikes (coarse-grained igneous rocks). There are more than 30 types of quartz, among which the most important are discussed as under.

Varieties of Quartz

Macrocrystalline Variety of Quartz Minerals

(1). Rock Crystal:

  • It is transparent and colorless quartz.
  • Rocky Crystal Quartz commonly occurs in quartz veins where it crytallizes inside rock cavities known as vugs. 
  • It also is common in vugs or pockets in pegmatite dikes.
  • Rock crystal often occurs as secondary quartz crystals on cryptocrystalline quartz in cavities and vugs, and in geodes.
  • It has a vitreous luster.
  • It is found in trigonal and hexagonal crystal systems.
  • It has an indistinct cleavage, thus having a conchoidal fracture.
  • It has brittle tenacity.
  • White streak.
  • Luster: Vitreous (waxy to dull when massive).
  • It has a specific gravity 2.65. But in the impure form the specific gravity ranges from 2.59 to 2.63.

 

(2). Amethyst:

  • A macrocrystalline form of quartz group of minerals.
  • A purple-colored gemstone is the world’s most popular mineral.
  • It has historical metaphysical importance and has been used for thousands of years.
  • Amethyst is a hard mineral, which has a hardness of 7 on moh’s scale.
  • The most common color of the amethyst is purple, however, in rare cases it is found in reddish purple color too.
  • It has a vitreous Luster.
  • It is harder than a streak plate.
  • It has a transparent to translucent diaphaneity.
  • It has a bad cleavage, and typically breaks with a conchoidal fracture.
  • Specific gravity= 2.6 -2.7
  • It can easily be diagnosed by its conchoidal fracture, glassy luster, hardness, purple color.
  • Like other silicate minerals, it has a chemical formula ; SiO2.
  • It has a hexagonal crystal system.

(3). Smoky Quartz:

  • Smoky quartz is macrocrystalline form of the quartz/ silicate group.
  • It is a brownish-grey colored mineral of silicate/ quartz group.
  • Its varieties have diaphaneity from completely transparent to opaque.
  •  Its streak is Colorless (harder than the streak plate).
  • It has a vistreous luster.
  • Its hardness is 7 on moh’s scale.
  • Its specific gravity lies between 2.6 and 2.7.
  • Smoky Quartz can be diagnosed by its conchoidal fracture, glassy luster, hardness, brownish color.
  • It is included in hexagonal crystals systems.
  • Smoky quartz is mainly found in quartz veins and pegmatite dikes that cut across metamorphic and igneous rocks.

(4). Rose Quartz:

  • The pink-colored quartz is known as rose quartz, which is found in the macro-crystalline form.
  • The chemical formula of the rose quartz is SiO2, which includes it in the group of quartz and silicate minerals.
  • The pink color of the rose quartz ranges from pale pink to vivid pink.
  • It has a colorless streak and harder than the streak plate.
  • It is found in vitreous luster, and transparent to translucent diapheniety.
  • Like most of the quartz minerals, it has a hardness about 7 on Moh’s scale.
  • Its specific gravity is from 2.6 – 2.7, like amethyst, and smoky quartz.
  • It can easily be diagnosed by its conchoidal fracture, glassy luster, hardness, pink color, translucence.
  • Like most of the quartz family minerals, it is also found in the hexagonal form of crystal systems.
  • Rose quartz is found in quartz cores of pegmatites and is believed to form at high temperatures, but it has also been found in hydrothermal veins.

(5). Milky Quartz:

  • The Milky quartz is commonly called as Milk Diamond/ or Quartz Milk/ or white wisdom quartz.
  • It is macrocrystalline form of the quartz/ silicate group.
  • It is probably the most common variety of quartz minerals and almost found everywhere in every country of the world.
  • It is a white milky colored mineral.
  • Its varieties have diaphaneity from translucent.
  •  Its streak is white. (harder than the streak plate).
  • It has a vistreous luster.
  • Its hardness is 7 on moh’s scale.
  • It is also found in hexagonal crystal shape.
  • Its specific gravity lies between 2.6 and 2.7.
  • Smoky Quartz can be diagnosed by its conchoidal fracture, glassy luster, hardness, brownish color.
  • It is included in hexagonal crystals systems.
  • Smoky quartz is mainly found in quartz veins and pegmatite dikes that cut across metamorphic and igneous rocks.

(6). Phantom Quartz:

  • Phantom quartz is recognized by its characteristic phantom crystal within itself. It can be identified as quartz by its crystal habit, transparency, hardness, glassy luster, conchoidal fracture, occurance and general lack of cleavage.
  • The phantom quartz crystal is actually formed on the earlier existing crystal for millions and millions of years.
  • It is actually a mineral deposit because it contains crystal within a crystal.
  • Phantom quartz is found in bright green color.
  • It has a specific gravity 2.65, but in raw form has lower specific gravity.
  • It has a glassy luster.
  • It has a lack of a cleavage, and breaks into the conchoidal fracture.

(7). Girasole:

  • Girasole is a quite clear, milky white, or extremely pale pink variety of quartz.
  • The word ”Girasole” has been derived from the Italian word, Girasole, which means sun.
  • The word was referred to as a milky blue glass in the early renaissance.
  • It is loved because of its internal floating glow, which moves with the changes of light thrown on it.
  • Its physical properties resemble opal, sapphire, and moon-stone.
  • Girosole quartz is excavated from the mines from where the rose quartz is mined.
  • It has a colorless streak.
  • It is hard as 7 on Moh’s scale, it is pretty harder than a streak plate.
  • Its specific gravity is 2.65 -2.69.
  • It is found in hexagonal crystal systems.
Quartz possessing needles of TiO2 (Rutile Mineral).

(8). Rutilated Quartz:

  • Rutilated quartz is another special microcrystalline variety of quartz.
  • This mineral includes needle-like threads of another mineral namely Rutile (Titanium Dioxide; TiO2) in it.
  • It is transparent and clear mineral, but due to the presence of the brown colored needle like structures of Rutiles make it half opaque.
  • Other properties of this mineral are same as those of the rest of the quarts group minerals, except some of the physical properties because of inclusion of rutile mineral.
  • It is hard at level 6-6.5 on Moh’s scale
  • Due to the presence of impurities (rutile) its specific gravity is higher than the rest of the quartz minerals, which is 4.2 -4.4.
  • It can easily be diagnosed by means of its luster, color, specific gravity, prismatic crystal habit.
  • It is used as an ore of titanium, pigments, inert coating on welding rods.
  • It has a tetragonal crystalline form.
  • Its chemical composition is SiO2 + TiO2.

(9). Jacare Quartz:

  • Jacare quartz is also known as Elestial quartz, skeletal quartz, and crocodile quartz.
  • It is included in microcrystalline quartz.
  • It has a light smoky, brown natural color.
  • It has a semi-transparent diaphaneity.
  • They are actually found in the smoky quartz, amethyst, citrine quartz, and in some rare instances, rose quartz.
  • This particular kind of crystal, which is etched and is multi-coated with some other crystals.
  • The specific gravity of jacare is 2.56 -2.61.

(10). Herkimer Diamond:

  • Herkimer diamond is named Herkimer Diamonds because they were originally discovered in the area of Herkimer County of New York.
  • This mineral is chemically classified in Silicate group of minerals.
  • It is sometimes colorless, while in rare case it is found in smoky color.
  • It give colorless streak on rubbing with streak plate, but it is harder than a streak plate.
  • It has a vitreous luster.
  • it is found in transparent diaphaneity.
  • It is no cleavage, thus having concoidal fracture.
  • On Moh’s scale, its hardness is calculated as 7.
  • It has a specific gravity from 2.6 to 2.7.
  • It can be diagnosed by means of its glassy, transparent color and vitreous lustre.
  • Its chemical formula is SiO2.
  • It is used in crystal jewelry.

(11). Citrine Quartz:

  • It is a microcrystalline variety of quartz mineral.
  • The word citrine has been taken from citrina, which means yellow.
  • It is found in yellow transparent to grey semi-transparent colors.
  • It has white steak on rubbing on a streak plate, however, it is pretty harder than the streak plate.
  • Hardness on Moh’s scale is 7.
  • It is found in trigonal crystalline form.
Strawberry Quartz

(12). Strawberry Quartz:

  • It is a silicate mineral, with a hardness 7 on Moh’s scale.
  • It has a trigonal crystal structure.
  • Hematite is included in this mineral.
  • As hematite mineral is included in this mineral, therefore the chemical composition of Strawberry Quartz is SiO2 + Fe2O3.
  • Strawberry Quartz is often combined with Rose Quartz to make pretty, feminine pieces of jewelry such as bracelets, earrings, and necklaces.
  • In India and countries influence by India, it may be included in chakra (spiritual powers in the human body) jewelry as the Base stone. It is also found in larger pieces carved into candle holders or other ornaments for use around the house.
Ametrine

(13). Ametrine:

  • Ametrine is a blend of Amethyst (a purple quartz) and Citrine (a yellow quartz).
  • Like most quartz gems, ametrine occurs in fairly large, clean pieces.
  • It has colors of Purple amethyst and golden-yellow citrine.
  • It has a vistreous luster, and a colorless streak.
  • It is harder than a streak plate.
  • It can be diagnosed by its conchoidal fracture, amethyst and citrine in a single crystal.
  • Its hardness on Moh’s scale is 7.
  • Its specific gravity is 2.6 to 2.7.
  • It is used in jewelry.
  • As it has no cleavage, therefore breaks into conchoidal fractures.

(14). Atlas Diamond:

  • It is a highly strong mineral of silica group, which has a hardness of 7.2 on Moh’s scale.
  • It is found in transparent whitish color.
  • It has a vistreous luster, and a colorless streak.
  • It is harder than a streak plate.
  • It can be diagnosed by its conchoidal fracture, amethyst and citrine in a single crystal.
  • Its hardness on Moh’s scale is 7.
  • Its specific gravity is 2.6 to 2.7.
  • It is used in jewelry.
  • As it has no cleavage, therefore breaks into conchoidal fractures.

(15). Tiger’s Eye Quartz:

  • A red-brown coloured, silky luster gemstone found mostly in metamorphic rocks.
  • A very famous member of quartz minerals, which resembles with a tiger’s eye.
  • Another related blue coloured mineral is known as a ”Hawk’s Eye”.
  • Tiger Eye quartz has golden to red colours.
  • Maximum hardness on Moh’s scale is 7.
  • Tiger Eye has a silky luster.
  • Specific gravity of tiger eye is 2.64 0 2.71.

(16). Cat’s Eye Quartz:

  • Another attractive quartz member resembles with a cat’s eye.
  • It is found in green and grey-green colours.
  • The maximum hardness on Moh’s scale is 7.
  • The specific gravity of Cat’s eye is 2.65 to 2.71.
  • It includes parallel fibers of asbestos in it.

 

Cryptocrystalline forms of quartz

(1). Chalcedony: 

  • A chalcedony is a cryptocrystalline form of silica mineral/quartz. Cryptocrystalline means very fine and minute crystals, which can not be seen with a naked eye.
  • Chalcedony has a variety of colors, i.e. black, green emerald, grey smoky, chalky color, purple (Amethystine Chalcedony), golden yellow, white, light green, red-orange, lapis blue, light pink rose quartz, and blue topaz.
  • Chalcedony is mixture of quartz and moganite (moganite is another silicate mineral, which was discovered in 1984, that differs from quartz because it has a monoclinic crystal form, while quartz has trigonal crystal form).
  • The chemical formula of Chalcedony is SiO2.
  • Physically Chalcedony is found in Hexagonal crystal systems.
  • It has a waxy luster and has a transparent or semi-transparent diaphaniety.
  • It is less hard than most of the quartz members. Chalcedony has a hardness equal between 6 and 7 on Moh’s scale.
  • It has no cleavage, and if broken, it breaks into uneven splintery conchoidal fractures.
  • Chalcedony gives a white streak, it is not harder than the streak plate.
  • The specific gravity of Chalcedony is 2.59–2.61.
  • Chalcedony is classified into two groups, natural chalcedony, and dike chalcedony. A natural Chalcedony has translucent diaphaneity, while the dike chalcedonies are semi-transparent.

(2). Agate:

  • Agate is another microcrystalline form of silicate minerals, which is a blend of Chalcedony and Quarts.
  • Agate has a wired variety of colors i.e black, blue, natural, purple, green, pink, teal, and orange.
  • It has a cryptocrystalline habit in the Silicate group.
  • Like other quartz minerals, it also has no cleavage, and on breaking it give a conchoidal fracture with very sharp edges.
  • Its hardness on Moh’s scale is 6.5 – 7.
  • It has wavy luster, and white streak.
  • It is heavier than the rest of the silica minerals and has a specific gravity 2.58 -2.64.
  • Unlike other quartz minerals, it is not included in the hexagonal crystal lattice, it is in the Rhombohedral crystal lattice.

(3). Jasper:

  • It is an opaque and impure variety of silicate minerals.
  • It is a blend of microgranular quartz and other minerals like Chalcedony.
  • It has a variety of colours: blue, green, red, white, yellow, black, grey, and orange.
  • It is found in hexagonal crystal systems.
  • Hardness on Moh’s scale is 6.5-7.
  • Included in Chalcedony mineral class.
Flint

(4). Flint:

  • It is a sedimentary cryptocrystalline form of quartz minerals.
  • Flint was historically used to make stone tools and make fire.
  • In nature, it occurs as nodules and interbedded inclusions in sedimentary deposits as a result of digenesis processes when calcium carbonate is replaced with Cilicia.
  • It has a waxy dull luster.
  • Its transparency is translucent to opaque.
  • Flint has Gray, black, brown, red, white, and other colors due to staining.
  • Streak: White.
  • Hardness: 6½ – 7 on Mohs scale.
  • Tenacity: Brittle.
  • Cleavage: None Observed.

(5). Chert:

  • Chert is a sedimentary rock consisting entirely of SiO2.
  • It is a very hard microcrystalline form of the quartz mineral, which forms in a variety of ways. Biochemical chert is formed from siliceous skeletons of marine plankton which are dissolved during diagenesis, with silica being precipitated from the resulting solution. When other material is replaced by silica, e.g. petrified wood forms when silica-rich fluids percolate through deadwood and the silica precipitates to replace the wood, it is called ”Replacement Chert”. Chert can also form through direct precipitation from silica-rich fluids, e.g. agate is formed by the precipitation of silica in voids within a rock.
  • Chert has the general physical properties of quartz.
  • Chert has general Physical Properties of Quartz.
  • It has a non-clastic texture.
  • Grain size: cryptocrystalline, cannot be seen except under very high magnification.
  • Hardness: hard, has a hardenss more than 7 on Moh’s scale.
  • Colours of chert dependent on impurities present when precipitated.
  • It has a glassy luster and opaqe diaphaniety.

(6). Opal:

  • Opal is a hydrated amorphous form of silcate mineral, which contains normally 6-11 % of water.
  • Color: Colorless, white, yellow, red, orange, green, brown, black, blue.
  • Luster: Subvitreous to uneven.
  • Transparency: Opaque, translucent, transparent.
  • Crystal System: Amorphous.
  • Crystal Habits: Irregluar veins, in masses, in nodules.
  • Cleavage: None.
  • Fracture: Concoidal to uneven.
  • Hardness: 5.5 – 6.
  • Opal’s specific gravity is 2.15.
  • It is used for ornamental purpose, and in Indian region it is used to activate several chakras. (Chakra means the presence of spiritual beings in human body)
Aventurine

(7): Aventurine:

  • Aventurine is known for its tranlucency.
  • It contains other platy minerals in it, which give it glistening effect termed aventurescence.
  • Aventurine has no cleavage and breaks into the conchoidal fracture. Abundant mica inclusions with a common orientation can cause a preferential direction of easy breakage.
  • Its hardness is equal to 7 on Moh’s scale, but the inclusion of large quantity of the mica may reduce its hardness.
  • It is found in hexagonal crystalline form.
  • It can be diagnosed by means of its Aventurescence, often green, hardness, conchoidal fracture. Microscopic examination with darkfield illumination will show tiny color-causing inclusions.
  • Aventurine’s specific gravity is 2.6 to 2.7, but it can rise with the inclusion of heavy substances.
Coloured layers of Onyx mineral

(8). Onyx:

  • Onyx is a banded agate  (a variety of Chalcedony) with layers in parallel lines, which gives alternating colors, especially black and white (sardonyx).
  • It is trigonal crystalline system.
  • Chemical composition is Silicon dioxide.
  • Colours; There are different layers of colours in an Onyx, mostly white and black layers.
  • Maximum hardness on Moh’s scale is 7.
  • Onyx has a vitreous silky luster.
  • Onyx gives a white streak on grinding with a streak plate.
  • It is found in translucent diaphaniety.
  • Onyx has no cleavage and on breaking is gives sharp conchoidal fractures.

How are Macro- and Cryptocrystalline Quartz Different?

Macrocrystalline quartz differs from the cryptocrystalline and microcrystalline forms of quartz in a way that

  • A microcrystalline minerals give an outside crystalline appearance, while the micro- and cryptocrystals can not be seen with naked eye from outside.
  • Macrocrystalline quartz like amethyst and rose quartz are generally transparent-translucent, whereas cryptocrystallines like agates are more often opaque.
  • A cryptocrystallines are commonly softer than the microcrystalline.
  • Most of the micro and cryptocrystallines have a hardness about 6.5 to 7 on Moh’s scale.
  • Cryptocrystallines have a duller, wax-like luster while the microcrystallines have a shiny, vitreous luster.
  • Cryptocrystallines have a higher water content as well as other non-quartz ingredients, up to 20% more.Tree Agate cabochon.
  • A cryptocrystalline quartzes can include several non-quartz minerals, many of these quartzes are considered “rocks,” rather than macrocrystalline amethyst, citrine, tiger eye, and smoky quartz, which are more likely called “gems” or “stones” due to their purity.

Occurrence of Quartz

Quartz occurs in most igneous rocks, metamorphic rocks, and sedimentary rocks. Some sandstones and quartz are composed almost entirely of quartz. It is also found abundantly as a gangue mineral in mineral veins. Agate occurs in volcanic lavas as a cavity filling.

Uses of Quartz

  1. Colored varieties of quartz are used as semi-precious stones.
  2. In the form of sandstone, it is used as construction material.
  3. Quartz is used as a flux or abrasive in industries.
  4. Quartz crystals are used as oscillators in electrical equipment, such as watches.

 

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