| Barytocalcite | |
|---|---|
 Barytocalcite fromEngland | |
| General | |
| Category | Carbonate mineral |
| Formula | BaCa(CO3)2 |
| IMA symbol | Bcal[1] |
| Strunz classification | 5.AB.45 |
| Dana classification | 14.02.06.01 |
| Crystal system | Monoclinic |
| Crystal class | Prismatic (2/m) (sameH-M symbol) |
| Space group | P21/m |
| Unit cell | a = 8.1 Å, b = 5.2 Å, c = 6.5 Å, β = 106°; Z = 2 |
| Identification | |
| Formula mass | 297.42 g/mol[2] |
| Colour | Colourless, white, greyish, greenish, yellowish |
| Crystal habit | Prismatic, drusy, massive – granular |
| Cleavage | Perfect in at least two directions[2][3] |
| Tenacity | Brittle |
| Mohs scale hardness | 4 |
| Luster | Vitreous to resinous |
| Streak | White |
| Diaphaneity | Translucent to transparent |
| Specific gravity | 3.64 to 3.71 |
| Optical properties | Biaxial (−) |
| Refractive index | nα = 1.525, nβ = 1.684, nγ = 1.686 |
| Birefringence | δ = 0.161[4] |
| 2V angle | Measured: 15°, calculated: 10° |
| Dispersion | Relatively weak, r>v |
| Ultravioletfluorescence | Fluorescent dull yellow under LW and SW UV[4] |
| Solubility | Soluble in dilute HCl[4] |
| References | [2][3][4][5] |
Barytocalcite is an anhydrousbariumcalciumcarbonate mineral with the chemical formulaBaCa(CO3)2. It istrimorphous withalstonite andparalstonite, that is to say the three minerals have the same formula but different structures.[3]Baryte andquartzpseudomorphs after barytocalcite have been observed.[4]
Barytocalcite crystallizes in themonoclinic crystal system, typically as massive todruzy accumulations of transparent white to yellow to grey aggregates of slenderprismatic crystals. It has aMohs hardness of 4 and aspecific gravity of 3.64 to 3.71.[4]
It was first described in 1824 for an occurrence in theBlagill Mine inNorth Pennines,Cumbria (Cumberland),England, and named for its composition.
The mineral crystallises in themonoclinic crystal system. Most sources[3][4][5] put it in theprismatic class 2/m, withspace group P21/m, but Webmin[2] puts it in thesphenoidal class 2 with space group P21. The structure is similar to that ofdolomite, but with two crystallographically different CO3 groups.[3] Thecation layers are stacked in an ABCABC... stacking pattern, repeating every 3 layers. The Caion iscoordinated to 7 oxygens.[5]
All sources giveunit cell parameters whose values rounded down are a = 8.1 Å, b = 5.2 Å, c = 6.5 Å and β = 106°, with two formula units per unit cell (Z = 2). The primary source[5] has a = 8.092 Å, b = 5.2344 Å, c = 6.544 Å, β = 106.05° and Z=2.
Barytocalcite often forms oriented growths onbaryte, andcalcite,witherite and baryte can beepitaxial on barytocalcite.[3][4] Crystals are normally short to long prismatic and striated. They are transparent to translucent, colourless, white, greyish, greenish or yellowish with a whitestreak and a vitreous to resinouslustre.
Barytocalcite is abiaxial (-) mineral withrefractive indices nα = 1.525, nβ = 1.684 and nγ = 1.686. The maximumbirefringence (the difference in refractive index between light travelling through the crystal with differentpolarizations) is δ = 0.161.[4] The optic angle 2V is the angle between the twooptic axes in abiaxial crystal. The measured values of 2V for this mineral is 15°. It is also possible to calculate a theoretical value of 2V from the measured values of therefractive indices. The calculated value for barytocalcite is 10°. If the colour of the incident light is changed, then the refractive indices are modified, and the value of 2V changes. This is known as dispersion of theoptic axes. For barytocalcite the effect is weak, with 2V larger for red light than for violet light (r>v).[3]
The optical directions X, Y and Z are the directions of travel of light with refractive indices nα, nβ and nγ respectively. In general they are not the same as the directions a, b and c of the crystallographic axes. For barytocalcite Z is parallel to the b axis, X is at an angle of 64° to the c axis and Y is at an angle of 26° to the c axis.[3]
Sources disagree aboutfluorescence. Webmin[2] describes the mineral as non-fluorescent, Dana[3] categorises it as weakly fluorescent and Mindat[4] states that it is fluorescent dull yellow under both longwave and shortwaveultraviolet light.
Barytocalcite has at least one perfect and one imperfectcleavage.[2][3] It is a brittle mineral, and breaks with an uneven toconchoidal fracture. It is quite soft, with ahardness 4, the same as that offluorite.Calcite CaCO3 anddolomite CaMg(CO3)2 arecarbonates wherecalcium Ca andmagnesium Mg replace thebarium in the formula for barytocalcite, BaCa(CO3)2. These two minerals are even softer than barytocalcite, and much less dense. Barytocalcite hasspecific gravity 3.7. This is higher than that of calcite (2.7) or dolomite (2.85) because the barium atom is very heavy, withatomic mass 137, which is much more than magnesium at 24 and calcium at 40. Barytocalcite is soluble in dilutehydrochloric acid HCl[4] and it is notradioactive.[2]
Thetype locality is the Blagill Mine, Nent Valley,Alston Moor District, North Pennines, North and Western Region, Cumbria, England. This mine is no longer operative, and it has been designated a site ofSite of Special Scientific Interest so collecting is no longer allowed. In the past lustrous, transparent, prismatic crystals of barytocalcite have been found there[6] in veins inlimestone, associated withfluorite,calcite andbaryte.[4]
