It would seem that John and Isabella probably separated between 1851 and 1861.
In 1861 John 49, shown as married, living with mother Ann 80 at 147 Barron Street, Old Machar and brothers Alexander 45, Joseph 39 and Francis 34. Shown as Quarry Contractor.
In 1871 still with mother (91) and brothers at 129 Barron Street - he is now a General Labourer.
In 1881 at Bridge Street living alone and 'Labourer General'.
Couldn't find him in 1891 and eventually discovered his death - died 8/4/1889 at Westfield, Balgowrie (?), Old Aberdeen - General Labourer aged 78 - parents shown as William Black, Snuff-Miller (deceased) and Ann, formerly Skinner (deceased). Shown as married to Isabella Brown.
Annette