Hydrogrossular is a subspecies of grossular. When SiO2 is insufficient, 4 (OH) will replace 1 [SiO4] to form Hydrogrossular, with the chemical formula of Ca3 Al2 (SiO4) 3-x (OH) 4x.
Hydrogrossular jade is a kind of jade with Hydrogrossular as the main mineral composition, which can contain a small amount of diopside, calcite, Vesuvianite, chlorite and other minerals with granular structure.
As a polycrystalline aggregate, Hydrogrossular is translucent to opaque and is often used as an imitation of jadeite.
The color of gem-grade Hydrogrossular is mainly green, and there are also a few varieties of blue-green, white, colorless and pink. The color is distributed unevenly in dots, blocks and irregular spots.
Hydrogrossular is usually in the form of massive aggregate, growing with superior jadeite, with a refractive index of 1.70~1.73 and a relative density of 3.35.
The polished surface is glassy, and the fracture surface is greasy to glassy. The hardness is the same as that of jadeite.
The absorption spectrum shows a narrow absorption band at 461 nm in the blue region.
The green part of the Hydrogrossular is pink to red under the Charles filter; It is inert under ultraviolet fluorescent lamp.
There is strong yellow and orange fluorescence under X-ray. When magnification, black dot inclusions can be seen in Hydrogrossular.
Hydrogrossular is green due to its chromium content and light pink due to its manganese content.
Because of its water content, its refractive index and density are lower than that of grossular.
Because its color is similar to that of jadeite, and its toughness is good, it is known as “African Jade”, “Delanswa Jade,” and “Arizona Jade.” It is loved by people, especially Asians.
Among them, emerald green Hydrogrossular is the most beautiful.
Basic Property of Hydrogrossular
- Mohs hardness: 7.
- Density: 3.15~3.56g/cm, generally 3.4~3.5g/cm.
- Optical property: Hydrogrossular is homogeneous, and the aggregate often contains heterogeneous minerals.
- Refractive index: 1.670 ～ 1.734, generally 1.720.
- Gloss: glassy, fracture glassy to greasy.
- Color: green, blue-green, yellow, pink, maroon, white, etc.
- Transparency: translucent to slightly transparent.
- The crystalline state: crystalline or crystalline aggregate. Isometric.
- Cleavage; None.
- Mohs hardness: 7.
- Optical characteristics: homogeneous.
- Multicolor: None.
- Birefringence: None.
- Ultraviolet fluorescence: None.
Dark green: complete absorption below 460nm;
Other colors: Absorbed near 463 nm (because it contains Vesuvianite).
Enlarged inspection: black dotted inclusion.
Special properties: It is pink to red under the Charles filter.
Origin of Hydrogrossular
It occurs in norite, plagioclase and gabbro and belongs to altered minerals. It can also be found in the contact metamorphic zone between magmatic rock and marl. Delanswa, South Africa, the former Soviet Union’s Caucasus, the United States and New Zealand are the main producing areas.
The difference between Hydrogrossular and jadeite
Hydrogrossular is similar to jadeite in appearance and is often used to process into pendants and bracelets for sale as jadeite.
They have the following differences:
A) The density of Hydrogrossular is 3.47g/cm, which is higher than that of jadeite. Compared with the same size, Hydrogrossular is slightly heavier than jadeite on hand.
B) Hydrogrossular contains chromite, often with black spots, which is an essential basis to distinguish it from jadeite.
C) The green color of the Hydrogrossular is distributed with spots, with black spots or patches, and granular structure, Most of them are good transparency, but a small part is poor transparency. The green of jadeite jade is distributed in sheets and filaments.
D) The refractive index of Hydrogrossular is about 1.72, which is obviously higher than that of jadeite,
E)The primary mineral of Hydrogrossular is Hydrogrossular, followed by epidote, Vesuvianiteand amphibole.
The most significant difference is the green part of the Hydrogrossular will turn dark purplish red under the Charles filter, and the dark green of the Hydrogrossular is observed to be fully absorbed below 460nm under the spectroscope.
To sum up, the differences between Hydrogrossular and jadeite are:
- The Hydrogrossular is a granular structure, while the jadeite is a typical fiber-interlaced structure;
- The refractive index of Hydrogrossular is higher than that of jadeite;
- The relative density of Hydrogrossular is also higher than that of jadeite;
- The absorption spectrum of Hydrogrossular is also different from that of jadeite. It is translucent, green, with black color block inclusions, and there is no characteristic absorption spectrum line in the red area of Hydrogrossular with grease luster;
- Hydrogrossular is red under the Charles filter, while natural jadeite does not change color under the Charles filter.
From the perspective of color, jadeite jade has uneven color distribution, ranging from dark to light, which is relatively natural. The glass luster of jadeite jade after polishing is strong;
While the color distribution of Hydrogrossular is uneven, the color depth and tone are the same.
It is generally the green hydrogrossular used as imitated jadeite for a long time. However, in the past two years, another variety of grossular on the market also acts as fake jadeite. Through testing and analysis, it is determined that it is yellow grossular imitated yellow jadeite.
Therefore, in the process of jade detection, we generally use magnifying observation to check the structure of the test sample and visual observation of its color and luster. The structure and luster of the polycrystalline yellow grossular are very close to yellow jadeite. The accurate identification of the Hydrogrossular pendant can be determined by Raman spectroscopy.
The appearance characteristics of Hydrogrossular are similar to jadeite. Yellow Hydrogrossular is often sold as yellow jadeite, and green Hydrogrossular is used as green jadeite.
The distinguishing features of Hydrogrossular are that it has a granular structure,and granular minerals can be seen at the fracture;
The green color on the surface of the Hydrogrossular is distributed in round, water-drop-shaped spots. There are usually one or several black dots in the center of the punctate green, which is caused by the iron precipitated when the Hydrogrossular is subjected to chloritization.
The Hydrogrossular mineral is mainly formed in the sand karsts and also in the regionally metamorphosed calcareous rocks.