Identity HelpChromite and green unknown?
Greetings,
On a recent collecting trip, a friend found a chunk of what we believe to be chromite (first photograph). It is black in color, shiny, very hard to scratch with a steel knife (estimated hardness of >4), when it is scratched into powder, the powder (streak) is brown in color, and the specimen is slightly magnetic. It looks like there are small serpentinite slickensides and possible chrysotile (or altered from such) veins in the chromite.
There is an apple-green mineral that appears to form veins in the chromite (second photograph). It is somewhat hard (maybe 4 or higher). Where exposed, it looks like rounded and smooth blobs and crusts (third photograph).
The area this was found is mostly serpentinite and small chromite mines were operated there in the past. Much of the chromite was mined as float.
Does it sound like this is pretty obviously chromite? Any ideas for what the green mineral could be?
Thanks!
Kyle
Where was this collected? 
I can only offer an opinion from sight, but it certainly looks like it might be chromite, a slight but not strong magnetic response would be expected, but the hardness should be around five, so double check to make sure you're doing the hardness test correctly. The green looks rather apple green rather than emerald green, so it's more likely what was once referred to as "genthite or "nickel gymmite" (defined here as a mix of Antigorite & Pimelite) or a deweylite serpentine colored by nickel impurities. The presence of nickel is not uncommon in many such deposits. Deweylite has a poorly ordered crystalline structure, generally defined as a layered composite of primarily lizardite and stevensite.
Serpentine can be very complex and specific species cannot really be determined without diagnostic work in the lab.
MRH
Genthite on Chromite (West Nottingham twp.,Chester Co., PA) old 19th C. collection 3.5 x 3.2 x 2.2cm 
Thanks for the feedback!
Keith: This was collected in the Clear Creek Management Area, San Benito County, California.
Mark: The >4 hardness was an estimate; all I can say is it is more difficult to scratch than, say, chalcopyrite, but still possible with a knife. That is interesting that this might be a serpentine mineral! If so, would this have formed during the serpentinization of the peridotite that contained the chromite?
Kyle Not exactly, formation of both are a part of the process. Serpentinization is essentially low-temperature metamorphic process involving heat and water in which low-silica mafic and ultramafic rocks are oxidized and hydrolyzed with water into serpentinite. Peridotite, including dunite is converted to serpentine, brucite, magnetite, chromite and a host of other minerals. In the process, large amounts of water are absorbed into the rock increasing the volume and destroying the structure. The reaction is highly exothermic and the final mineralogy depends both on rock and fluid compositions, temperature, and pressure.
I've done some reading on the northern california serpentine belt, but little to nothing on what you have down south. Here is an excellent report on the region I'm more familiar with, if you want to riffle through it for some expansion on details of the process, but you may be better off looking for a publication more specific to your area. It is a complex process with many variables from deposit to deposit, but I must admit I've always been fascinated by serpentine zones in general!
http://pubs.usgs.gov/bul/1546a-d/report.pdf
MRH
